sign in sign up
<<
Home , Braided river, Channel pattern

Latin American Journal of Sedimentology and Basin Analysis E-ISSN: 1851-4979 [email protected] Asociación Argentina de Sedimentología Argentina

Bridge, John S. Characterization of fluvial hydrocarbon reservoirs and : problems and solutions Latin American Journal of Sedimentology and Basin Analysis, vol. 8, núm. 2, diciembre, 2001, pp. 87-114 Asociación Argentina de Sedimentología Buenos Aires, Argentina

Available in: http://www.redalyc.org/articulo.oa?id=381747741005

How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative AAS Revista (2001), vol. 8 nº 2: 87-114. ISSN 0328 1159 Asociación Argentina de Sedimentología

Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions

John S. BRIDGE

Department of Geological Sciences, Binghamton University, P.O. Box 6000, Binghamton, NY 13902-6000, USA. Email: [email protected]

Abstract. Fluvial deposits are important hydrocarbon reservoirs and aquifers in many parts of the world. In order to assess, develop and manage these resources, it is necessary to determine the three-dimensional geometry, orientation, spatial distribution and total volume of the reservoirs/aquifers. Such characterization of reservoirs/aquifers normally involves (1) analysis of well logs, cuttings, cores and seismic data (2) correlation of strata between wells, and (3) modeling of the three-dimensional volume between wells. These procedures require a great deal of geological knowledge. Unfortunately, approaches to reservoir/ characterization are problematical in many cases. Specific fluvial depositional forms (e.g., bars, crevasse splays, lacustrine deltas) are commonly interpreted from well logs and cores, because it is thought that deposits associated with particular depositional forms will have particular geometries and reservoir quality. However, it is very difficult to unambiguously interpret fluvial depositional forms from such one-dimensional data, and it is questionable that a particular depositional form will have a distinctive stratal geometry. In particular, it is not possible to distinguish braided deposits from meandering river deposits based on their vertical facies sequences. Also, it is a common myth that there is a relationship between channel-belt width/thickness, , grain size of load, and stability. However, with very careful analysis, the lateral extent of subsurface channel-belt deposits and overbank deposits can be estimated from their facies, thickness, and proportion observed in well logs and cores. Correlation of fluvial lithostratigraphic units (e.g., channel-belt sandstone bodies) between wells is commonly based on untenable assumptions. The result is normally that stratigraphic units appear much more laterally continuous than they really are. Before attempting correlation, it is desirable to estimate the likely lateral extent and thickness variation of stratigraphic units, based on the interpretation of available well logs and cores mentioned above. Then, it is wise to set up multiple working hypotheses for the correlation. Estimation of the geometry of stratigraphic units is aided by use of modern analogs, ancient analogs, and depositional models. Modern analogs are superior aids because it is possible to describe fluvial morphology and deposits directly and unambiguously. Ancient analogs are less reliable, because their origin must be interpreted, and because outcrops are commonly not large enough to allow definition of the facies geometry (e.g., channel-belt width). Depositional models can be used to help estimate the lateral extent of stratigraphic units (e.g., zones of high or low net-to-gross). However, such models may be only two-dimensional, not quantitative, or do not represent depositional processes and products correctly. Sequence stratigraphic models are particularly problematical in these regards. In order to characterize the three-dimensional volume between wells, it is common to make use of ancient analogs in combination with object-based stochastic models. The geometry and proportion of stratigraphic objects such as channel-belt sandstone bodies are determined statistically from well data and ancient analogs, and these objects are located in space using their known positions in wells and using quasi-random placement between wells. Although this method honors well data, the shapes and locations of objects can be very unrealistic. Process-based (forward) models are capable of producing much more realistic stratigraphy, and have much greater predictive value, but it is very difficult to fit such models to well data exactly. For this reason, process-based models are not routinely used for reservoir/aquifer modeling. However, it has been shown recently that hybrid process-based and stochastic models can potentially be fitted exactly to well data using a trial-and-error approach with new optimization procedures (genetic algorithms).

87 John S. BRIDGE

Keywords: Fluvial reservoirs and aquifers, well logs and cores, stratigraphic correlation, 3-D alluvial architecture models.

Palabras claves: Reservorios y acuíferos fluviales, perfiles de pozo y testigos corona, correlación estratigráfica, modelos de arquitectura aluvial 3-D.

RESUMEN EXPANDIDO los obtenidos de análogos modernos porque en ellos es posi- ble describir sin ambigüedades la morfología y los depósitos Caracterización de reservorios de hidrocarburos y acuíferos fluviales. Los análogos antiguos son menos confiables debi- fluviales: Problemas y Soluciones. Los depósitos fluviales do a que su origen debe ser interpretado y porque los aflora- son importantes reservorios de hidrocarburos y acuíferos en mientos no son lo suficientemente extensos como para per- muchas regiones del mundo. A fin de valorar, desarrollar y mitir la definición de las geometrías de las facies (por utilizar estos recursos es necesario determinar la geometría ejemplo: ancho de la faja de canales). Los modelos tridimensional, la orientación, la distribución espacial y el depositacionales pueden ser utilizados para ayudar a esti- volumen total de los reservorios y acuíferos. Generalmente, mar la extensión lateral de las unidades estratigráficas (por este tipo de caracterización de reservorios y acuíferos ejemplo: zonas de alta o baja relación porcentual entre espe- involucra, (1) el análisis de perfiles de pozos, cuttings, testi- sor útil y espesor total del reservorio, net-to-gross). Sin em- gos corona y datos sísmicos, (2) la correlación de estratos bargo, tales modelos pueden ser análogos inapropiados, ya entre pozos, y (3) el modelado tridimensional de los depósi- que son solamente 2-D, no cuantitativos, o no representan tos entre los pozos. Estos procedimientos requieren una ela- correctamente los procesos y productos depositacionales. En boración del conocimiento geológico. Desafortunadamente, estos análisis, los modelos de secuencia estratigráfica son en muchos casos las propuestas actuales para la caracteriza- particularmente problemáticos. ción de los reservorios y acuíferos son problemáticas. A fin de caracterizar el volumen de los depósitos entre los Las formas fluviales de depositación (ejemplos: barras de pozos, es común utilizar los análogos antiguos en combina- , crevasse splays, deltas lacustres) son interpretadas ción con objetos basados en modelos estocásticos. La geome- comúnmente a partir de perfiles de pozo y testigos corona, tría y la proporción de los objetos estratigráficos, tales como ya que se piensa que los depósitos asociados con formas cuerpos de areniscas de faja de canal, son determinados depositacionales determinadas tendrán geometrías particu- estadísticamente a partir de datos de pozo y análogos anti- lares y una calidad de reservorio determinada. Sin embargo, guos, estos objetos son ubicados en el espacio utilizando sus es muy difícil interpretar inequívocamente las formas posiciones conocidas en los pozos y un emplazamiento cuasi depositacionales fluviales a partir de estos datos aleatorio entre los pozos. Aunque este método señala la im- unidimensionales y es cuestionable suponer que una forma portancia de los datos de pozos, las formas y ubicaciones de depositacional particular tendrá una geometría estratal dis- los objetos son muy poco realistas. Los modelos basados en tintiva. En especial, no es posible distinguir depósitos de procesos (ver más adelante) son capaces de producir ríos enlazados de depósitos de ríos meandrantes en base a estratigrafías mucho más realistas y tienen un mayor valor sus secuencias verticales de facies. Además, es un mito po- predictivo pero es muy difícil adecuar exactamente estos pular que hay una relación entre ancho/profundidad de la modelos a los datos de pozos. Por esta razón, los modelos faja de canales, diseño del canal, tamaño de grano de la car- basados en procesos no son usados rutinariamente en el ga de lecho y estabilidad de los márgenes. Sin embargo, modelado de reservorios y acuíferos. Sin embargo, reciente- mediante un análisis muy detallado de las facies, el espesor mente ha sido demostrado que potencialmente los modelos y la proporción observados en los perfiles de pozo y testigos híbridos, entre estocásticos y basados en procesos, pueden corona puede ser estimada la extensión en subsuelo de los ser ajustados con exactitud a los datos de pozos utilizando depósitos de las fajas de canales y de los depósitos de una aproximación por prueba y error con nuevos procedi- albardón. mientos de optimización (algoritmos genéticos). La correlación de unidades litoestratigráficas fluviales (por Los depósitos fluviales son importantes reservorios de hi- ejemplo: cuerpos de areniscas de faja de canales) entre po- drocarburos y acuíferos en muchas regiones del mundo. A zos está basada comúnmente en suposiciones insostenibles. fin de valorar, desarrollar y utilizar estos recursos es necesa- Normalmente, el resultado de estas correlaciones es que las rio determinar la geometría tridimensional, la orientación, unidades estratigráficas parecen mucho más continuas late- la distribución espacial y el volumen total de los reservorios ralmente de lo que son en realidad. Antes de intentar la y acuíferos. Generalmente, este tipo de caracterización de correlación, es conveniente estimar la probable extensión reservorios y acuíferos involucra, (1) el análisis de perfiles lateral y las variaciones de espesor de las unidades de pozos, cuttings, testigos corona y datos sísmicos, (2) la estratigráficas, basándose en el análisis de los perfiles de correlación de estratos entre pozos, y (3) el modelado pozo y testigos corona mencionados anteriormente. A partir tridimensional de los depósitos entre los pozos. Estos proce- de estas estimaciones es necesario establecer múltiples hi- dimientos requieren una elaboración del conocimiento pótesis de trabajo para realizar la correlación. La estimación geológico. Desafortunadamente, en muchos casos las pro- de la geometría de las unidades estratigráficas es factible puestas actuales para la caracterización de los reservorios y mediante el uso de análogos actuales, análogos antiguos y acuíferos son problemáticas. modelos depositacionales. Los estimadores más útiles son Las formas fluviales de depositación (ejemplos: barras de

88 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions canal, crevasse splays, deltas lacustres) son interpretadas Los análogos antiguos son menos confiables debido a que su comúnmente a partir de perfiles de pozo y testigos corona, ya origen debe ser interpretado y porque los afloramientos no que se piensa que los depósitos asociados con formas son lo suficientemente extensos como para permitir la defi- depositacionales determinadas tendrán geometrías particu- nición de las geometrías de las facies (por ejemplo: ancho lares y una calidad de reservorio determinada. Sin embargo, de la faja de canales). Los modelos depositacionales pueden es muy difícil interpretar inequívocamente las formas ser utilizados para ayudar a estimar la extensión lateral de depositacionales fluviales a partir de estos datos las unidades estratigráficas (por ejemplo: zonas de alta o unidimensionales y es cuestionable suponer que una forma baja relación porcentual entre espesor útil y espesor total depositacional particular tendrá una geometría estratal dis- del reservorio, net-to-gross). Sin embargo, tales modelos tintiva. En especial, no es posible distinguir depósitos de ríos pueden ser análogos inapropiados, ya que son solamente 2- enlazados de depósitos de ríos meandrantes en base a sus D, no cuantitativos, o no representan correctamente los pro- secuencias verticales de facies. Además, es un mito popular cesos y productos depositacionales. En estos análisis, los que hay una relación entre ancho/profundidad de la faja de modelos de secuencia estratigráfica son particularmente pro- canales, diseño del canal, tamaño de grano de la carga de blemáticos. lecho y estabilidad de los márgenes. Sin embargo, mediante A fin de caracterizar el volumen de los depósitos entre los un análisis muy detallado de las facies, el espesor y la pro- pozos, es común utilizar los análogos antiguos en combina- porción observados en los perfiles de pozo y testigos corona ción con objetos basados en modelos estocásticos. La geome- puede ser estimada la extensión en subsuelo de los depósitos tría y la proporción de los objetos estratigráficos, tales como de las fajas de canales y de los depósitos de albardón. cuerpos de areniscas de faja de canal, son determinados La correlación de unidades litoestratigráficas fluviales (por estadísticamente a partir de datos de pozo y análogos anti- ejemplo: cuerpos de areniscas de faja de canales) entre pozos guos, estos objetos son ubicados en el espacio utilizando sus está basada comúnmente en suposiciones insostenibles. Nor- posiciones conocidas en los pozos y un emplazamiento cua- malmente, el resultado de estas correlaciones es que las uni- si aleatorio entre los pozos. Aunque este método señala la dades estratigráficas parecen mucho más continuas lateral- importancia de los datos de pozos, las formas y ubicaciones mente de lo que son en realidad. Antes de intentar la corre- de los objetos son muy poco realistas. Los modelos basados lación, es conveniente estimar la probable extensión lateral en procesos (ver más adelante) son capaces de producir y las variaciones de espesor de las unidades estratigráficas, estratigrafías mucho más realistas y tienen un mayor valor basándose en el análisis de los perfiles de pozo y testigos predictivo pero es muy difícil adecuar exactamente estos corona mencionados anteriormente. A partir de estas esti- modelos a los datos de pozos. Por esta razón, los modelos maciones es necesario establecer múltiples hipótesis de tra- basados en procesos no son usados rutinariamente en el bajo para realizar la correlación. La estimación de la geome- modelado de reservorios y acuíferos. Sin embargo, reciente- tría de las unidades estratigráficas es factible mediante el mente ha sido demostrado que potencialmente los modelos uso de análogos actuales, análogos antiguos y modelos híbridos, entre estocásticos y basados en procesos, pueden depositacionales. Los estimadores más útiles son los obteni- ser ajustados con exactitud a los datos de pozos utilizando dos de análogos modernos porque en ellos es posible descri- una aproximación por prueba y error con nuevos procedi- bir sin ambigüedades la morfología y los depósitos fluviales. mientos de optimización (algoritmos genéticos).

INTRODUCTION entails the following steps: (1) analysis of well logs, cuttings and cores in order to describe and interpret Fluvial deposits are important hydrocarbon the nature and origin of the rocks; (2) stratigraphic reservoirs in many parts of the world (e.g., Alaska, correlation of well logs and cores in order to assess Argentina, southern USA and Gulf of Mexico, North the continuity of distinctive rock types (facies) Sea, China, Venezuela). Also, fluvial deposits are between wells; (3) use of seismic data to assess the commonly important aquifers. In order to assess, orientation and structural continuity of relatively develop, and manage these resources, it is necessary thick (> 100 m thick) sequences of strata, and to to determine: (1) the total volume of reservoir or recognize distinctive seismic patterns that can be aquifer in a particular volume of the sedimentary related to distinctive sedimentary facies; (4) basin (the net-to-gross), and; (2) the three-dimen- modeling of the geometry and distribution of sional geometry, orientation and spatial distribution lithofacies in the three-dimensional space between of the sedimentary rocks that form the reservoirs or wells, and; (5) distribution of rock properties such aquifers. In fluvial deposits, reservoir or aquifer as porosity and permeability as a function of rocks are mainly channel-belt sandstones and lithofacies or using stochastic models. Successful gravelstones (conglomerates), with subordinate completion of these steps involves a great deal of sandstones attributable to crevasse splays and geological knowledge, the use of a range of . Determination of the stratigraphic properties stratigraphic models, and many assumptions that (characterization) of a reservoir or aquifer generally are difficult to test. Some methods of fluvial

AAS Revista 8 (2), 2001 89 John S. BRIDGE reservoir/aquifer characterization that are in thin parts of isolated channel-belt sandstone bodies common use are ill-conceived and unjustifiable. In (e.g., parts of channel fills, or shallow parts of the this review, I will explain and justify this criticism, channel) from the thicker overbank sandstone and attempt to offer constructive alternatives to bodies; (2) floodbasin muds from muddy channel reservoir/aquifer characterization. The comments in fills, and; (3) the different types of overbank this review also have general implications for sandstone bodies (Fig.1). describing and interpreting fluvial deposits. The folly of trying to interpret paleochannel pattern from well logs and cores ANALYSIS OF WELL LOGS AND CORES Sandstone bodies interpreted from well logs Description and interpretation of distinctive are commonly classified as fining upward, rock types (facies) coarsening upward, or “blocky”. Fining-upward sandstone bodies might be interpreted as point- Well logs and cores are used to describe or deposits or channel-fill deposits of a meandering interpret the spatial distribution of sedimentary rock river, whereas blocky sandstone bodies (no clear composition, texture, , vertical trends in grain size) are commonly assigned porosity and permeability, the nature of the fluids to bars (e.g., Galloway and Hobday, in the void spaces, and the thickness of the different 1983, 1996; Flores et al., 1985). Coarsening-upward rock types (e.g., sandstone, shale, coal). It is sandstone bodies might be interpreted as deposits common practice to interpret the specific fluvial of -mouth bars, lacustrine deltas or (e.g., channel bar, progradational crevasse splays/levees. The reality , , floodbasin) of the various is that all of these vertical sequence patterns can lithofacies or petrofacies using vertical trends in form in all of these depositional environments. Fi- grain size or some other sedimentary or gures 2 and 3 show how the vertical variation of petrophysical property. The objective is to use this grain size, sedimentary structure, and thickness of interpretation to help estimate the three-dimensio- single channel-belt deposits varies depending on the nal geometry and connectivity of the reservoir rocks topographic features of the bars and channels, the and permeability barriers. The assumptions in this position within a channel bar or channel fill, and practice are: (1) it is possible to interpret specific the nature of channel migration (determining what fluvial depositional environments from vertical logs, parts of channel bars are preserved). Fining-upward, and; (2) there is an unambiguous relationship coarsening-upward, and “blocky” sequences can between the geometry and sedimentary processes occur in any type of channel deposit. Therefore, it of a specific depositional environment and the three- is not possible to distinguish the deposits of the dimensional geometry of the deposits formed in that different river patterns (braided, meandering) from environment. Both of these assumptions are single vertical logs. suspect, as explained below. Why is it so important to interpret ancient channel patterns such as braided, meandering, and Difficulties interpreting fluvial depositional anastomosing anyway? This preoccupation with environments from well logs and cores determining ancient channel pattern is based on the misguided notion that different channel patterns are A simple view of fluvial deposits, as seen in associated with distinctive sedimentary facies and well logs and cores, is that the thickest (say, meters geometry (e.g., Allen, 1965; Collinson, 1996; thick) sandstone-gravelstone bodies are deposits of Galloway and Hobday, 1996; Miall, 1996; Selley, channel belts, and very thick bodies (say, tens of 1996). For example, meandering river deposits are meters thick) may be superimposed (amalgamated) commonly thought of as relatively fine-grained channel belts. Thinner sandstones bodies (dm to m sandstones with ribbon-like geometries (low width/ thick) interbedded with shales might be taken as thickness) set in voluminous muddy overbank deposits such as crevasse splays, levees, deposits. In contrast, braided river deposits are lacustrine deltas, or fills of floodplain-drainage commonly thought of as relatively coarse-grained channels. Thick sequences of shale with minor sandstones and/or gravelstones with sheet-like sandstones would be floodbasin deposits (Fig. 1). geometry (large width/thickness) with very little However, it is difficult to distinguish: (1) relatively associated floodplain deposits. Anastomosing river

90 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions

Figure 1. Example of interpreted gamma ray logs from the Travis Peak Formation, East Texas Basin (from Tye, 1991). Interpretations of depositional environment were aided by cores from well S.F.E. No.2. The sandstone bodies vary in thickness, and the thicker ones are in most cases interpreted as channel-belts. The thinner, overbank sandstones were interpreted by Tye (1991) as either crevasse splays or lacustrine deltas. The sandstone bodies may fine upward, coarsen upward or appear “blocky”, irrespective of their thickness and interpretation. It is possible that some of the overbank sandstones are channel-belt sandstones, and vice versa. Also, it is possible that some of the mudstone could be deposited in channel fills or as upper-bar deposits within channel belts (Bridge and Tye, 2000). Notice also that, in correlating the stratigraphic units between wells, the tops and bases of sandstone bodies were not assumed to be horizontal.

Figura 1. Ejemplo de perfiles de rayos gama interpretados para la Formación Travis Peak, Cuenca East Texas (de Tye, 1991). Las interpretaciones del ambiente depositacional fueron apoyadas por testigos corona del pozo S.F.E. Nº2. Los cuerpos de areniscas varian de espesor y en la mayoría de los casos los más espesos se interpretaron como fajas de canales. Los cuerpos más delgados, las areniscas de albardón fueron interpretadas por Tye (1991) como crevasse splays o deltas lacustres. Los cuerpos de arenisca pueden ser granodecreciente, granocreciente o no presentar cambios (blocky) independientemente de su espesor e interpretación. Es posible que algunas de las areniscas de albardón sean areniscas de faja de canal y viceversa. Es posible además que algunos de los estratos de pelitas podrían estar depositados en los rellenos de canal o como depósitos de tope de barra dentro de las fajas de canales (Bridge and Tye, 2000). Note que también, en las correlaciones de las unidades estratigráficas entre pozos, las bases y techos de los cuerpos de areniscas no fueron tomados como horizontales. deposits have been thought of as a connected pended load tend to have relatively low network of channel sandstone bodies with low and high degree of braiding. Such bed-load width/thickness set in a matrix of floodplain mud. have been associated with relatively easily eroded There are some very serious misconceptions in all banks of sand and gravel, large channel slope and of this. large power, such that they are laterally unstable. In contrast, with relatively large Attempts to distinguish meandering and suspended loads were postulated to be cha- braided river deposits based on their grain racteristic of undivided rivers of higher sinuosity. size and geometry Such suspended-load streams were associated with cohesive muddy banks, low and This practice owes its origin to Schumm’s power, and lateral stability. Schumm defined sus- (1963, 1971, 1972, 1977, 1981, 1985) classification pended-load channels as those carrying more than of channel patterns. Schumm (followed by many 97% suspended-sediment load (presumably at sedimentologists such as Galloway and Hobday, stage), whereas bed-load channels are defined as 1983, 1996; Miall, 1996) postulated that rivers that those carrying less than 89% suspended sediment. transport large amounts of relative to sus- Such a definition of bed-load channels is misleading

AAS Revista 8 (2), 2001 91 John S. BRIDGE

Figure 2. Typical channel deposits, with gamma ray logs, from different parts of sandy channel bars and channel fills (from Bridge and Tye, 2000).

Figura 2. Depósitos típicos de canal, con perfiles de rayos gama, de distintos sectores de las barras arenosas y de los rellenos de canal (tomado de Bridge and Tye, 2000). to say the least! The correlation between channel is braided or meandering are the amounts of water pattern, type of sediment load and bank stability is and sediment supplied during seasonal (Fig. not generally supported by data. This mythical 4). correlation probably arose because early studies of To appreciate the fact that channel pattern may braided rivers were in mountainous areas of sandy- not have a major influence on the geometry of a gravelly outwash and those of single-channel single channel-belt sandstone body, consider the sinuous streams were from temperate lowlands (e.g., Mississippi and Brahmaputra Rivers, classic the U.S. Great ). In fact, many braided rivers examples of meandering and braided rivers, are sandy and silty (e.g., Brahmaputra in Bangladesh, respectively. Both of these rivers flow through Yellow in China, Platte in ), and many sin- extensive . The width of the Mississippi gle-channel, sinuous rivers are sandy and gravelly River belt in the lower Mississippi (Madison in Montana, South Esk in Scotland, Yukon (Yazoo Basin) is 15 to 25 km, and the maximum in Alaska) (Rust, 1978; Jackson, 1978; Bridge, thickness of the channel-belt deposits ranges from 1985). The key controls on whether or not a river 20 to 40 m, giving an average channel-belt width/

92 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions thickness of about 650 (Bridge, 1999). In the scale trough cross strata formed by the that Atchafalaya Basin and delta , the Mississippi migrate over the unit bars during high flow stages. channel-belt width is 10 to 15 km and the maximum Planar cross strata associated with migration of a thickness is about 50m, giving a maximum width/ unit bar with an avalanche face apparently only thickness of about 300. The Brahmaputra channel occurs at the margins of unit bars. However, this is belt is about 10 km wide, and 40 m in maximum a moot point when dealing with well logs or cores, thickness, giving a minimum width/thickness of because it is not possible to distinguish planar cross about 250 (Bristow, 1987). It appears that the width/ strata from trough cross strata when the sets are thickness ratio of the meandering Mississippi dm to m thick (i.e., medium-scale cross strata). channel belt is greater than that of the braided Brahmaputra, or at least comparable. The reason Relationship between variability, for the similar width/thickness ratios is not hard to channel pattern and deposits understand. A braided river occupies several channels across the width of its channel belt Another common myth (perpetuated by Miall, simultaneously. A meandering river may only 1977,1996, and many others) is that discharge occupy one channel at a time, but the channel still variability is greater for braided rivers than for wanders through a channel belt that may be com- meandering rivers. This myth probably originated parable in width to a braided channel belt of the from the early studies of pro-glacial braided rivers same thickness. in mountainous regions of North America, where discharge varied tremendously during snowmelt. In The significance of planar cross strata contrast, many single-channel rivers were studied in temperate lowland regions where discharge It is commonly stated in the literature (e.g., variations were moderated by groundwater supply. Collinson, 1996; Miall, 1977, 1996) that planar cross In fact, discharge variability does not have a major strata produced by transverse unit bars is charac- influence on the existence of the different channel teristic of braided rivers, but not meandering rivers. patterns, because all patterns can be formed in labo- This is quite wrong (review in Bridge, 2002; Fig. 3). ratory channels at constant discharge. Moreover, Transverse unit bars occur in all types. many rivers with a given discharge regime show Furthermore, it appears from recent studies that the along-stream variations in channel pattern. deposits of most unit bars are composed of medium- However, variability of water and sediment supply

Figure 3. Topographic features of braided and meandering rivers: curved channels, unit bars, compound bars, cross-bar channels (modified from Bridge, 1993a).

Figura 3. Rasgos topográficos de ríos enlazados y meandrantes: canales curvados, barras unidad, barras compuestas, canales transversales en el tope de las barras (modificado de Bridge, 1993a).

AAS Revista 8 (2), 2001 93 John S. BRIDGE

(anastomosing or anabranching) (Lane, 1957; Brice, 1964, 1984; Chitale, 1970; Smith, 1976; Schumm, 1977, 1985; Knighton and Nanson, 1993; Nanson and Knighton, 1996; Makaske, 2001). The characteristic and definitive features of anas- tomosing (anabranching) channel segments are that they are longer than a curved channel segment around a single braid or , and the patterns of flow and in adjacent anastomosing segments are essentially independent of each other. This means that each anastomosing segment contains bars appropriate to the imposed discharge and sediment load, enabling assignment Figure 4. Qualitative variation of equilibrium channel patterns with channel-forming water discharge, valley slope, and sediment of its own channel pattern, based on degree of size. Valley slope can be thought of as a measure of sediment channel splitting around bars and sinuosity. This transport rate, because sediment transport rate increases with means also that the terms anastomosing and increasing slope and water discharge and with decreasing sediment braiding are not mutually exclusive, as implied in size. the channel-pattern classifications of Rust (1978) Figura 4. Variación cualitativa de los diseños de equilibrio de los and Miall (1992, 1996). In fact, many rivers are canales con respecto al caudal de agua del canal formador, la pen- braided and/or meandering as well as anastomosing diente del valle y el tamaño de sedimento. La pendiente del valle (Fig. 5). The term anastomosing belongs in puede ser analizada como una medida de la tasa de transporte de classifications that describe how channel belts split sedimento debido a que la tasa de transporte de sedimento se and join on floodplains, such as distributive and incrementa con el incremento de la pendiente, el caudal de agua y tributive. The number of coexisting anastomosing con el decrecimiento del tamaño del sedimento. or distributive channels is controlled by the nature of channel-belt and rather than the water and sediment discharge in channels (which during floods does have an influence on the nature controls whether channels are meandering or of flood sedimentation units, and these units might braided). The nature of channel-belt deposition and be recognizable in well logs and cores (Fig. 2). avulsion has a major control on the superposition Related to this issue of discharge variability is of channel belts, the preservation of overbank the claim that extremes of discharge will have a deposits, and the overall net-to-gross, as discussed strong influence on the nature of the river deposits. below. Whether or not the ancient channel belt was Specifically, the deposits of ephemeral rivers are braided or meandering is largely irrelevant when considered to have certain distinctive features, such analyzing net-to-gross in the subsurface. as a predominance of planar laminated and low- angle cross-stratified sandstone, and either a lack Superimposed channel bars and channel belts of well-defined channels or channels with high width/depth ratios (e.g., North and Taylor, 1996). Channel bars and channel fills can be These features are not restricted to ephemeral rivers. superimposed within a single channel belt and by The key features that distinguish deposits of ephe- superposition of different channel belts (Fig. 6). meral rivers from those of perennial rivers are those Recognition of such superposition is not a simple that indicate that the channel flow ceased (i.e., plant task. Within a single channel belt, simple and roots and desiccation cracks in the bottom of chan- compound bars can be superimposed in complicated nels). In reality, the main characteristics of channel ways that cannot be interpreted easily from well deposits are controlled by flood conditions, and logs. It is very important to recognize superimposed floods occur in ephemeral and perennial rivers alike. channel belts in well logs and cores, because superimposed channel belts generally produce Are anastomosing river deposits distinctive? sandstone-gravelstone bodies with much larger width/thickness than single channel belts. It is not A distinction has been made between rivers possible to rationally correlate channel-belt where channels split around bars (braided) and sandstone bodies between wells without assessing those where channels split around floodplain areas their degree of superposition, as discussed below.

94 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions

Spatial variations in proportion of channel- belt deposits (net-to-gross)

The proportion and grain size of channel-belt deposits commonly vary vertically in wells, and vary between wells at a given stratigraphic level. For example, an increase in channel-deposit proportion may be associated with a zone of superimposed channel bars or channel belts. Variations in net-to-gross can occur over different spatial scales. For example, vertical variations in channel-deposit proportion over scales of tens of meters, hundreds of meters and kilometers have been documented (e.g., Willis, 1993a,b; Khan et al., 1997; Zaleha, 1997a,b). Spatial variations in alluvial architecture have been explained by a variety of different kinds of stratigraphic models (e.g., Leeder, 1978; Allen, 1978; Bridge and Leeder, 1979; Paola et al, 1992; Bridge and Mackey, 1993a; Shanley and McCabe, 1993, 1994; Wright and Marriott, 1993; Mackey and Bridge, 1995). There may be many different explanations for a given alluvial architecture, and great care must be taken to keep an open mind about the possible explanatory hypotheses. This is an important point, because choice of any particular interpretation of alluvial architecture has far-reaching implications for late- ral correlation of stratigraphic units.

Proposed procedure for interpreting channel deposits in well logs, image logs, and cores

When interpreting channel deposits in vertical logs, it is important to estimate maximum and mean bankfull-channel depth with some confidence, because these parameters are commonly used to estimate the width of subsurface channels and channel belts (Bridge and Tye, 2000). Quantitative estimation of channel depth from subsurface data requires three preliminary steps. First, major channel-belt sandstones and gravelstones must be Figure 5. immediately north of its distinguished from floodplain sandstones. This is with the Ganges River, showing braided channels and undivided not always an easy task, because of the gradation sinuous channels that are also anastomosed. Scale bar is 20 km. Photo courtesy of C.S.Bristow, also published in Bridge (1993a). in thickness and facies between these types of sediment bodies. Second, it is essential to try to Figura 5. Río Brahmaputra inmediatamente al norte de su con- recognize the different scales of channel deposits: fluencia con el río Ganges, se observan canales enlazados y cross sets, flood sedimentation units, unit-bar monocanales sinuosos que también son anastomosados. La barra deposits, compound-bar deposits, individual de escala es 20 km. La fotografía es cortesía de C.S. Bristow, tam- bién publicada en Bridge (1993a). channel belts, and compound channel belts. This must be done by inspection of spatial variations in grain size, sedimentary structures, paleocurrents and degree of disruption. It is very difficult to do this to confuse flood-sedimentation units with bar using data from one well. As shown above, it is easy deposits, and to confuse individual channel belts

AAS Revista 8 (2), 2001 95 John S. BRIDGE with superimposed channel belts. Third, the generally increases with formative flow depth, d. It thickness of as many untruncated channel bars or appears that, for all types of river dunes (including fills (from the tops of channel belts) as possible must those not in equilibrium with the flow), d/Hm be measured to get an idea of the range of maximum averages between 6 and 10. Although estimation channel depths. of flow depth from height is imprecise, such Maximum bankfull-channel depth is commonly an estimate is still a useful complement to flow estimated from the (decompacted) thickness of depth calculated from channel-bar thickness. untruncated channel-bar and channel-fill deposits. It is not always easy to correctly identify untrunca- Re-interpretation of the geometry and spatial ted channel-bar and channel-fill deposits in logs or distribution of subsurface fluvial sandstone cores. Furthermore, the thickness of such channel bodies sandstones is generally less than the bank-full channel depth (Bridge and Mackey, 1993b). The There are many examples in the literature of presence of sandy-muddy upper-bar deposits, and interpretation of river-channel deposits using data the uncertainty in distinguishing these from proximal from cores and well logs (e.g., various papers in the overbank deposits makes it difficult to identify volumes edited by Barwis et al., 1990; Ethridge and paleo-bankfull level. Thick channel-fill mudstones Flores, 1981; Ethridge et al., 1987; Galloway and above thin channel-bar sandstones can look very Hobday, 1996; Lomando and Harris, 1988; Miall similar to overbank deposits. Furthermore, and Tyler, 1991; Selley, 1996). In most examples, a maximum channel depth and bar thickness vary in range of possible interpretations of well logs is not space quite markedly (by at least a factor of two) considered, and the cores are not described in in channel belts, such that limited data from a sin- sufficient detail to allow use of the methods gle well may not be representative. An independent proposed here. In particular, the thickness of means of estimating bankfull flow depth would be medium-scale cross sets is rarely presented. beneficial. This is possible using an understanding However, Bridge and Tye (2000) demonstrated how of the relationship between dune height and application of the new techniques discussed here thickness of associated cross sets, and of the known had an impact upon previous interpretations of relationship between dune height and water depth. paleochannel depths, channel-belt widths, and flu- The method for calculating the distribution of vial sandstone-body dimensions and connectedness. dune height from the distribution of cross-set thickness is described by Bridge (1997), Leclair et al. (1997) and Leclair and Bridge (2001). The STRATIGRAPHIC CORRELATION OF WELL method is based on the justifiable assumption that LOGS AND CORES: USE OF SEISMIC DATA the distribution of cross-set thickness is due primarily to variability in dune height, and that Method of correlation variation in deposition rate plays a minor role. This method is also limited to homogeneous cosets of Correlation of lithofacies between wells is cross strata, meaning that there are no obvious normally accomplished within a framework of re- spatial changes in the type of strata or mean grain gional stratigraphic markers established using size. The assumption is, therefore, that such cross seismic sections, well logs and cores. In order to sets were formed by migration of dunes whose mean facilitate correlation, it is common practice to plot geometry did not vary appreciably in time and the stratigraphic markers as horizontal surfaces. space. To use this method, the thickness, s, of as Lithostratigraphic correlation must entail certain many cross sets as possible should be measured, assumptions about, or models of, the geometry and such that the mean set thickness, sm, can be lateral continuity of lithofacies such as channel-belt calculated. Mean dune height, Hm , is approximately sandstone bodies or floodplain shales (Fig. 1, 7 and equal to 3 sm . To avoid confusing cross strata of 8). For example, must the tops and bases of dune origin with solitary sets formed by unit bars, correlated sandstone bodies be horizontal and abnormally thick, isolated cross sets should be parallel? If they are not horizontal and parallel, how avoided. As dune height is expected to vary with inclined can the boundaries be relative to the datum, position on channel bars, cross-set thickness and how variable can the thickness of a sandstone measured in different positions in the vertical profile body be? What is a reasonable lateral extent in any should be grouped into subsets. Mean dune height given direction? These are very difficult questions

96 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions

Figure 6. Vertical sequences of lithofacies and gamma-ray logs for superimposed channels bars and channel belts (from Bridge and Tye, 2000).

Figura 6. Secuencias verticales de litofacies y perfiles de rayos gama correspondientes a barras de canal superpuestas y fajas de canales (tomado de Bridge and Tye, 2000). to answer. In many cases, the assumptions made in lateral extent of reservoirs or aquifers correlation are either not stated explicitly, or are based on various kinds of stratigraphic models. If It is ironic, but not surprising, that the most correlating stratigraphic sequences that are less than common method for estimating channel-belt widths the order of 100 m thick, seismic data will be of and orientations is by correlating specific channel- limited use in resolving the lateral continuity of belt sandstone bodies between well logs (e.g., Nanz, strata and the presence of small faults. Stratigraphic 1954; Berg, 1968; Cornish, 1984; Tye, 1991). With resolution using seismic data improves as interval this technique, there is generally no attempt to thickness and impedance contrast increase. predict reasonable geometries and lateral extents Therefore, an infinite number of correlations is of different lithofacies from analysis of well logs. possible, but only one will be used in practice. The minimum possible width of sediment bodies that can be resolved by this method is the well Using well-to-well correlation to estimate spacing. This approach to estimating lateral extent

AAS Revista 8 (2), 2001 97 John S. BRIDGE

through channel deposits indicate the paleochannel pattern and hence the geometry of channel-belt sandstone bodies. Concerning assumption (1), the depositional models discussed above clearly show that the basal surfaces and tops of channel belts are not generally flat. Concerning assumption (2), sand- stone bodies at the same stratigraphic level in adjacent wells are not necessarily connected, and some assessment of the probability of connection is required, perhaps using empirical data on channel- belt width/maximum channel depth. However, if two sandstone bodies are indeed continuous between wells, they are not necessarily from a sin- gle channel belt. This is of particular concern if sandstone-body proportion exceeds 0.4 (Bridge and Mackey, 1993b). Assumptions (3) and (4) were shown to be wrong above. In order to rationalize the correlation process as much as possible, it is desirable to use interpretations of depositional environments and stratigraphic models to constrain the correlation assumptions. In the case of isolated channel belts, it is unlikely that the tops and bases will be hori- zontal and parallel. Data from modern and ancient analogs can be used to assess these characteristics. Models and real-world examples of channel belts indicate that their thickness may vary laterally by a factor of at least two, and that there are systematic lateral variations in thickness. A range of likely channel-belt widths can be obtained from the thickness of single channel belts, as indicated below. It may turn out that sandstone bodies at the same stratigraphic level should not be correlated over the distance between two wells if this distance greatly exceeds the expected lateral extent of a channel belt. There needs to be much more work Figure 7. Different ways of correlating rock units between two done on the geometry of different lithofacies in wells, depending on assumptions about the physical continuity modern fluvial environments, if modern analogs are and orientation of unit boundaries. to be used to predict the geometry of subsurface Figura 7. Diferentes formas de correlacionar unidades de roca fluvial sediment bodies. entre dos pozos, dependiendo de las suposiciones de la continui- dad física y de la orientación de los límites de la unidad. Geometry of modern channel-belt deposits

Several attempts have been made to predict of lithofacies is commonly compromised by channel-belt width in the subsurface using empirical simplistic or erroneous assumptions, such as: (1) equations derived from modern rivers that relate basal erosion surfaces and tops of channel-belt maximum channel depth, channel width, and sandstone bodies are flat; (2) sandstone bodies channel-belt width (Collinson, 1978; Lorenz et al., positioned at the same stratigraphic level must be 1985, 1991; Fielding and Crane, 1987; Davies et connected between adjacent wells; (3) sandstone al., 1993; discussed by Bridge and Mackey, 1993b). body width/thickness ratios are closely related to This approach requires reliable estimates of paleochannel pattern, and; (4) vertical sequences maximum bankfull-channel depth from one-dimen-

98 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions sional subsurface data. As discussed previously, correct estimation of maximum bankfull-channel depth is not straightforward, because complete channel-bar or channel-fill sequences may be difficult to identify, and the thickness of the sandstone-gravelstone parts of these coarse members is not always as great as the bankfull- channel depth. Furthermore, within a single channel belt, maximum channel depth and bar thickness can vary spatially by a factor of at least 2 (Bridge, 1993a; Salter, 1993). Therefore, limited data from a single well may not be representative. Empirical regression equations relating maximum channel depth, channel width, and channel-belt width have large standard errors, and are dependent on channel-pattern parameters such as channel-bend wavelength and sinuosity (Bridge and Mackey, 1993b). Most of the empirical Figure 8. Correlation of two wells within the Ness Formation of equations used to date (Collinson, 1978; Lorenz et the Brent Field, UK North Sea (from Bryant and Flint, 1993). GR = al., 1985, 1991; Fielding and Crane, 1987; Davies Gamma Ray; FDC = Density Log; CNL = Compensated Neutron Log. Coals have been used as correlation markers. There has been et al., 1993) do not include such dependencies. some imaginative definitions of the shapes and lateral extents of Channel-bend wavelength and sinuosity are actually channel sands and crevasse splay sands. very difficult to reconstruct from outcrops and are impossible to determine from well data. Lorenz et Figura 8. Correlación de dos pozos en la Formación Ness del Distri- al. (1985) used empirical equations derived from to Brent, Mar del Norte-Inglaterra (de Bryant and Flint, 1993). GR = Rayos Gama; FDC = Perfil de Densidad; CNL = Perfil Compensa- rivers with sinuosity greater than 1.7, and assumed do de Neutrones. Las capas de carbón han sido utilizadas como that sandstone bodies in both outcrops and niveles de correlación. Algunas de las definiciones de las formas y subsurface strata were deposited in highly sinuous extensiones laterales de las arenas de canal y de las arenas de crevasse rivers. Equations presented in Bridge and Mackey splay han sido imaginativas. (1993b: their Table 2) are based on larger data sets than previous equations or on theoretical principles. Channel-belt widths predicted by these equations difficult to make detailed interpretations of agreed with those observed in outcrops by Bridge depositional environments using typical subsurface et al. (2000). data. Because our understanding of modern depositional environments is incomplete, most Use of outcrop analogs to aid well-to-well depositional models are qualitative, lacking in correlation detail, and not fully three-dimensional (Bridge, 1985, 1993b). The deficiency of most depositional The geometry and lithofacies of channel and models severely limits their use in detailed channel-belt deposits determined in outcrops are interpretation of ancient deposits. Rarely are commonly used as analogs for subsurface strata outcrops extensive enough to allow unambiguous (Collinson, 1978; Walderhaug and Mjos, 1991; determination of the three-dimensional geometry Lowry and Raheim, 1991; Cuevas Gozalo and and orientation of channels and channel belts. Martinius, 1993; Dreyer, 1993; Dreyer et al., 1993; Channel-belt width is particularly difficult to de- Robinson and McCabe, 1997; Bridge et al., 2000). termine, even in large outcrops (Geehan and Despite the popularity of this approach, it has many Underwood, 1993). Moreover, channel-belt width pitfalls. First, it must be established that the and thickness are known to vary spatially within depositional setting interpreted for the subsurface one channel belt and between different channel strata is indeed analogous to that interpreted for belts. In general, limited data from a few large the outcrops. This requires two interpretation steps, exposures are unlikely to be generally representative the reliability of which depends on the quality of of fluvial-deltaic channel belts. This is why it is the outcrops, of the subsurface data, and of the desirable to use analog data from Holocene depositional models used for interpretation. It is depositional environments, where channel-belt

AAS Revista 8 (2), 2001 99 John S. BRIDGE dimensions can be determined easily, and the bodies between wells, it is critical to be able to relationship between the nature of the deposits and distinguish connected channel belts. Two connected the geometry, flow and sedimentary processes of channel belts may be up to twice the width of a the environment can be established unambiguously. single channel belt. Bridge and Mackey (1993b) used their revised two-dimensional model of alluvial Amplitude analysis of 3-D seismic horizon architecture (Bridge and Mackey, 1993a) to study slices the width and thickness of sandstone (or gravelstone) bodies comprising single or connected Amplitude analysis of 3-D seismic horizon slices channel belts. The width and thickness of sandstone (Weber, 1993; Hardage et al., 1994, 1996; Burnett, bodies depend critically on the proportion of 1996) is the only method capable of yielding channel-belt deposits in the stratigraphic interval, directly the width of channel belts, and imaging the as this proportion controls the degree of channel pattern (sinuosity, channel splitting) of connectedness of individual channel belts. For low subsurface sandstone bodies (Fig. 9). This is also values of channel-deposit proportion (less than the only method that can be used to predict the about 0.4), channel belts are unconnected, spatial distribution of channel-belt thickness and sandstone-body width equals channel-belt width, lithofacies. However, high quality 3-D seismic data and sandstone-body thickness equals aggraded are rarely available, the most sandstone bodies of channel-belt thickness (Fig. 10). As channel-deposit interest are too thin and buried too deeply to be proportion increases, some channel belts become completely resolved using seismic data. connected, the mean and standard deviation of sandstone-body width and thickness increase, and Estimating width of superimposed channel their frequency distributions become polymodal belts from subsurface data with the largest modes at the lowest values of width and thickness. As channel-deposit proportion When correlating channel-belt sandstone continues to increase, the distributions are still

Figure 9. Amplitude analysis of a 3-D seismic horizon slice showing the width of a channel belt and channel pattern of subsurface channel sandstone bodies. The cross section shows correlated logs and the position of the horizon slice. Log 3 cuts through the variable- width, straight channel belt trending north-south. Logs 4 to 6 cut through a point bar and channel fill of a slightly older channel belt.

Figura 9. Análisis de amplitud de una sección horizontal de sísmica 3-D mostrando el ancho de la faja de canales y el diseño de los canales de los cuerpos de areniscas de canal en subsuelo. Las secciones transversales muestran los perfiles correlacionados y la posición de la sección horizontal. El perfil 3 atraviesa longitudinalmente la faja de canal de ancho variable en dirección norte-sur. Los perfiles 4 y 6 atraviesan una barra de punta y el relleno de canal de una faja de canal un poco más antigua.

100 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions

Figure 10. Relationships between channel-belt deposit proportion and connectedness, and the width and thickness of channel-belt sandstone bodies (based on Bridge and Mackey, 1993b).

Figura 10. Relaciones entre la proporción de los depósitos de la faja de canales, la interconexión, el ancho y el espesor de los cuerpos de areniscas de la faja de canales (basado en Bridge and Mackey, 1993b).

polymodal but the larger modes are at higher values The three-dimensional model of alluvial of width and thickness (Fig. 10). If channel-deposit architecture of Mackey and Bridge (1995) gives proportion exceeds about 0.75, all channel belts are results that are generally similar to those described connected, and the single sandstone body has a above for two-dimensional models. However, the width equal to floodplain width and a thickness three-dimensional model predicts that channel- equivalent to the whole section. deposit proportion and connectedness, and the Channel-deposit proportion, width and dimensions of channel sandstone bodies, vary with thickness increase as bank-full channel depth and distance from points of channel-belt splitting (due channel-belt width increase, and as floodplain to avulsion). Upstream of avulsion points, width, rate and avulsion period sandstone bodies have lower than average width/ decrease. Channel-deposit proportion is also thickness because of aggradation in a fixed channel influenced by depth of burial (degree of belt. Immediately down valley from avulsion points, compaction) and tectonic tilting of the floodplain. channel belts are connected, resulting in sandstone For example, channel-deposit proportion and bodies with higher than average width/thickness. connectedness of channel belts increase on the Because of this, relationships between channel- down-tilted sides of floodplains but are reduced on deposit proportion and connectedness and the up-tilted sides. sandstone-body dimensions derived from 2-D

AAS Revista 8 (2), 2001 101 John S. BRIDGE models are strictly applicable only to parts of the 1993; Gibling and Bird, 1994; Miall, 1996; Fig. 11). floodplain located some distance down valley from This is ascribed to arising from relative avulsion points. fall in and exposure of a slope that is steeper than the river slope upstream. Although Correlation using sequence-stratigraphic incision may be associated with base-level fall near models the coast, incision further inland may not be associated with falling base level (Blum, 1994; Blum Correlation of lithofacies associations between and Price, 1998: Blum and Tornqvist, 2000). wells may be aided by other kinds of stratigraphic Therefore, the basal erosion surface of the sequence models, such as sequence-stratigraphic models may not be coeval with, or coincident with, erosion (Fig.11). Sequence-stratigraphic models (e.g., surfaces inland. The erosional base of the sequence Jervey, 1988; Posamentier and Vail, 1988; in these models is overlain by amalgamated fluvial Posamentier et al., 1988; Ross, 1990; Shanley and channel deposits (the so-called lowstand systems McCabe, 1993, 1994; Wright and Marriott, 1993) tract: LST) that are ascribed to deposition under predict the effects of relative sea-level change on conditions of low deposition rate and restricted alluvial deposition rate and alluvial architecture, and floodplain width (due to valley incision). Many are therefore potentially applicable to near-coastal workers assume that zones of high channel-deposit fluvial successions. There are substantial differences proportion (high net-to-gross) in alluvial deposits between these models, and all of them can be represent these basal parts of sequences, and that criticized. Most of these models are severely limited the basal erosion surface of the lowest sandstone because they are qualitative, essentially two-dimen- body represents an incised valley (e.g., Aitken and sional, and do not account for all of the factors that Flint, 1995). In many cases, evidence for an incised influence alluvial architecture. Actually, eustatic valley is lacking. Criteria for incised valleys include sea-level change may not have the influence on (Dalrymple et al., 1994): (1) erosional relief that is alluvial systems that has been assumed in the past. greater than the thickness of a single channel fill; Factors such as climate, vegetation and tectonism (2) multiple, vertically stacked channel bars within may also have an important influence on rivers and the valley; (3) evidence for extended periods of non floodplains during eustatic sea-level change deposition (mature paleosols) on interfluves; (4) (Schumm, 1993; Wescott, 1993; Blum, 1994; Koss alluvial channel deposits resting erosively upon et al., 1994; Leeder and Stewart, 1996; Miall, 1996; shallow marine sands and muds. Commonly, a large Blum and Price, 1998; Ethridge et al., 1998; Blum amount of erosional relief on the base of a single and Tornqvist, 2000). Miall (1986, 1991, 1996) has channel deposit can be misinterpreted as an incised criticized some of the earlier models (e.g., Jervey, valley margin (Salter, 1993; Best and Ashworth, 1988; Posamentier et al., 1988) for the effects of 1997). sea-level change on near-coastal alluvial deposition. Falling sea level (marine regression) does not His main point is that a relative fall in sea level is necessarily result in valley incision near the coast. not normally associated with alluvial aggradation, The land exposed by marine regression may except for the newly exposed part of the sea bed, experience erosion or deposition depending on the and even then only under special circumstances. In slope of the exposed surfaces (Schumm, 1993; fact, whether or not a river valley is incised or Wescott, 1993; Wood et al., 1993). Erosion will aggraded during sea-level fall depends, among other occur on relatively steep slopes given enough time things, on the slope of the exposed shelf relative to and flow competence. However, erosion of channels that of the river valley (Pitman, 1986; Pitman and and floodplains is a long-term process, and the short- Golovchenko, 1988; Schumm, 1993; Leeder and term response may be to increase sinuosity of rivers. Stewart, 1996). In general, the effects of sea-level Also, upstream avulsion may cause channel change are expected to decrease up-valley (Saucier, abandonment before incision is complete (Leeder 1981, 1994; Autin et al., 1991; Schumm, 1993; and Stewart, 1996). Incision of channels and Shanley and McCabe, 1994). Only three-dimensio- floodplains is associated with terrace formation, nal models can describe these effects (Bridge, 1999: reduction of valley width, and up-valley migration Karssenberg et al, 2001b). of knick-points. Avulsion frequency is expected to Most alluvial sequence-stratigraphic models be low in areas of erosion. Channel-deposit have an erosional base to the sequence (Shanley proportion and connectedness may increase during and McCabe, 1993, 1994; Wright and Marriott, sea-level lowstand because of reduced deposition

102 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions

Figure 11. Examples of alluvial sequence-stratigraphic models (upper model from Shanley and McCabe, 1993; lower model from Wright and Marriott, 1993).

Figura 11. Ejemplos de modelos de secuencia estratigráfica aluvial (modelo superior de Shanley and McCabe, 1993; modelo inferior de Wright and Marriot, 1993).

AAS Revista 8 (2), 2001 103 John S. BRIDGE rate (or erosion) and reduced floodplain width. width of floodplains decrease them. It is also However, channel-deposit proportion may decrease necessary to know how channel-belt geometry as a result of reduced avulsion frequency, or if the changes during sea-level change in order to channel-belt width and thickness were reduced as adequately predict channel-deposit proportion. In a result of a decrease in water discharge. Sequence the case of the Mississippi coastal plain, the channel- stratigraphic models must account for changes in deposit proportion in the Holocene deposits is low all of these factors that influence alluvial mainly because of high floodplain width relative to architecture. channel-belt width. However, in the coeval deposits Thick and laterally extensive amalgamated of the lower Mississippi Valley, the channel-deposit channel deposits are commonly important oil and proportion is very high because of low floodplain gas reservoirs. Therefore, it is important to interpret width relative to channel-belt width (Bridge, 1999). such deposits accurately when predicting their The highstand systems tract (HST) is also thickness, lateral extent, and bounding facies. If associated with relatively low channel-deposit zones of high channel-deposit proportion are proportion according to Shanley and McCabe incorrectly interpreted as incised valley fills, their (1993). However, Wright and Marriott (1993) extent normal to the valley direction will be predict an increase in channel-deposit proportion underestimated, and their extent parallel to the in the HST, and an increase in soil maturity, both valley will be overestimated. There are many related to reduced deposition rate. examples in the literature where zones of high Based on the comments above, it is unlikely channel-deposit proportion are not associated with that extant alluvial sequence-stratigraphic models filling of incised valleys. For example, they may be are generally applicable. Karssenberg et al (2001b) associated with high deposition rates and high have attempted to improve upon this situation by avulsion frequencies of large channels on megafans producing a 3-D model that takes account of most (Willis, 1993a,b; Khan et al, 1997; Zaleha, 1997a,b). of the factors that control alluvial architecture In this case, their lateral extent may be considera- during changing base level. ble. According to the sequence-stratigraphic models, the deposits above the lowstand systems MODELING OF THE THREE-DIMENSIONAL tract were deposited under conditions of relatively GEOMETRY AND DISTRIBUTION OF high deposition rate on a broad alluvial plain, FACIES BETWEEN WELLS associated with rising relative base level and drowning of incised valleys (the so-called Information derived from seismic profiles, transgressive systems tract: TST). The channel- cores, well logs, and well tests is rarely sufficient to deposit proportion and connectedness are taken to provide comprehensive three-dimensional be relatively low as a result. According to Gibling description and understanding of subsurface and Bird (1994), coal is likely to occur at the top of reservoir rocks. As a result, recourse is commonly the TST, immediately below the so-called maximum- made to outcrop analogs and quantitative flooding surface. Paleosols are likely to reflect high depositional models. A common approach is to use groundwater table. Deposits associated with the interpreted outcrop analogs (discussed above) to maximum-flooding surface may contain evidence provide supplementary data on sedimentary of marine influence. architecture, and to use stochastic, structure- During relative sea-level rise, slopes of rivers imitating models conditioned by subsurface data and floodplains are reduced near shore due to to distribute the sediment types in 3-D space backwater effects, and the width and depth of (reviews by Bryant and Flint, 1993; Koltermann and valleys increase due to drowning. These changes Gorelick, 1996; North, 1996; Anderson, 1997: result in reduced grain size of transported sediment, Fig.12). Structure-imitating stochastic models do not deposition, change in channel pattern, and increased simulate processes of deposition: they directly frequency of avulsion. Tornqvist (1993, 1994) simulate the stratigraphy. Methods used are associated high avulsion frequency and indicator geostatistics (e.g., Journel, 1983; Bierkens anastomosing river patterns with periods of rapid and Weerts, 1994), simulated annealing (e.g., base-level rise and deposition. Increases in avulsion Deutsch and Cockerham, 1994), Markov chains frequency increase channel-deposit proportion and (e.g., Doveton, 1994, Carle et al., 1998), and connectedness, but increases in deposition rate and Boolean object models that use probabilistic rules

104 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions

Figure 12. An example of use of a two-dimensional, object-based stochastic model (from Srivastava, 1994). The width and thickness of channel-belt sandstone bodies are normally obtained from Monte Carlo sampling from empirical distributions derived from outcrop analogs. Notice that the “sand bodies” have been given a channel-shaped form. Channel-belt sandstone bodies do not normally have this form.

Figura 12. Un ejemplo del uso de un modelo bidimensional estocástico basado en objetos (de Srivastava, 1994). El ancho y el espesor de los cuerpos de areniscas de faja de canales son obtenidos del muestreo Monte Carlo a partir de las distribuciones empíricas derivadas de análogos de afloramiento. Note que los «cuerpos de arena» han sido adjudicados a una forma de canal. Los cuerpos de areniscas de faja de canales normalmente no tienen esta forma.

to define the geometry and location of stratigraphic imitating models) simulate the sedimentary units (e.g., Budding et al., 1992; Deutsch and Wang, processes acting to produce a deposit (Koltermann 1996; Hirst et al., 1993; Holden et al., 1998). An and Gorelick, 1996; Anderson, 1997). Process-based advantage of these models is that they match the models can be deterministic and/or stochastic, and available data exactly. However, input parameters empirical and/or theoretical. Examples of such are very difficult to obtain, and the alluvial models include random-walk sedimentation models stratigraphy simulated is commonly very unrealistic of braided rivers (Webb, 1994), models based on (e.g., Tyler at al., 1994; Deutsch and Wang 1996; the fundamental equations of fluid flow and Holden et al., 1998). In the case of object-based sediment transport (e.g., Bridge, 1977, 1992; Tetzlaff models, the lack of realism is associated with the and Harbaugh, 1989; Stam, 1996), and avulsion- choices available for object shapes, and the based alluvial stratigraphy models (e.g., Bridge and essentially random placement of objects where well Leeder, 1979; Mackey and Bridge, 1995; Heller and data are not available. For example, channel belts Paola, 1996). Process-based models are forward are modeled as one or more sinuous ribbons with models in the sense that they predict the nature of channel-shaped bases (Fig. 13). deposits given a set of initial starting parameters. It Unlike the structure-imitating models, process- is not known a priori what the deposits will look based models (sometimes referred to as process- like. Advantages of process-based models are that

AAS Revista 8 (2), 2001 105 John S. BRIDGE

Figure 13. (A) Example from the MOHERES software of a single channel belt consisting of four “channel beds” (from Tyler et al., 1994). In this software, internal elements such as channel bars and channel fills can be placed randomly within these “channel beds”. This is not a realistic representation of the geometry of deposits in channel belts. Notice that the “channel beds” are not even in contact with eachother in places. (B) Example of a channel belt from Patagonia showing a single active channel and many abandoned channel bars and channel fills. The channel-bar deposits comprise most of the volume of this channel belt.

Figura 13. (A) Ejemplo del programa MOHERES consistente en una faja de canales simple con cuatro “lechos de canales” (de Tyler et al., 1994). En este programa, los elementos internos tales como barras de canal y relleno de canales pueden estar ubicados al azar dentro de los “lechos de canales”. Esta no es una representación relista de la geometría de los depósitos en las fajas de canales. Note que los “lechos de canales” no están en contacto entre sí en otros sectores (figura inferior). (B) Ejemplo de una faja de canales de Patagonia mostrando un canal activo simple y muchas barras de canal abandonadas y rellenos de canal. Los depósitos de barras de canal conforman la mayor parte del volumen de esta faja de canales.

106 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions they utilize fundamental physical principles to generate stratigraphy. Therefore, process-based models can help provide genetic interpretations of deposits, and can predict more realistic stratigraphy than structure-imitating (stochastic) models. However, a perceived disadvantage of process-based models is that it is difficult or impossible to make the simulated deposits fit observational data in sufficient detail (Clemetsen et al., 1990; North, 1996; Koltermann and Gorelick, 1996; Anderson, 1997). Therefore, process-based models have had limited application in quantitative simulation of reservoir stratigraphy. Recent work shows, however, that fitting of process-based models to well data is possible in principle, using a trial-and-error approach with some optimization (Karssenberg et al., 2001a). Karssenburg et al. (2001a) fitted a simplified version of the Mackey-Bridge (1995) three-dimensional model of alluvial stratigraphy to five hypothetical wells (Fig. 14). The approach is to run the model many times, each time with different input defined Figure 14. (A) Hypothetical model area with location of five wells using Monte Carlo simulation techniques. (B) Hypothetical well data used for model fitting (from Karssenberg Successful runs are those where output fits well data et al., 2001a). within certain tolerance limits. Figure 15 shows the temporal evolution of the floodplain for one run of Figura 14. (A) Área hipotética del modelo con la ubicación de cinco pozos, (B) Datos hipotéticos del pozo usados para ajustar el the process-based model with the simulated modelo (de Karssenberg et al., 2001a). deposits fitting the five wells. On the basis of all of the realizations of the model that fit the wells, the probability of occurrence of channel-belt deposits within the area can be calculated (Fig. 16). expresses the difference between model output and It was concluded that using this trial-and-error well data. The objective is to minimize the approach in practice will require: (1) refinement of difference between the model output and well data, the process-based model; (2) improved algorithms or to maximize the fitness function. Since the fit- for efficiently fitting the model to the data, and; (3) ting procedure will put a high demand on overcoming the anticipated large amounts of computational resources, the choice of the fitting computing time. Recent progress on these three method and the software implementation of this problems has facilitated the implementation of this method and the process-based model is critical in inverse modeling approach and its application to order to minimize run times. real-world data. Moreover, the effectiveness of process-based models in simulating alluvial stratigraphy can now be compared with stochastic, Acknowledgements. Sincere thanks to my friend and structure-imitating models. The 3-D process-based colleague Bo Tye for many discussions of the issues model can be fitted to well data exactly by inverse raised in this review. Bo also generously reviewed the manuscript. Sincere thanks also to another friend and modeling (Cross and Lessenger, 1999; Bornholdt et colleague, Sergio Georgieff, who invited me to write al, 1999; Karssenberg et al., 2001a). With inverse this paper, and who was responsible for the Spanish modeling, fitting of the model to well data is translations. achieved by repeatedly running the model, comparing its output with well data, and adjusting REFERENCES the model input parameters until the model output fits the well data within specified tolerance limits. Aitken, J.F. and S. S. Flint, 1995. The application of high- For each model run, a fitness function (called resolution sequence stratigraphy to fluvial systems: a case objective function or goal function by others) study from the Upper Carboniferous Breathitt Group,

AAS Revista 8 (2), 2001 107 John S. BRIDGE

Figure 15. Model run that fits the well data. Each map shows position of the old and new channel belt, and the surface elevation (m) at the time of an avulsion (from Karssenberg et al., 2001a).

Figura 15. Modelo simulado que se ajusta a los datos de los pozos. Cada mapa muestra la posición de la faja de canales antigua y nueva, y la elevación de la superficie (m) en el momento de una avulsión (de Karssenberg et al., 2001a).

108 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions

Figure 16. 3-D image of probability of occurrence of channel belt deposits based on model runs that fit the well data. Top figure shows volume with probability > 0.8. Bottom diagram shows transects through the 3-D block, with grayscale representing probability of occurrence of channel-belt deposits (from Karssenberg et al., 2001a).

Figura 16. Imagen 3-D de probabilidad de ocurrencia de los depósitos de faja de canales basada en los modelos simulados que se ajustan a los datos de los pozos. La figura en la parte superior muestra el volumen con probabilidad > 0,8. El diagrama de la base muestra las transectas a través del bloque 3-D, la escala de grises representa la probabilidad de ocurrencia de los depósitos de la faja de canales (de Karssenberg et al., 2001a).

AAS Revista 8 (2), 2001 109 John S. BRIDGE

eastern Kentucky, USA. Sedimentology 42: 3-30. dimensional model. Earth Surface Processes 2: 401-416. Allen, J.R.L., 1965. A review of the origin and characteristics Bridge, J.S., 1985. Paleochannel patterns inferred from of recent alluvial . Sedimentology 5: 89-191. alluvial deposits: A critical evaluation. Journal of Allen, J.R.L., 1978. Studies in fluviatile sedimentation: an Sedimentary Petrology 55: 579-589. exploratory quantitative model for architecture of Bridge, J.S., 1992. A revised model for water flow, sediment avulsion-controlled alluvial suites. Sedimentary Geology transport, bed topography and grain size sorting in natu- 21: 129-147. ral river bends. Water Resources Research 28: 999-1023. Anderson, M.P., 1997, Characterization of geological Bridge, J.S., 1993a. The interaction between channel heterogeneity. In: G. Dagan, and S.P. Neuman (Eds.), geometry, water flow, sediment transport and deposition Subsurface Flow and Transport: A Stochastic Approach. in braided rivers. In: J.L. Best and C.S. Bristow (Eds.), Cambridge University Press, Cambridge, 23-43. Braided Rivers. Geological Society of London, Special Autin, W.J., S.F. Burns, R.T. Miller, R. T. Saucier and J. I. Publication 75: 13-72. Snead, 1991. Quaternary Geology of the lower Mississippi Bridge, J.S., 1993b. Description and interpretation of fluvial valley. In: R. B. Morrison (Ed.), Quaternary nonglacial deposits: a critical perspective. Sedimentology 40: 801- geology: conterminous U.S. The Geology of North America. 810. Geological Society of America K-2: 547-582. Bridge, J.S., 1997. Thickness of sets of cross strata and planar Barwis, J. H., J. G. Mcpherson and R. J. Studlick, 1990. strata as a function of formative bed-wave geometry and Sandstone Petroleum Reservoirs. Springer-Verlag, New migration, and aggradation rate. Geology 25: 971-974. York, 583 pp. Bridge, J. S., 1999, Alluvial architecture of the Mississippi Berg, R.R., 1968. Point-bar origin of Fall River Sandstone Valley: predictions using a 3D simulation model. In: S.B. reservoirs. American Association of Petroleum Geologists Marriott and J. Alexander (Eds.), Floodplains: Bulletin 52: 2116-2122. Interdisciplinary Approaches. Geological Society of Best, J.L. and P. J. Ashworth, 1997. Scour in large braided London, Special Publication 163: 269-278. rivers and the recognition of sequence stratigraphic Bridge, J.S., 2002. Alluvial Rivers and Floodplains. boundaries. Nature 387: 275-277. Blackwells, Oxford Bierkens, M.F.P. and H.J.T Weerts, 1994. Application of Bridge, J.S., G. A. Jalfin and S. M. Georgieff, 2000. Geometry, indicator simulation to modelling the lithological lithofacies and spatial distribution of Cretaceous fluvial properties of a complex confining layer. Geoderma 62: sandstone bodies, San Jorge Basin, Argentina: outcrop 265-284. analog for the hydrocarbon-bearing Chubut Group. Blum, M.D. and D.M. Price, 1998. Quaternary alluvial plain Journal of Sedimentary Research 70: 341-359. construction in response to glacio-eustatic and climatic Bridge, J.S. and M. R. Leeder, 1979. A simulation model of controls, Texas Gulf Coastal Plain. In: Shanley, K.J. and P. alluvial stratigraphy. Sedimentology 26: 617-644. J. McCabe (Eds.), Relative Role of Eustasy, Climate and Bridge, J.S. and S. D. Mackey, 1993a. A revised alluvial Tectonism on Continental Rocks. Society of Economic stratigraphy model. In: Marzo, M. and C. Puidefabregas Paleontologists and Mineralogists Special Publication 59: (Eds.), Alluvial sedimentation. International Association 31-48. of Sedimentologists Special Publication 17: 319-337. Blum, M.D., 1994. Genesis and architecture of incised valley Bridge, J.S. and S, D. Mackey, 1993b. A theoretical study of fill sequences: a late Quaternary example from the Colo- fluvial sandstone body dimensions. In: S.S. Flint and I.D. rado River, Gulf Coastal Plain of Texas. In: P. Weimer and Bryant (Eds.), Geological modeling of hydrocarbon H.W. Posamentier (Eds.), Siliciclastic sequence reservoirs. International Association of Sedimentologists stratigraphy. American Association of Petroleum Special Publication 15: 213-236. Geologists Memoir 58: 259-283. Bridge, J.S. and R. S. Tye, 2000. Interpreting the dimensions Blum, M.D. and T.E. Törnqvist, 2000. Fluvial responses to of ancient fluvial channel bars, channels, and channel climate and sea-level change: a review and look forward. belts from wireline-logs and cores. American Association Sedimentology 47 (Suppl.1): 2-48. of Petroleum Geologists Bulletin 84: 1205-1228. Bornholdt, S., U. Nordlund and H. Westphal, 1999. Inverse Bristow, C.S., 1987. Brahmaputra River: Channel migration stratigraphic modeling using genetic algorithms. In: and deposition. In: Ethridge, F.G., Flores, R. M., and M. D. Harbaugh, J.W. et al. (Eds.), Numerical Experiments in Harvey (Eds.), Recent developments in fluvial Stratigraphy: Recent Advances in Stratigraphic and sedimentology. Society of Economic Paleontologists and Sedimentologic Computer Simulations. Society of Mineralogists Special Publication 39: 63-74. Economic Paleontologists and Mineralogists Special Bryant, I.D., and S. Flint, 1993. Quantitative clastic reservoir Publication 62: 85-90. geological modeling: problems and perspectives. In: Flint, Brice, J.C., 1964. Channel patterns and terraces of the Loup S. and I. D. Bryant (Eds.), Geological modeling of Rivers in Nebraska. U.S. Geological Survey Professional hydrocarbon reservoirs. International Association of Paper 422D: 1-41. Sedimentologists Special Publication 15: 3-20. Brice, J.C., 1984. Planform properties of meandering rivers. Budding, M.C., A. H. M. Paardekam and S. J. Van Rossem, In: C.M. Elliot (Ed.), River Meandering. American Society 1992. 3D connectivity and architecture in sandstone of Civil Engineers, 1-15. reservoirs. SPE Technical Paper 22342, presented at SPE Bridge, J.S., 1977. Flow, bed topography, grain size and International Meeting on Petroleum Engineering, Beijing, sedimentary structure in open channel bends: a three- China, 24-27 March 1992.

110 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions

Burnett, M., 1996. 3-D seismic expression of a shallow flu- simulation. Mathematical Geology 26: 67-82. vial system in west central Texas. In: P. Weimer and T.L. Deutsch, C.V. and L. Wang, 1996. Hierarchical Object-Based Davis (Eds.), Applications of 3-D Seismic Data to Stochastic Modeling of Fluvial Reservoirs. Mathematical Exploration and Production. AAPG Studies in Geology Geology 28: 857-880. No. 42 and SEG Geophysical Developments Series No. 5, Doveton, J.H., 1994, Theory and applications of vertical AAPG/SEG, Tulsa: 45-56. variability measures from Markov Chain analysis. In Carle, S.F., E. M. Labolle, G. S. Weissmann, D. Van Brocklin Yarus, J.M. and R. L. Chambers (Eds.), Stochastic Modeling and G. E. Fogg, 1998. Conditional simulation of and Geostatistics. American Association of Petroleum hydrofacies architecture: a transition probability/Markov Geologists Computer Applications in Geology 3: 55-64. approach. In: Fraser, G.S. and J.M. Davis (Eds.), Dreyer, T., 1993. Quantified fluvial architecture in Hydrogeologic models of sedimentary aquifers. Concepts ephemeral stream deposits of the Esplugafreda in Hydrogeology and Environmental Geology No.1, Formation (Palaeocene), Tremp-Graus Basin, northern Society of Economic Paleontologists and Mineralogists: Spain. In: M. Marzo and C. Puigdefábregas (Eds.), Alluvial 147-170. Sedimentation. International Association of Chitale, S.V., 1970. River channel patterns. Journal of the Sedimentologists Special Publication 17: 337-362. Division, American Society of Civil Engineers Dreyer, T., L. M. Falt, T. Hoy, R. Knarud, R. Steel and J. L. 96: 201-221. Cuevas, 1993. Sedimentary architecture of field Clemetsen, R., A. R. Hurst, R. Knarud and H. Omre, 1990. A analogues for reservoir information (SAFARI): a case computer program for evaluation of fluvial reservoirs. study of the fluvial Escanilla Formation, Spanish Pyrenees. In: Buller, A.T., E. Berg, O. Hjemeland, J. Kleppe, O. In: Flint, S. and I. D. Bryant (Eds.), The geological Torsaeter and L. O. Aasen (Eds.), North Sea oil and gas modeling of hydrocarbon reservoirs and outcrop reservoirs-II. Graham and Trotman, London: 373-385. analogues. International Association of Sedimentologists Collinson, J.D., 1978. Vertical sequence and sand body shape Special Publication 15: 57-80. in alluvial sequences. In: A.D. Miall (Ed.), Fluvial Ethridge, F. G. and R. M. Flores (Eds.), 1981. Recent and Sedimentology. Canadian Society of Petroleum Geologists ancient nonmarine depositional environments: models Memoir 5: 577-586. for exploration. Society of Economic Paleontologists and Collinson, J. D., 1996. Alluvial sediments. In: H. G. Reading Mineralogists Special Publication 31. (Ed.), Sedimentary Environments and Facies, 3rd edition, Ethridge, F. G., R. M. Flores and M. D. Harvey (Eds.), 1987. Elsevier, New York: 37-82. Recent developments in fluvial sedimentology. Society Cornish, F.G., 1984. Fluvial environments and of Economic Paleontologists and Mineralogists Special palaeohydrology of the Upper Morrow “A” (Pennsylvanian) Publication 39, Tulsa, 389 pp. meander belt sandstone, Beaver County, Oklahoma. Shale Ethridge, F.G., L. J. Wood and S. A. Schumm, 1998. Cyclic Shaker, 34: 70-80. variables controlling fluvial sequence development: Cross, T.A. and M. A. Lessenger, 1999. Construction and problems and perspectives. In: Shanley, K.J. and P. J. application of a stratigraphic inverse model. In: McCabe (Eds.), Relative Role of Eustasy, Climate and Harbaugh, J.W. et al. (Eds.), Numerical Experiments in Tectonism on Continental Rocks. Society of Economic Stratigraphy: Recent Advances in Stratigraphic and Paleontologists and Mineralogists Special Publication 59: Sedimentologic Computer Simulations, Society of 17-29. Economic Paleontologists and Mineralogists Special Fielding, C.R. and R. C. Crane, 1987. An application of Publication 62: 69-83. statistical modelling to the prediction of hydrocarbon Cuevas Gozalo, M.C. and A. W. Martinius, 1993. Outcrop recovery factors in fluvial reservoir sequences. In: data-base for the geological characterization of fluvial Ethridge, F.G., R. M. Flores and M. D. Harvey (Eds.), Recent reservoirs: an example from distal fluvial fan deposits in developments in fluvial sedimentology. Society of the Loranca Basin, Spain. In C.P. North and D.J. Prosser Economic Paleontologists and Mineralogists Special (Eds.), Characterization of Fluvial and Aeolian Publication 39: 321-327. Reservoirs. Geological Society of London Special Flores, R. M., F. G. Ethridge, A. D. Miall, W. E. Galloway and Publication 73: 79-94. T. D. Fouch, 1985. Recognition of fluvial depositional Dalrymple, R. W., R. Boyd and B. A. Zaitlin (Eds.), 1994. systems and their resource potential. Society of Economic Incised-valley systems: origin and sedimentary Paleontologists and Mineralogists Short Course 19, 290 sequences. Society of Economic Paleontologists and pp. Mineralogists Special Publication 51. Galloway, W. E. and D. K. Hobday, 1983. Terrigeneous Clastic Davies, D. K., B. P. J. Williams and R. K. Vessell, 1993. Depositional Systems: Applications to Petroleum, Coal, Dimensions and quality of reservoirs originating in low and Uranium Exploration, 1st ed., Springer-Verlag, New and high sinuosity channel systems, Lower Cretaceous York, Travis Peak Formation, East Texas, USA. In: C. P. North Galloway, W. E. and D. K. Hobday, 1996. Terrigeneous Clastic and D. J. Prosser (Eds.), Characterization of Fluvial and Depositional Systems: Applications to Petroleum, Coal, Aeolian Reservoirs. Geological Society of London Special and Uranium Exploration, 2nd ed., Springer-Verlag, New Publication 73: 95-121. York, 489 p. Deutsch, C. and P. Cockerham, 1994. Practical considerations Geehan , G. and J. Underwood, 1993. The use of shale length in the application of simulated annealing to stochastic distributions in geological modeling. In: Flint, S. and

AAS Revista 8 (2), 2001 111 John S. BRIDGE

Bryant, I.D. (Eds.), The geological modeling of process-imitating and descriptive approaches. Water hydrocarbon reservoirs and outcrop analogues. Resources Research 32: 2617-2658. International Association of Sedimentologists Special Knighton, A.D. and G. C. Nanson, 1993. Anastomosis and Publication 15: 205-212. the continuum of channel pattern. Earth Surface Gibling, M.R. and D. J. Bird, 1994. Late Carboniferous Processes and Landforms 18: 613-625. cyclothems and alluvial paleovalleys in the Sydney Basin, Koss, J.E., F. G. Ethridge and S. A. Schumm, 1994. An expe- Nova Scotia. Geological Society of America Bulletin 106: rimental study of the effects of base-level change on flu- 105-117. vial, coastal plain and shelf systems. Journal of Hardage, B.A., R. A. Levey, V. Pendleton, J. Simmons and R. Sedimentary Research B64: 90-98. Edson, 1994. A 3-D seismic case history evaluating Lane, E.W., 1957. A study of the shape of channels formed by fluvially deposited thin-bedded reservoirs in a gas- natural streams flowing in erodible material. Missouri producing property. Geophysics 59: 1650-1665. River Division Sediment Series No. 9, U.S. Army Engineer Hardage, B.A., R. A. Levey, V. Pendleton, J. Simmons and R. Division, Missouri River, Corps of Engineers, Omaha, Edson, 1996. 3-D seismic imaging and interpretation of Nebraska. fluvially deposited thin-bed reservoirs. In: P. Weimer and Leclair, S.F. and J. S. Bridge, 2001. Quantitative interpretation T.L. Davis (Eds.), Applications of 3-D Seismic Data to of sedimentary structures formed by river dunes. Journal Exploration and Production. AAPG Studies in Geology of Sedimentary Research.71: 713-716. No. 42 and SEG Geophysical Developments Series No. 5, Leclair, S. F., J. S. Bridge and F. Wang, 1997. Preservation of AAPG/SEG, Tulsa: 27-34. cross-strata due to migration of subaqueous dunes over Heller, P.L. and C. Paola, 1996. Downstream changes in aggrading and non-aggrading beds: comparison of expe- alluvial architecture: an exploration of controls on rimental data with theory. Geoscience Canada 24: 55- channel-stacking patterns. Journal of Sedimentary 66. Research 66: 297-306. Leeder, M.R., 1978. A quantitative stratigraphic model for Hirst, J. P. P., C. R. Blackstock and S. Tyson, 1993. Stochastic with special reference to channel deposit modeling of fluvial sandstone bodies. In: S. S. Flint and I. density and interconnectedness. In: Miall, A.D. (Ed.), Flu- D. Bryant (Eds.), The Geological Modeling of Hydrocarbon vial sedimentology. Canadian Society of Petroleum Reservoirs and Outcrop Analogues. International Geologists Memoir 5: 587-596. Association of Sedimentologists Special Publication 15: Leeder, M.R. and M. Stewart, 1996. Fluvial incision and 237-251. sequence stratigraphy: alluvial responses to relative base Holden, L., R. Hauge, Ø. Skare and A. Skorstad, 1998. level fall and their detection in the geological record. In: Modeling of Fluvial Reservoirs with Object Models. Hesselbo, S.P. and D. N. Parkinson (Eds.), Sequence Mathematical Geology 30: 473-496. stratigraphy in British Geology. Geological Society of Jackson, R.G., 1978. Preliminary evaluation of lithofacies London Special Publication 103: 47-61. models for meandering alluvial streams. In: Miall, A.D. Lomando, A. J. and P. M. Harris, 1988. Giant oil and gas (Ed.), Fluvial Sedimentology. Canadian Society of fields: a core workshop. Society of Economic Petroleum Geologists Memoir 5: 543-576. Paleontologists and Mineralogists Core Workshop 12, 853 Jervey, M.T., 1988. Quantitative geological modeling of pp. siliciclastic rock sequences and their seismic expression. Lorenz, J.C., D. M. Heinze, J. A. Clark and C. A. Searls, 1985. In: C.K. Wilgus, B. Hastings, C. Kendall, H.W. Posamentier, Determination of widths of meander-belt sandstone C.A. Ross and J.C. Van Wagoner (Eds.), Sea-level changes: reservoirs from vertical downhole data, Mesaverde an integrated approach. Society of Economic Group, Piceance Creek Basin, Colorado. American Paleontologists and Mineralogists Special Publication 42: Association of Petroleum Geologists Bulletin 69: 710-721. 47-69. Lorenz, J. C., N. R. Warpinski and P. T. Brannagan, 1991. Journel, A.G., 1983. Nonparametric estimation of spatial Subsurface characterization of Mesaverde reservoirs in distributions. Mathematical Geology 15: 445-468. Colorado: geophysical and reservoir-engineering checks Karssenberg, D., T. E. Tornqvist and J. S. Bridge, 2001a. on predictive sedimentology. In: A. D. Miall and N. Tyler Conditioning a process-based model of sedimentary (Eds.), The three-dimensional facies architecture of architecture to well data. Journal of Sedimentary terrigenous clastic sediments and its implications for Research 71: 868-879. hydrocarbon discovery and recovery. Society of Economic Karssenberg, G, D., J. S. Bridge and T. E. Tornqvist , 2001b. Paleontologists and Mineralogists Concepts in Modeling cycles of fluvial aggradation and Sedimentology and Paleontology 3: 57-79. due to base-level change using a revised process-based, Lowry, P. and A. Raheim, 1991. Characterisation of delta 3-D alluvial stratigraphy model. VII International front sandstones from a fluvial-dominated delta system. Conference on Fluvial Sedimentology, Lincoln, Nebraska. In: L. W. Lake, H. B. Carroll Jr., and T. C. Wesson (Eds.), Khan, I.A., J. S. Bridge, J. Kappelman and R. Wilson, 1997. Reservoir characterisation - II. Academic Press, London: Evolution of Miocene fluvial environments, eastern 665-676. Potwar plateau, northern Pakistan. Sedimentology 44: Mackey, S.D. and J. S. Bridge, 1995. Three dimensional 221-251. model of alluvial stratigraphy: theory and application. Koltermann, C.E. and S. M. Gorelick, 1996. Heterogeneity in Journal of Sedimentary Research B65: 7-31. sedimentary deposits: a review of structure-imitating, Makaske, B., 2001. Anastomosing rivers: a review fo their

112 AAS Revista 8 (2), 2001 Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions

classification, origin and sedimentary products. Earth- In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Science Reviews 53: 149-196. Posamentier, C.A. Ross and J.C. Van Wagoner (Eds.), Sea- Miall, A.D., 1977. A review of the braided river depositional level changes: an integrated approach. Society of environment. Earth Science Reviews 13: 1-62. Economic Paleontologists and Mineralogists Special Miall, A.D., 1986. Eustatic sea level changes interpreted from Publication 42: 125-154. seismic stratigraphy: a critique of the methodology with Robinson, J. W. and P. J. McCabe, 1997, Sandstone-body and particular reference to the North Sea Jurassic record. shale-body dimensions in a braided fluvial system: Salt American Association of Petroleum Geologists Bulletin Wash Sandstone Member (Morrison Formation), Garfield 70: 131-137. County, Utah. American Association of Petroleum Miall, A.D., 1991. Stratigraphic sequences and their Geologists Bulletin 81: 1267-1291. chronostratigraphic correlation. Journal of Sedimentary Ross, W.C., 1990. Modeling base-level dynamics as a control Petrology 61: 497-505. on basin-fill geometries and facies distribution: a con- Miall, A.D., 1992. Alluvial deposits. In: Walker, R.G. and N. ceptual framework. In: Cross, T.A. (Ed.), Quantitative P. James (Eds.), Facies models: response to sea level Dynamic Stratigraphy, Prentice-Hall, Englewood Cliffs, changes. Geological Association of Canada, St Johns, New Jersey: 387-399. Newfoundland: 119-142. Rust, B.R., 1978. A classification of alluvial channel systems. Miall, A.D., 1996. The geology of fluvial deposits. Springer- In: Miall, A.D. (Ed.), Fluvial Sedimentology Canadian Verlag, New York, 582 p. Society of Petroleum Geologists Memoir 5: 187-198. Miall, A.D. and N. Tyler (Eds.), 1991. The Three-Dimensio- Salter, T., 1993. Fluvial scour and incision: models for their nal Facies Architecture of Terrigeneous Clastic Sediments influence on the development of realistic reservoir and Its Implications for Hydrocarbon Discovery and geometries. In: C. P. North and D. J. Prosser (Eds.), Recovery. Society of Economic Paleontologists and Characterization of Fluvial and Aeolian Reservoirs. Mineralogists Concepts in Sedimentology and Geological Society of London Special Publication 73: 33- Paleontology 3, 309 pp. 51. Nanson, G.C. and A. D. Knighton, 1996. Anabranching rivers: Saucier, R.T., 1981. Current thinking on riverine processes their cause, character and classification. Earth Surface and geologic history as related to human settlement in Processes and Landforms 21: 217-239. the southeast. Geoscience and Man 22: 7-18. Nanz, R.H. JR., 1954. Genesis of Oligocene sandstone Saucier, R.T., 1994. Geomorphology and Quaternary Geologic reservoir, Seeligson field, Jim Wells and Kleberg Counties, History of the Lower Mississippi Valley. Mississippi River Texas. American Association of Petroleum Geologists Commission, Vicksburg, 364 pp. Bulletin 38: 96-117. Schumm, S.A., 1963. Sinuosity of alluvial channels on the North, C.P., 1996. The prediction and modelling of subsurface . Geological Society of America Bulletin 74: fluvial stratigraphy. In: P.A. Carling and M.R. Dawson 1089-1100. (Eds.), Advances in Fluvial Dynamics and Stratigraphy. Schumm, S.A., 1971. Fluvial geomorphology. In: Shen, H.W. John Wiley, Chichester: 395-508. (Ed.), River Mechanics I. Water Resources Publications, North, C. P. and K. S. Taylor, 1996. Ephemeral-fluvial deposits: Colorado, Chapter 5: 1-22. integrated outcrop and simulation studies reveal Schumm, S.A., 1972. Fluvial paleochannels. In: J.K. Rigby complexity. American Association of Petroleum and W.K. Hamblin (Eds.), Recognition of ancient Geologists Bulletin 80: 811-830. sedimentary environments. Society of Economic Paola, C., P. L. Heller and C. L. Angevine, 1992. The large- Paleontologists and Mineralogists Special Publication 16: scale dynamics of grain-size variation in alluvial basins. 98-107. 1 - Theory. Basin Research 4: 73-90. Schumm, S.A., 1977. The Fluvial System. John Wiley and Pitman, W.C. III, 1986. Effects of sea level change on basin Sons, New York, 338 p. stratigraphy. American Association of Petroleum Schumm, S.A., 1981. Evolution and response of the fluvial Geologists Bulletin 70: 1-62. system, sedimentologic implications. In Ethridge, F.G. Pitman, W.C. III and X. Golovchenko, 1988. Sea-level and R. M. Flores (Eds.), Recent and ancient nonmarine changes and their effect on the stratigraphy of Atlantic- depositional environments: models for exploration. type margins. In: Sheridan, R.E. and J. A. Grow (Eds.), Society of Economic Paleontologists and Mineralogists The geology of North America, vol I-2, The Atlantic con- Special Publication 31: 19-29. tinental margin, United States. Geological Society of Schumm, S.A., 1985. Patterns of alluvial rivers. Annual America: 429-436. Reviews Earth and Planetary Science 13: 5-27. Posamentier, H.W., M. T. Jervey and P. R. Vail, 1988. Eustatic Schumm, S.A., 1993. River response to base level change: controls on clastic deposition I - Conceptual framework. implications for sequence stratigraphy. Journal of Geology In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. 101: 279-294. Posamentier, C.A. Ross and J.C. Van Wagoner (Eds.), Sea- Selley, R.C., 1996. Ancient Sedimentary Environments and level changes: an integrated approach. Society of their Subsurface Diagnosis, 4th ed., Chapman and Hall, Economic Paleontologists and Mineralogists Special London, 300 pp. Publication 42: 109-124. Shanley, K. W. and P. J. McCabe, 1993. Alluvial architecture Posamentier, H.W. and P.R. Vail, 1988. Eustatic controls on in a sequence stratigraphic framework: a case history clastic deposition II - Sequence and systems tract models. from the Upper Cretaceous of southern Utah, USA. In:

AAS Revista 8 (2), 2001 113 John S. BRIDGE

Flint, S. and Bryant, I.D. (Eds.), The geological modeling distribution of sediment units in braided stream deposits. of hydrocarbon reservoirs and outcrop analogues. Journal of Sedimentary Research B64: 219-231. International Association of Sedimentologists Special Weber, K. J., 1993. The use of 3-D seismic in reservoir Publication 15: 21-56. geological modelling. In: S.S. Flint and I.D. Bryant (Eds.), Shanley, K. W. and P. J. McCabe, 1994. Perspectives on the The Geological Modelling of Hydrocarbon Reservoirs and sequence stratigraphy of continental strata. American Outcrop Analogues. International Association of Association of Petroleum Geologists Bulletin 78: 544-568. Sedimentologists Special Publication 15: 181-188. Smith, D.G., 1976. Effect of vegetation on lateral migration Wescott, W.A., 1993. Geomorphic thresholds and complex of anastomosed channels of a glacier meltwater river. response of fluvial systems - some implications for Geological Society of America Bulletin 87: 857-860. sequence stratigraphy. American Association of Srivastava, R.M., 1994. An overview of stochastic methods Petroleum Geologists Bulletin 77: 1208-1218. for reservoir characterization. In: Yarus J.M. and R. L. Willis, B.J., 1993a. Ancient river systems in the Himalayan Chambers (Eds.), Stochastic modeling and Geostatistics. foredeep, Chinji village area, northern Pakistan. American Association of Petroleum Geologists Computer Sedimentary Geology 88: 1-76. Applications in Geology 3: 3-16. Willis, B.J., 1993b. Evolution of Miocene fluvial systems in Stam, J.M.T., 1996. Migration and Growth of Aeolian the Himalayan foredeep through a two kilometer-thick . Mathematical Geology 28: 519-536. succession in northern Pakistan. Sedimentary Geology Tetzlaff, D.M. and J. W. Harbaugh, 1989. Simulating Clastic 88: 77-121. Sedimentation. New York, Van Nostrand Reinhold. Wood, L.J., F. G. Ethridge and S. A. Schumm, 1994. An Törnqvist, T.E., 1993. Holocene alternation of meandering expermental study of the influence of subaqueous shelf and anastomosing fluvial systems in the Rhine-Meuse angles on coastal plain and shelf deposits. In: Weimer, P. Delta (central Netherlands) controlled by sea-level rise and H. W. Posamentier (Eds.), Siliciclastic sequence and subsoil erodibility. Journal of Sedimentary Petrology stratigraphy. American Association of Petroleum 63: 683-693. Geologists Memoir 58: 381-391. Törnqvist, T.E., 1994. Middle and late Holocene avulsion Wright, V.P. and S. B. Marriott, 1993. The sequence history of the River Rhine (Rhine-Meuse Delta, stratigraphy of fluvial depositional systems: the role of Netherlands). Geology 22: 711-714. floodplain sediment storage. Sedimentary Geology 86: Tye, R. S., 1991. Fluvial-sandstone reservoirs of the Travis 203-210. Peak Formation, East Texas Basin. In: A. D. Miall and N. Zaleha, M.J., 1997a. Fluvial and lacustrine Tyler (Eds.), The Three-Dimensional Facies Architecture palaeoenvironments of the Miocene Siwalik Group, of Terrigeneous Clastic Sediments and Its Implications Khaur area, northern Pakistan. Sedimentology 44: 349- for Hydrocarbon Discovery and Recovery. Society of 368. Economic Paleontologists and Mineralogists Concepts in Zaleha, M.J., 1997b. Intra- and extrabasinal controls on flu- Sedimentology and Paleontology 3: 172-188. vial deposition in the Miocene Indo-Gangetic basin, Tyler, K., A. Henriquez and T. Svanes, 1994. Modeling northern Pakistan. Sedimentology 44: 369-390. heterogeneities in fluvial domains: a review of the influence on production profiles. In Yarus J.M. and R. L. Chambers (Eds.), Stochastic modeling and Geostatistics. American Association of Petroleum Geologists Computer John S. BRIDGE Applications in Geology 3: 77-89. Department of Geological Sciences Walderhaug, O. and R. Mjos, 1991. Sandbody geometry in Binghamton University fluvial systems - an example from the Jurassic of P.O. Box 6000 Yorkshire, England. In: L. W. Lake, H. B. Carroll Jr. and T. Binghamton, NY C. Wesson (Eds.), Reservoir characterisation - II. 13902-6000, USA Academic Press, London: 701-703. Webb, E.K., 1994. Simulating the three-dimensional Email: [email protected]

114 AAS Revista 8 (2), 2001