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Generation of a Synthetic Vertical Profile of a Fluvial Sandstone Body

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Generation of a Synthetic Vertical Profile of a Fluvial Sandstone Body Generation of a Synthetic Vertical Profile of a Fluvial Sandstone Body INDIANA U. PAUL 'OTTER INDIANA GEOLOGICAL SURVEY F' BLAKELY I BLOOMINGTON, IND. ABSTRACT depositional processes. From the petroleum engi- neer's viewpoint it seems reasonable to believe Downloaded from http://onepetro.org/spejournal/article-pdf/7/03/243/2152790/spe-1778-pa.pdf by guest on 30 September 2021 Any stratigraphic section or bedding sequence that the synthetic generation of rock properties and can be synthesized if there is a transition procedure their distribution in a reservoir should be relevant from one lithology or bedding type to another, and in the study of reservoirs. if thickness distributions of the different lithologies Any stratigraphic section or bedding sequence are known. Stratigraphic sections of a fluvial can be generated provided there is a transition sandstone body were synthesized with five bedding procedure from one lithology or bedding type to types: cross- bedding, massive beds, parting another, and provided the thickness distributions of lineation, ripple mark and mudstone. The transition the different units are known. The transition procedure from one bedding type to another used procedure involves random processes that are dependent, Markovian random processes which have either independent or dependent. If the depositional a memory that extends one step backward in the process is independent, previous deposition will depositional process. As observed in nature, median have no influence on present deposition. However, grain size and sand wave thickness (cross-bedding if it is dependent, past deposition will influence and ripple mark) decline upward in the synthesized either present or future deposition. Such a dependent sections as proportions of the different bedding depositional process can be thought of as having a types change. Grain size and permeability were memory that extends backward in time through one also incorporated into the sections. By changing or more pulses of deposition. A process with a the transition procedures, bed thickness distribu- memory can be described by a Markov process. tions, rate of upward decline or sand wave height Because the concept of memory or dependence and length, different types of sections can be appears to be in accord with our understanding of synthesized, thus making it possible to model many many depositional processes, Markov processes different sedimentation problems. were used to synthesize the bedding sequences of this study (see Appendix). INTRODUCTION The above methods are perfectly general and are This paper describes a general method for appropriate for any stratigraphic section or bedding synthesizing stratigraphic sections and bedding sequence: bedding types in a beach deposit, an sequences of sedimentary, metamorphic or igneous evolving carbonate bank or the changing lithologic origin. Synthetic is of interest for fill of a thick geosyncline sequence. We chose to several reasons. close correspondence between synthesize a vertical profile of a fluvial sandstone real and synthetic sections suggest that the factors body because its characteristics were well docu- used in the synthesizing model may indeed be the mented, much was known about fluvial processes and fluvial-deltaic sandstone bodies constitute an correct ones, thus giving the investigator a check important class of petroleum reservoirs. on his assumptions. Rapid, inexpensive simulation of many stratigraphic sections permits one to CHARACTERISTICS AND ORIGIN OF synthesize a rock body (sandstone or carbonate FLUVIAL CYCLE reservoir) or, on a larger scale, the fill of a sedimentary basin. Harbaughl gives an example of The fluvial cycle has been well documented in mathematical simulation of a carbonate basin. He recent years by ~ersier,2Allen3-5 and ~isher.6.~ simulated the basin in the hope that improved Deposits from fluvial cycles range from 10 to 150 ft prediction would follow better understanding of the or more in thickness and are characterized by a "fining upwards": coarse sandstones with occa- Original manuscript received in Society of Petroleum Engineers office Jan. 13, 1967. Revised manuscript of SPE 1778 received sional conglomerates grade upward into medium- to May 12, 1967. Publication authorized by permission of State fine-grained sandstone, and hence into siltstone and Geologist, Indiana Dept. of Natural Resources, Geological Survey. @ Copyright 1967 American Institute of Mining, mudstone. The dominant sedimentary structure is Metallurgical, and Petroleum Engineers. Inc. inclined bedding: thick cross-beds in the lower half '~eferencesgiven at end of paper. SEPTEMBER, 1967 become thinner upward and pass into ripple mark caps the cycle. The conglomerates commonly consist and parting lineation. Also present are some of fragments of peat, coal, shale or clay ironstone. seemingly massive beds which, in reality, are Casts and carbonized impressions of logs in the probably either thinly laminated or ripple marked,8 basal part of the cycle, as well as rootlets and and a few mudstone or shale partings. Mudstone finely macerated plant debris in the overlying Downloaded from http://onepetro.org/spejournal/article-pdf/7/03/243/2152790/spe-1778-pa.pdf by guest on 30 September 2021 -. 0 90 I80 270 (mm) THICKNESS ORIENTATION BEDDING (cm) (degrees) TYPES MEDIAN GRAIN SIZE CROSS-BEDDING EXPLANATION ~.:v -0 . - Cross- bedded sondstone Floser structure: Thin beds of ripple Massive" sondstone marked siltstone ond fine-grained - .... sondstone interlominoted with shale. PIG. 1 - UPWARD DECLINE OF GRAIN SIZE AND THICKNESS OF CROSS-BEDDING IN SANDSTONE, MANSFIELD FORMATION, SOUTHWESTERN INDIANA. 22 244 SOCIETY OF PETROLEUM ENGINEERS JOURNAL siltstones and mudstones, are common. Although in fluvial sandstone bodies. other sedimentary structures may be present, they Figs. 2 and 3 link depositional processes to the are not present in great abundance. Most fluvial empirically observed fining upwards fluvial cycle. sandstones are faunally barren, but concentra- This linkage provides the rational in using Markov tions of vertebrate remains are not uncommon processes to generate synthetic sections. in Mesozoic and later sandstones. Fig. 1 shows a vertical profile of the Pennsylvanian Mansfield METHODS formation that illustrates a fairly typical fluvial The upward change in kinds and thickness of cycle. Excellent documentation of the vertical sedimentary structures in a sandstone body were profile of an ancient fluvial sandstone body is approximated by dividing the sandstone body into given by Ilewitt and M0r~an.9 three parts: a lower, dominantly cross-bedded zone, Although there are some differences in details, a middle rippled and horizontally bedded zone and the fluvial succession is fairly well established and an upper zone transitional to mudstone. The has been related to four different facies. 101117677 following assumptions were made: (1) the deposi- These are (1) poorly bedded basal thalweg sands tional process is Markovian with a one -step that may contain granules, pebbles and even dependence, (2) the sandstone body consists of cobbles, (2) cross-bedded sands of point bars, (3) Downloaded from http://onepetro.org/spejournal/article-pdf/7/03/243/2152790/spe-1778-pa.pdf by guest on 30 September 2021 five bedding types or states: cross-beds (S1), massive rippled and horizontally bedded fine sands and silts, beds (S2), parting lineation (S3), ripple (wavy) usually of the inner flood plain, and (4) backwater bedding (S4) and mudstone (S5), (3) thicknesses of silts and clays. Origin of this fining upwards the different bedding types are assumed to have a sequence is shown in Fig. 2, a block diagram of a log-normal distribution and (4) the upward decline point bar at a river bend. Basal thalweg sands in current competence is reflected in sand wave usually constitute only a small portion of the total thickness, which decreases linearly upward. The section and may be indistinguishable from the last assumption implies that most of the sandstone point bar sands, almost all of which predominate. body was principally deposited in the lower flow Some parting lineation may be found in any of the regime. four facies, but it is best developed in the rippled The dynamics described in Figs. 2 and 3 provide and horizontally bedded fine sands and silts. These the justification for the assumption that the four facies are deposited simultaneously in geo- depositional process is Markovian with a one-step graphically distinct areas and are superimposed in dependence. Average hydraulic conditions at the a regular manner as the stream migrates laterally. sediment interface in the channel change but slowly The velocity distribution within the channel at as deposition proceeds; hence, there is a strong the stream bend provides more insight into the probability of a continuity or dependence in the origin of the fining upward (Fig. 3). The highest resultant types of bedding and sedimentary structures velocity is proximal to the outside of the bend. The deposited. This implies a memory. Such-a memory inset graph of Fig. 3 shows a plot of the decline of is, moreover, fully consistent with short-term, day velocity along the bottom, from the thalweg at X to to day variations in discharge. It is these short-term the inside of the bend at Y. This approximately perturbations that introduce the random, probabilistic linear decline in velocity shoreward is accompanied elements in most sedimentary processes. Of course, by an upward decline of both
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