Comparison of Modern Fluid Distribution, Pressure and Flow In

Marine and Petroleum Geology 51 (2014) 210e229

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Marine and Petroleum Geology

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Comparison of modern ﬂuid distribution, pressure and ﬂow in sediments associated with anticlines growing in deepwater (Brunei) and continental environments (Iran)

Chris K. Morley a,*, John Warren b, Mark Tingay c, Phathompat Boonyasaknanon a, Ali Julapour d a PTTEP, EnCo, Soi 11, Vibhavadi-Rangsit Road, Chatuchak, Bangkok 10900, Thailand b Dept. of Geology, Faculty of Science, Chulalongkorn University, Bangkok, Thailand c Chevron, Perth, Australia d National Iranian Oil Company, Calle Barachavi, No 510 B, Santa Cruz, Bolivia article info abstract

Article history: Differences in fluids origin, creation of overpressure and migration are compared for end member Received 1 August 2013 Neogene fold and thrust environments: the deepwater region offshore Brunei (shale detachment), and Received in revised form the onshore, arid Central Basin of Iran (salt detachment). Variations in overpressure mechanism arise 13 November 2013 from a) the availability of water trapped in pore-space during early burial (deepwater marine environ- Accepted 16 November 2013 ment vs arid, continental environment), and b) the depth/temperature at which mechanical compaction Available online 11 December 2013 becomes a secondary effect and chemical processes start to dominate overpressure development. Chemical reactions associated with smectite rich mud rocks in Iran occur shallow (w1900 m, smectite to Keywords: Brunei illite transformation) causing load-transfer related (moderate) overpressures, whereas mechanical fl Iran compaction and in ationary overpressures dominate smectite poor mud rocks offshore Brunei. The basal Overpressure detachment in deepwater Brunei generally lies below temperatures of about 150 C, where chemical Blow out processes and metagenesis are inferred to drive overpressure development. Overall the deepwater Halite Brunei system is very water rich, and multiple opportunities for overpressure generation and fluid Smectite leakage have occurred throughout the growth of the anticlines. The result is a wide variety of fluid Fluid-escape migration pathways and structures from deep to shallow levels (particularly mud dykes, sills, laccoliths, volcanoes and pipes, fluid escape pipes, crestal normal faults, thrust faults) and widespread inflationary- type overpressure. In the Central Basin the near surface environment is water limited. Mechanical and chemical compaction led to moderate overpressure development above the Upper Red Formation evaporites. Only below thick Early Miocene evaporites have near lithostatic overpressures developed in carbonates and marls affected by a wide range of overpressure mechanisms. Fluid leakage episodes across the evaporites have either been very few or absent in most areas. Locations where leakage can episodically occur (e.g. detaching thrusts, deep normal faults, salt welds) are sparse. However, in both Iran and Brunei crestal normal faults play an important role in the transmission of fluids in the upper regions of folds. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction understand that a very complex interplay exists in folds between fluids, stratigraphy, stages of fold development and fold style. Fluids In the early days of hydrocarbon exploration the identification of comprise three key properties: temperature, pressure and chemical surface anticlines associated with oil seeps, contains the implicit composition, each of which is controlled by a number of processes. understanding that anticlines are sites of focused fluid flow, where These properties can vary considerably even around a single fold some fluids are at least temporarily trapped, and others escape to (Evans and Fischer, 2012). Fluids exert an effect on rock strength, the surface. As reviewed by Evans and Fischer (2012), today we (by permitting pressure solution and other chemical reactions to occur, stiffening the rock by depositing cements, and reducing rock strength by overpressure) and impact structural style. Conversely * Corresponding author. Tel.: þ66 (0)22352464. the structural and stratigraphic permeability architecture in- E-mail address: [email protected] (C.K. Morley). fluences the location of fluids and fluid migration pathways (e.g.

Figure 1. Location maps for the study areas. A) Regional map, B) Central Iran, C) Brunei. 1 ¼ Silak Anticline, 2 ¼ Sarajeh Anticline, 3 ¼ Alborz Anticline, 4 ¼ Mil-Zangar Anticline, 5 ¼ Yazdan Anticline.

Sibson, 1996, 2005). These fluid pathways typically involved how fluids are distributed at a snapshot in time of fold and thrust downward percolation of meteoric waters through fractures, and development. For example the modern relationships between upwards movement of warm fluids (particularly along thrusts, e.g. folds, depth to overpressure, and structural evolution led Cobbold Barker et al., 2000; Beaudoin et al., 2011). Fluid filled zones, pre- et al. (2009) to suggest that the deepwater thrust front of the dominantly in fractures, are commonly transient features and evi- Niger Delta tracks the onset of the hydrocarbon maturation win- dence for their existence is likely to be lost if cements are not dow, and that hydrocarbon generation and chemical compaction, deposited. So, while understanding the development of folds by not burial-disequilbrium compaction, facilitates the detachment studying veins is very important and allows the sequential devel- development. opment of foldefluid relationships through time to be determined This paper compares the modern distribution of fluids, (e.g. Barbier et al., 2012; Beaudoin et al., 2012; Evans and Fischer, including hydrocarbons associated with two young (Late Miocenee 2012), it is also important to study active folds, and to understand Recent) regions of detachment and fault propagation fold-type 212 C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229 anticlines and considers the fluid development in terms of the and clusters of quartz form a pervasive network when burial rea- types and timing of overpressures developed with respect to burial ches temperatures around 80e85 (Thyberg et al., 2010). This during sedimentation and fold development. The depositional en- mechanism will retard further mechanical, but not chemical vironments are very different; one is a shale-prone deepwater fold compaction. and thrust belt, offshore Brunei; the other is a shallow marine to Diagenetically and catagenically induced changes can also semi-arid continental sequence of deposits (evaporites, clastics, weaken parts of the rock framework, which responds by com- and carbonates) in Central Iran (Fig. 1) (see Hall, 2013 and pacting (e.g. Meissner, 1981; Osborne and Swarbrick, 1997; Mouthereau et al., 2012 for reviews of the tectonic settings of these Swarbrick et al., 2002). Key changes include: solid kerogen hydro- areas). These represent end members of the types of depositional carbon conversion to liquids, gases and residuals; and grain-size environment in which folds grow and hence permit comparison of changes associated with the smectite to illite and kaolinite to a broad spectrum of the processes that influence fluid flow, and illite transformations. If fluid cannot escape, and the rock cannot fluid migration pathways as fluids are driven from regions of continue to compact, then the load is partially transferred to the relatively high pressure to lower pressure. Understanding the fluids (‘load transfer’ type disequilbrium compaction, Swarbrick mechanisms creating overpressure, and the timing of these et al., 2002). This process for the smectite-illite transition occurs mechanisms relative to anticline growth are very important to at temperatures starting between 60 and 80 C depending upon understanding the origin of the fluids and their availability/ability the carbonate phases present, and ends around 110 C(Hower et al., to influence structural development during deformation. 1976; Buller et al., 2005). Key permeability-reducing processes include the dissolution of smectite and re-precipitation as fibrous 2. Development of overpressures in basins illite that intensely subdivides existing pore space (Nadeau et al., 2002), and precipitation of microquartz (Thyberg and Jahren, Zones of overpressured fluids in sedimentary basins are gener- 2011). Kaolinite undergoes dissolution and precipitation as illite ally transient features that exist until some environmental change at temperatures around 130e140 C, but an extra source of potas- causes fracturing of the seals which initiates migration of the fluids sium (such as feldspar) is needed for the reaction (Bjorlykke, 1998). towards regions of lower pressure (e.g. Morley et al., 2008). As ‘Load transfer’ can result in fluid pressures that are 1500e3000 psi reviewed by Swarbrick et al. (2002) overpressures in basins are higher than pressures interpreted from shallow compaction trends caused by: 1) an increase in applied load (disequilibrium compac- (Lahann and Swarbrick, 2011). Goulty et al. (2013) have described tion), 2) changes in the volume of the pore fluid within a confined high-temperature overpressures from the Kutai Basin, eastern volume of sedimentary rock (fluid expansion), 3) fluid movement Borneo that fit with chemically-induced overpressures. or buoyancy related to density differences, and 4) redistribution of It should be noted that the load transfer mechanism in the overpressured fluids from one site in a basin to a different area smectite-illite transition is a different overpressuring mechanism (fluid transfer or inflationary overpressures). from that originally proposed for this transition, which is a volume Disequilibrium compaction encompasses a range of mecha- expansion mechanism. In the volume expansion mechanism nisms where overpressures arise from the failure of fluids to be interlayer water in the clay crystal is released during diagenesis as expelled rapidly enough in response to an applied load including: free water, which causes expansion of the volume of pore water increasing vertical stress during burial, lateral compression due to (Powers, 1967; Burst, 1969; Bruce, 1984). This mechanism is tectonic stress, and load transfer due to diagenetic changes. These thought to be a relatively minor factor in development of over- and other mechanisms are discussed below. pressure (Osborne and Swarbrick, 1997). The conversion of kerogen Burial of mud produces a number of important changes to its to oil and/or gas, and particularly cracking of oil to gas is regarded constituent minerals, which affect how overpressures are gener- as the most significant volume expansion mechanisms occurring in ated with increasing temperature and depth. Initially pressure is the subsurface (e.g. Osborne and Swarbrick, 1997; Tingay et al., the most important control on compaction (mechanical compac- 2013). tion). In basins where smectite is the dominant early-stage clay mineral, such as the Gulf of Mexico and the North Sea, the 60e90 C 3. Overpressure and fluid flow in folds and thrusts offshore temperature range marks the zone where the effects of mechanical Brunei compaction become less important and temperature-controlled chemical changes become significant or even dominant (see re- The latest Early Miocene- Holocene deltaic deposits offshore view in Nadeau, 2011). However, other important diagenetic Brunei formed a thick (>10 km) depocentre that lies both onshore changes can occur shallower. For example in biogenic, silica-rich and offshore, and extends into the deepwater area of the NW sediments shallow (typically about 300e600 m below the sea Borneo Trough. Uplift and rapid erosion of the interior of Borneo floor) diagenetic transformation of Opal A to Opal C/T can reduce provided the sediment source area (Hall and Nichols, 2002; Morley porosity in the order of 15e20% (Negeau et al., 2010), and trigger and Back, 2008). Deformation and uplift commenced as a conse- fluid loss and volume reduction mechanisms that lead to the cre- quence of the Dangerous Grounds continental crust entering and ation of polygonal faults (e.g. Cartwright, 2011). Likewise, in a jamming the Palaeogene to Early Miocene subduction zone of the megasulphate evaporite basin, the loss of structural water as a body Proto-South China Sea (see Hall, 2013 for a review of the tectonic of thick primary gypsum (CaSO4.2H2O) converts to anhydrite development). The post-collisional Miocene sand-prone shallow (CaSO4), via loss of structural water, can lead to overpressure at marine deposits are called the Belait Formation, while the Pliocene- depths of less than a kilometre (Jowett et al., 1993). Recent equivalent deposits are called the Liang Formation. Shale- Chemical processes change mineralogy, volume, increase shale prone sequences deposited more seaward on the shelf, or in density, cause dissolution of load-bearing grains, convert bound deepwater are known as the Setap Shale Formation (e.g. Sandal, water to mobile water, and increases in the preferred orientation of 1996). The basin that received these sediments along the NW clay mineral grains (Ho et al., 1999; Aplin et al., 2006; Day-Stirrat margin of Borneo is known as the Baram-Balabac Basin (Cullen, et al., 2008; Lahann and Swarbrick, 2011). Some chemical 2010). changes may strengthen the rock framework. For example the Deformation of the Baram-Balabac Basin comprises in part smectite-illite transition releases fine-grained quartz that helps to classic gravity-driven delta-style growth faults on the shelf and toe stiffen mudstone. 1e3 mm spherical, discrete grains or short chains folds in the deepwater area (Morley et al., 2008, Figs. 1C, 2). There is C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229 213

also a lithospheric stress component to the compressional deformation, which has resulted in: a) inversion of some of the growth- fault depocentres particularly onland and along the inner shelf, b) propagation of folds and thrusts under the deltaic deposits, and c) caused an additional component of shortening to the deepwater fold and thrust belt (Morley et al., 2003, 2008, 2011; King et al., 2009b; Sapin et al., 2013). The amount of extension in the shelf region is less than the shortening in the deepwater area (Hesse et al., 2008; King et al., 2009b). Consequently deformation in the deepwater fold and thrust belt is thought to be driven both by lithospheric stresses and by gravity (Morley et al., 2008, 2011). The

. exact cause of the lithospheric stress component is uncertain, one possibility is crustal-scale gravity collapse (Hall, 2011). This mixture of gravity and lithospheric stress-driven deformation is seen in the modern stress ﬁeld where the maximum horizontal stress is sub- orthogonal to the shoreline (appropriate for compression) along the inner shelf and deepwater area, and sub-parallel to the shelf (appropriate for extension) in the outer shelf-upper slope region (Tingay et al., 2009b; King et al., 2009a). McGilvery and Cook (2003)

and 3.1. General model for ﬂuid migration in Brunei

In deepwater basins the shallow sediments are very under- compacted, with up to 60e80% of the sediment volume comprising seawater trapped in pore spaces (see review by Day-Stirrat et al., 2010). From the deposition of turbidites and mass transport com-

Morley et al. (2011) plexes, to burial and deformation, structures associated with dewatering (e.g. ﬂame and dish structures, ﬂuid escape pipes, sand

ed from injectites, polygonal faults) occur at a variety of scales and are fi important features of the deepwater environment (e.g. Hurst et al., 2005; Cartwright et al., 2007). In addition to the vertical escape of fluids, a component maybe expelled oceanwards, laterally from the thicker section of the deltaic shelf (Van Rensbergen and Morley, 2000). An additional source of fluids may come from the under- ection data. Modi fl thrust Dangerous Grounds block. Although the setting is not as dynamic as a classic accretionary prism setting Zielinski et al. (2007) noted that the mean high heat flow (83 66.5 mW/m2) in the landward half of the deepwater fold and thrust belt compared with the seaward half (59 22.6 mW/m2; Fig. 3) followed a similar pattern to accretionary prisms, where fluid is expelled laterally (e.g. Barnes et al., 2010). In Brunei, overpressures arising from burial-induced disequilibrium compaction cause relatively low density and low sonic velocity responses on well logs in the zone of overpressure (Tingay et al., 2003, 2007, 2009a). The overpressure zones plot on the burial loading curve for normally pressured intervals on the porosity- effective stress cross-plot (Tingay et al., 2009a). These characteristics reflect the halting of mechanical compaction at some point along the burial curve, and the preservation of anomalously high porosity, for a particular depth of burial. The occurrence of burial-

Regional cross-section across Brunei based on seismic re induced disequilbrium compaction has been extensively documented for the shelf area of offshore Brunei (Tingay et al., 2009a). However, some overpressures in the shelf area, particularly shallow Figure 2. overpressures, are interpreted as caused by fluid expansion (infla- tion) because they have no large associated porosity anomaly, and plot above the burial loading curve on sonic velocity-effective stress cross plots (Fig. 4; Tingay et al., 2007). The overpressures migrate from regions of primary overpressure development via carrier beds, faults, fractures or fluid pipes and are forced into a confined pore volume laterally or higher in the section. In the preceding “Development of Overpressures” section several key chemical reactions are related to the transformation of smectite to illite. The chemical composition of shales in NW Borneo is not well documented, but the data that exist indicate smectite is a minor proportion of the clay minerals. Hence the smectite-illite 214 C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229

Figure 3. Results of bottom sampling of the deepwater area of Brunei. A) Heat flow, B) Hydrocarbon content from piston cores. Data from Zielinski et al. (2007), modified from Morley et al. (2011). transition is not regarded as a significant factor in overpressure Figure 6 shows a plot of the depth and temperature at top of generation in the basin. In surface outcrops of the Jerudong area, overpressure in Brunei. The top of overpressure for many wells Middle Miocene Setap Shale samples from both the country rock, occurs at relatively low temperatures (40e60), at depths between and from shale dykes (e.g. Morley et al., 1998; Morley, 2003)are 1000 m and 2300 m. In some cases, for example in the Mergani-1, composed of about 40e45% kaolinite, 35e50% illite and 10e20% Laksamana-1 and Merpati-1 wells on the outer shelf (Tingay et al., mixed layer smectite-illite (Fig. 5). Recently deposited clay minerals 2009a), the top of overpressure is related to burial-induced sampled from the Malay Peninsula, North Borneo and South Borneo disequilibrium compaction. Conversely, the top of overpressure in are predominantly kaolinite and illite þ chlorite, with less than 14% wells on the inner shelf is related to inflationary overpressure (e.g. smectite (Wang et al., 2011; Liu et al., 2012). These data suggest the Peragam-1, and Bugan-1 location a, Fig. 6), and the Bugan-1 well low percentage of smectite in the Jerudong samples is a charac- also shows a deeper onset of disequilibrium compaction over- teristic feature of weathering in the region and not because pressures (Fig. 6, location b). smectite has been converted to illite. In the absence of a smectite- The top of 6 overpressure points plot in the temperature field rich mud rocks, the onset of significant overpressures due to (>60 C) where mixed chemical and mechanical compaction chemical effects is unlikely to occur in the 60e90 C range dis- operate (if the smectite content of the mud rocks is high; Fig. 6 A). cussed by Nadeau (2011) and shown for illustrative purposes in However, there is no strong support in Figure 6A for temperature Figure 6. Kaolinite to illite transformation can trigger load transfer control of overpressure development following the predictions of overpressure, and will also release bound water (about 8% of the Nadeau (2010). A comprehensive analysis of wells throughout the rock volume for 40% kaolinite). However, this reaction is very slow below temperatures in the range of 120e140 C(Bjorlykke, 1998).

Figure 4. Sonic velocity-effective stress plot for 1400 repeat formation tests from 31 fields across Brunei. Grey dots and normally pressured points that define the loading curve. Black triangles are overpressured points (>11.5 MPa/km). Overpressured points lie both on and off the loading curve. Points that plot on the loading curve likely represent disequilibrium compaction, while points that lie off the loading curve are Figure 5. Clay composition of outcrop samples of Setap Shale, Brunei Darussalam. The likely to be the result of fluid expansion or vertical transfer. Modified from Morley et al. samples are from a mud diapir, intrusive shales, and country rock, in the Jerudong area (2008). (described by Morley et al., 1998; Morley, 2003). C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229 215

Figure 6. A) Plots of the depth to the top overpressure in various areas of the Brunei shelf and slope. B) Location map for wells named in A), map shows depth to top overpressure for offshore Brunei (modified from Morley et al., 2008). B ¼ Bugan-1, L ¼ Laksamana-1, M ¼ Merangi-1, Me ¼ Merpati-1, ML ¼ Maharaja Leyla-1, P ¼ Peragam-1 and S ¼ Seria-1. region clearly indicates that the depth of onset of overpressure is fracture. A very large loss of porosity occurs at shallow depths, and dependent on a range of variables, not just temperature and pres- by about 1.5 km depth the initial porosity (initially about 60e70% sure. For example the structural/stratigraphic location of a well is for deepwater shales) is reduced by half (e.g. Dawson and Almon, important, particularly in growth faults where drilling the depo- 2006). However, for overpressured shales the porosity loss will be centre (e.g. Maharaja Leyla-1, Fig. 6A) demonstrates the occurrence arrested at some point in the burial history and fluid expulsion will of a very thick, well-drained sequence of alternating sands and slow, or cease (fluid isolation depth), and overpressures begin to shales. Consequently the maximum depth to the top of over- build. pressure exceeds 4 km. In contrast on the outer shelf, overpressure Nadeau (2010) suggested that thermo-chemical reactions could in shale prone-sequences occurs much shallower in the section (1e explain the overpressure origins in the Baram-Balabac Basin. 1.5 km; Figs. 2 and 6A,B). The top of overpressure on the inner shelf However, in Brunei the temperatures at the top of disequilibrium is commonly associated with an inflationary mechanism, while the compaction overpressure can be as low as 40 C, as well as greater top of overpressure on the outer shelf is related to disequilibrium than 80 C(Fig. 6). The low-temperatures to the top of overpressure compaction (Tingay et al., 2007, Fig. 6B). This boundary coincides and smectite-poor mud-rocks clearly indicate that mechanical, not with the occurrence of Miocene-Pliocene inversion structures on thermo-chemical processes are driving overpressures in the basin. the inner shelf, and the absence of inversion on the outer shelf In an extensional stress environment hydraulic fracturing and a (Tingay et al., 2009a,b). Hence, there is the strong indication that pulse of fluid expulsion will occur when the pore fluid pressure inversion is promoting the migration of fluids upwards in the exceeds the minimum horizontal stress plus the tensile strength of section. Those fluids include hydrocarbons; the largest fields in the rock. Repeated pulses of burial, pressure build up, and frac- Brunei are associated with inversion anticlines along the shoreline turing will occur until the pore fluids are depleted. For offshore and inner shelf. The high overpressures encountered in the 1953 Brunei further burial causes increased overpressures and triggers Seria, 1974, 1979 Champion Field internal blowouts (Tingay et al., another cycle of fluid expulsion through chemical diagenesis and 2005) are associated with inversion anticlines. catagenesis (hydrocarbons). Chemical diagenesis includes kaolinite Figure 7 schematically shows how three main pulses of fluid to illite and a relatively minor smectite to illite transformation. In migration may have developed in the MioceneePliocene deep- the Golden Zone model the upper temperatures of this deeper zone water shales of Brunei. Initially expulsion of fluids under generally tends to be around 120 C (e.g. Nadeau, 2010). However, for hydrostatic conditions occurs during compaction. But, even in the offshore Brunei the low proportion of smectite, and high kaolinite shallow environment, local overpressures may develop for a variety content of the shales suggests that this zone begins and ends at of reasons including: a) local development of a low porosity zone higher temperatures than those typically used in the Golden Zone (perhaps by shallow cement development, or the presence of a gas model. Additionally load transfer-type disequilibrium compaction hydrate seal), b) rapid loading by mass transport deposits, and c) mechanisms are less important than volume expansion mecha- inflationary overpressures transported along a pipe, fault, or nisms related to hydrocarbons. 216 C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229

Another likely contributor to overpressures in the deepwater fold and thrust belt is tectonic (horizontal)-related disequilibrium compaction, which can produce a significant amount (up to 26%) of early ‘ductile’ shortening (e.g. Butler and Paton, 2010). Quantifying the contribution of this mechanism in mud rocks is difficult since compaction is not controlled only by the vertical effective stress, and typically tectonically-induced overpressures have no measur- able impact on compaction (e.g. Yassir and Addis, 2002). Tectonic loading can generate near lithostatic overpressure since in the general case the minimum principal stress is vertical. Qualification of this statement is necessary because the local stress state around folds can vary significantly from extensional through strike-slip to compressional. Overpressures in shales are developed (e.g. Cobbold et al., 2009), since widespread overpressure is the main facilitator of deformation. Hence extensional growth faults, and down-dip folds and thrusts will accelerate fluid depletion from the zone of disequilibrium compaction overpressures, into adjacent forma- tions, and ultimately to the surface (e.g. Morley et al., 2008).

3.2. Fluid migration along the basal detachment

Figure 2 shows a cross-section passing from the deepwater fold and thrust belt to the shelf counter-regional extensional fault systems. The approximate locations of the 60 C, 100 C and 150 C geotherms have been plotted. As discussed in Section 2, the interval between the 60 C and 100 C geotherms typically marks the transition where mechanical compaction starts to be replaced by chemical processes (although this is to a lesser degree than smectite-rich provinces). The detachment to the outer-most folds of the deepwater fold and thrust belt lies within the zone where overpressures are related to mechanical compaction-induced disequilibrium compaction, whereas the majority of the folds and growth faults detach within a zone with temperatures above 150 C that is now well within the zone of load transfer-induced

Figure 7. Schematic illustration of the general kinds of overpressure events, and ﬂuid disequilibrium compaction and volume-expansion mechanisms. expulsion events that would occur within the Setap Shales with increasing burial Although deltaic detachments are commonly said to be associated (indicated by temperature) for the Baram Delta area, Brunei. Temperature trends with mobile shales, the modern temperatures appear to be far to derived from Sandal (1996), Zielinski et al. (2007), and Laird and Morley (2011). high for the shales to be mobile in response to loads in the same

Figure 8. Schematic block diagram showing typical features associated with growing deepwater folds (modified from Laird and Morley, 2011). C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229 217 way that salt is mobile. The shales are likely to be well compacted, (Zielinski et al., 2007) that affects anticline VI (Fig. 3; anticline but weak due to lithology, high overpressures, and the develop- labelling from Morley, 2009). The megaseep has a maximum heat ment of low-friction polished (graphitic) slip surfaces (for the po- flow of 604 mWm2, which Zielinski et al. (2007) calculated requires tential importance of the latter see Rutter et al., 2013). The low that the fluids originated from around 6 km depth. If this source critical taper of the fold and thrust belt wedge indicates the pres- were located vertically beneath the anticline it would lie below the ence of near-lithostatic pore fluid pressures along the basal detachment. However, detachments are thought to act as barriers detachment (Morley, 2007a). to vertical fluid migration (e.g. Wawrzynice et al., 2003). Conse- As is well documented for accretionary prisms, fluids are likely quently it is more likely that the source is from within or above the to be pumped from the inner part of the wedge along the basal detachment zone, about 20 km to the SE of the surface flow (Fig. 2). detachment to the more external parts of the fold and thrust belt The distribution of hydrocarbons in the deepwater area falls into (e.g. Fisher and Hounslow, 1990; Moore et al., 1995; Ruppel and an oceanward zone where hydrocarbon seeps are very rare (2% of Kinoshita, 2000). An example of this plumbing is the megaseep all sites) and only one site showed a heat flow greater than

Figure 9. Seismic examples of folds, deepwater Brunei. A) Line drawing, and B) seismic line showing the difference in development between a BSR (bottom simulating reﬂection) developed on a simple anticline, and one affected by a mud pipe and degradation complex. The BSR over the younger, simpler anticline to the east is deeper than the BSR over the complex anticline. 1 ¼ Top (?) of gas hydrates appears to act as a shallow detachment for shallow minor normal faults. 2 ¼ BSR crossing mud pipe is closer to the surface than at 1, suggesting the mud pipe is episodically still active and warmer than the surrounding sediments. HA, HB, HC are locations of regionally mapped horizons(Morley, 2009). TWTT ¼ two way travel time. C) and D) details of BSR in anticline affected by crestal normal faults. C) Typical amplitude display. D) Spectral colour palette, which highlights amplitude variations along the BSR and underlying bright spots. Double BSR is evident as an alignment of bright spots parallelling and underlying the BSR. 218 C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229

99 mWm2, and a more landward zone where 16% of sites showed along the detachment so that the outer fold zone did not receive abundant oil-associated hydrocarbons and 20% of sites exhibited these fluids in abundance (Fig. 2). Temperatures beneath the outer high heat flows (Zielinski et al., 2007). Thermogenic gas is only folds probably permitted some generation of hydrocarbons, but present in the landward zone, but sites with mixed biogenic and volumes appear to have been relatively low. thermogenic gases are present in both zones (Warren et al., 2011). Warm active chimneys of fluids associated with thermogenic or- 3.3. Fluid migration within anticlines ganics are also present, (Warren et al., 2011). The high- displacement thrust associated with the Anticline VI and the The crests of anticlines are the confluence of fluid migration megaseep appears to have diverted the migration of hot fluids from depths of the basal detachment at around 6 km to more local

Figure 10. Stratigraphic column for the Central Basin, Iran (Morley et al., 2013). C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229 219

fluid migration in the upper 1 km. Fluids migrating from depth variations in shallow geothermal gradient, which for some anti- along faults commonly depart from the fault trace and rise as clines in the deepwater of Brunei range from 4.8 C to 6.4C/100 m vertical pipes through the crests of anticlines (Figs. 8 and 9), and (Laird and Morley, 2011). These unusually high gradients are may form a chain of mud volcanoes at the sea floor. interpreted to reflect the migration of hot fluids from depth into the As anticlines grow on the sea floor they are modified by gravity, anticlines (Laird and Morley, 2011). Detailed studies of shallow fluid material may slump from the forelimbs and crests of anticlines flow around gas-hydrates in deepwater folds shows fluid migration when they become too steep, and/or gravity-driven crestal normal occurs through existing conduits such as faults, and feeder beds faults develop (Morley, 2007b, 2009). The faults can be local until they reach a depth where fluids are able to fracture their way pathways for fluid migration in about the upper 1 km of section vertically to the surface (e.g. Morley, 2009; Sultan et al., 2011, Fig. 8). (Figs. 2 and 8), and since they dip away from the fault crest, can help feed gas towards the anticline crests to help form gas hydrate 4. Folds in central Iran layers. In the shallow deepwater subsurface carbonate cements are commonly developed in response to microbial degradation of The Central Basin of Iran lies in the upper plate of the Zagros methane, and gas hydrates can be associated with authigenic car- Mountains Collision Orogen, which formed during the Late bonate cements (Dela Pierre et al., 2010; Warren et al., 2011) Palaeogene-Neogene as a consequence Arabia-Eurasia collision Bottom simulating reflections (BSR) related to gas hydrates are (e.g. Morley et al., 2009; Mouthereau et al., 2012). The Central Basin common features of anticlines in the deepwater of Brunei (Laird of Iran displays many folds developed within the plateau region of and Morley, 2011, Fig. 9). Bright spots beneath the BSR show a an orogenic belt, in a large basin influenced by normal faults that positive AVO response and are related to free gas trapped by the gas was inverted in the Late Miocene to Pliocene (Morley et al., 2009). hydrate saturated layer (Laird and Morley, 2011, Fig. 9C). These gas The basin stratigraphy comprises a basal evaporite (halite-domi- accumulations occur mostly on the backlimbs of fault propagation nant) and red bed unit (Lower Red Bed Formation) of Oligocene age, folds, reflecting the effects fold asymmetry that focus fluid migra- which overlies Eocene volcanics (Fig. 10; Furrer and Sonder, 1955). tion onto the back limb. Differing depths to the BSR reflect The Lower Red Bed Formation is succeeded by a 1e2 km thick, Late

Figure 11. Location map for the Central Basin in the Qom area, Iran. A6, A5, A14 ¼ Alborz wells, S1 ¼ Sarajeh-1. 220 C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229

Oligocene to Early Miocene marine sequence of limestones, sandy subsurface after deposition. This has resulted in the Miocene- Up- limestones, marls with occasional black shale and anhydrite layers, per Red Formation being a compacted, dense formation even very called the Qom Formation. The sequence is capped by the Miocene shallow in the section. In the Silak-1 well from near surface to Upper Red Formation, the lowest section comprises halite- 500 m depth, bulk density increases from 2.1 g/cc to 2.4 g/cc; the dominated evaporites a few hundred metres thick, capped by red average density increases to 2.48 g/cc below 1500 m, and increases bed clastics. The thickness of the formation varies greatly, but can slowly to over 2.5 g/cc until about 4000 m. be up to 6 km (Morley et al., 2009). The composition of the Upper Red Formation is dominantly In the Central Basin the source of the ﬂuids is either meteoric, or volcanic lithics, with signiﬁcant contributions from carbonate formation waters. The Miocene to Present Day climate is semi-arid, lithics, opaque minerals, and feldspar. Cements are dominated by and consequently the continental sediments do not contain a high zeolites (including analcime), calcite and iron. With burial augite, percentage of low-salinity meteoric water in the shallow biotite and chlorite are replaced by hematite, and the Fe cement

Figure 12. Seismic line across the Silak Anticline (see Fig. 11 for location). A) Uninterpreted line, B) interpreted line. Hydrocarbons and overpressured fluids leak through weld in evaporites (I) and up a normal fault (II). C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229 221 content increases. This reflects a sediment source predominantly from the Eocene volcanic arc sequence (including interbedded carbonates, clastics and evaporites), which forms an extensive belt in central Iran. A study on the diagenesis of two clay-rich sections in the Upper Red Formation in central Iran was conducted by Amini (2001), the following description is based on their work. Early weathering reactions involve aluminosilicate glass þ water transforming to smectite þ alkaline solutions. The alkaline solutions can develop in three ways: a) (evaporative concentration) saline fluids þ zeolites, b) evaporative concentrations þ CO2 ¼ a) above þ calcite, and c) (drainage) e which leaves a smectite-dominated assemblage. In terrestrial environments with good drainage or relatively high precipitation zeolites would be lost, while in arid conditions pore water saturation and evaporite-derived solutions favour zeolite growth. Amini (2001) found that the URF contained stable smectite-dominated assemblages between depths of 400 and 1900 m. In parts of the basin analcime and other zeolites helped buffer high pH pore fluids and stabilize K-smectite. In one thick section in the northern margin of the URF basin, starting around 1900 m depth, smectite was replaced by discrete illite and chlorite and by 4900 m smectite was virtually eliminated. Four anticlines Aran, Silak, Sarajeh and Alborz (Fig.11) have been drilled during hydrocarbon exploration. Three of them (Silak, Sar- ajeh, Alborz) are capped by Upper Red Formation halite seals, while halite is absent in Aran anticline and only thin metre-scale anhy- drites are present. The folds are detachment anticlines, which have developed on thick Lower Red Formation evaporites (Morley et al., 2009, 2013, Fig. 12). The Silak Anticline was drilled by Silak-1 during 2008 and 2009, and is the latest exploration well in the basin. The well encountered drilling problems related to flowing salt, overpressures, and highly fractured zones within the Qom Formation. Figure 12 is a dip 2D seismic line across the Silak Anticline that shows the fold detaches in the Lower Red Formation, above a thick, relatively strong Eocene volcanic-dominated sequence. The anticline was probably localized by the presence of a normal fault. Displacement on the normal fault is relatively low in the seismic section and increases in displacement passing eastwards. The fold Figure 13. Mud weightedepth plots for the Alborz-6, Alborz-14 and Aran-1 wells. was probably caused by the buttressing effect of the normal fault, Subtle bend in hydrostatic pressure gradient is due to much higher salinity and density but instead of inversion occurring along the normal fault, the formation waters associated with the Upper Red Formation evaporites. detachment fold style developed in the hanging wall (further east inversion does occur along the normal fault). The URF shows a local synclinal depocentre is developed in the lower part of the forma- overpressuring (Aran-1, Silak-1). The distribution and connectivity tion on what is now the northern limb of the fold. This depocentre of sands within the Upper Red Formation is highly variable laterally is interpreted to have resulted from local loading of the URF and vertically, which affects drainage and overpressure develop- evaporites and downbuilding during localized salt withdrawal. ment. The absence of associated porosity anomalies or density re- Subsequently the anticline began growing and the upper part of the versals in overpressure zones indicates burial-type disequilibrium URF shows thinning towards the fold crest. A number of normal compaction is an insignificant contributor to overpressure devel- faults, with convergent dips affect the crest of the anticline. opment. As discussed above, chemical reactions are very important within the formation and smectite to illite transformation is a very 4.1. Overpressure development significant factor starting at around 1900 m depth (Amini et al., 2011). Geothermal gradients are moderate w2.5e3C/100 m indi- The pressure gradients in wells down to the top of the Upper cating temperatures at this depth are around 70e80 C, assuming a Red evaporites ranges from hydrostatic to moderately over- 20 C surface temperature. In Aran-1 the onset of mild overpressure pressured (Fig. 13). The evaporites are mostly pure halite, but in- around 2000 m indicates load transfer disequilibrium compaction terbeds of anhydrite and clastics, mostly claystones, also occur. A arising from the smectite-illite transition would be a viable 10 m thick, overpressured, red, sticky clay zone within the evapo- mechanism. rites associated with a thrust presented a particular problem to In the Alborz Anticline a thrust in the Upper Red Formation drilling the Alborz Anticline (Abaie et al., 1963). The presence of detaches within the evaporites and has transmitted overpressured poorly lithified clays indicates that very locally disequilibrium fluids, including oil along the fault zone, both within the evaporites, compaction was operating due to the sealing potential of halite. and higher in the Upper Red Formation (Abaie et al., 1963). The only source rock intervals lie within the Qom Formation, hence despite 4.1.1. Upper Red Formation overpressures the evaporite seal, the presence of the thrust and/or hydraulic The Upper Red Formation above the evaporite unit can be nor- fracturing has enabled fluids to leak from the Qom Formation. Not mally pressured (Alborz wells) or exhibit some degree of surprisingly the anticline is under-filled with hydrocarbons. 222 C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229

In Silak-1 there is no obvious Alborz Anticline type-thrust on 4.1.2. Overpressures in the Qom Formation seismic reflection data, but gas and oil have still managed to leak Despite potential breaches to the Upper Red evaporite seal, very into the Upper Red Formation, where they were encountered in high overpressures occur within the Qom Formation in the Alborz, convergent, conjugate extensional faults at the crest of the struc- Sarajeh and Silak anticlines (Figs. 13 and 14). These overpressures ture (Fig. 14). The crestal normal fault zone coincides with flows of are separated from lower-magnitude overpressures in the Upper overpressured fluids into the wellbore between depths of 1450e Red Formation by the intervening evaporites, which can reach 2050 m depth (Fig. 14). The fluid is rich in calcium (w1000 ppm) thicknesses up to about 400 m. The most spectacular demonstra- and chlorine (w1000 ppm) and also sulphate, which caused tion of the high overpressures is the 1956 Alborz-5 well, which considerable expense since a large quantity of soda ash had to be exited the Upper Red evaporites by only a few centimetres and blew added to the drilling mud to neutralize the calcium; drilling rates out at a depth of around 2677 m. The mud column of 2.07 g/cc slowed. The origin of the calcium could be from diagenetic pro- (19 ppg) density was ejected and the well then initially ‘produced’ cesses within the Upper Red Formation. Alternatively, the fluids are about 80,000 barrels of oil a day, plus an unknown quantity of deeper CaCl2-basinal brines derived from within the lower part of water and gas for 25 days, the well was brought under partial the thick evaporites and associated hydrothermal/volcanic waters control and finally self sealed 80 days after the blow out (Abaie near the base of the sequence. According to Abaie et al. (1963) et al., 1963). In total an estimated 5 million barrels of oil was typical Qom Formation water samples are highly saline, with a expelled during the blow out. The formation pressure at the top specific gravity at 20 C of 1.21 (similar to the Dead Sea brines), and Qom Formation is estimated to be > 8000 psi (Gretener, 1982). are rich in calcium (25,000 ppm), sulphate (900 ppm) and chloride Three other wells (Alborz-9, 10 and 11) each produced about 30,000 (207,000 ppm). If the fluid in the crestal normal fault is partly barrels per day under less spectacular circumstances. Alborz-6 was derived from the Qom Formation then it has been considerably drilled 2.6 km away from Alborz-5, below the oil/water contact. The modified during migration by either chemical reactions and/or well encountered high pressures (w2.18 g/cc) at the top Qom For- mixing with fresher water (perhaps meteoric water migrating mation, indicating that there was no pressure relief from the down the crestal normal faults). Alborz-5 well, which blew out one year earlier. In Silak-1 overpressure begins shallow (w1600 m) and is associ- An intense blow-out also occurred when the first well (Sar- ated with crestal normal faults (Fig.14). There is no density reversal on ajeh-1) was drilled into the Sarajeh Anticline in 1958. The well well logs, and the overpressures are probably at least partially infla- only had surface casing at the time of the blow-out. Fractures and tionary as indicated by the presence of oil, which must have originated gas seeps occurred at the surface up to 3 km westwards, 1.5 km from the Qom Formation. Around 3300e3600 m the overpressure eastwards, 7.5 km northwards and 0.5 southwards of the well. magnitude decreases in a more sand-prone, and hence better-drained The subsurface conditions at the time of the Sarajeh-1 blow-out interval. However, a shallower decrease in overpressure from about are unknown, and subsequent wells drilled into the anticline 1.8 specific gravity (SG) to 1.45 SG around 2800e3000 m occurs within have not encountered high overpressures, indicating the blow- an interval of uniformly high density. Possibly the presence of faults is out depleted the pressures over a wider area than the Alborz responsible for this decline in overpressure (Fig. 14B). Anticline.

Figure 14. A) Formation pressureedepth plot for Silak-1. B) Cross-section through the Silak-1 drilling location based on 2D seismic data (see Fig. 12 for example), showing the geological context of plot A. See Fig. 11 for location. Subtle bend in hydrostatic pressure gradient is due to much higher salinity and density formation waters associated with the Upper Red Formation evaporites. C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229 223

The Qom Formation is a poor conventional reservoir, the upper high-angle to bedding (Fig. 15A). Hence a dense network of part is a marl, the lower section limestones and sandy limestones fractures is needed to connect any remaining porosity. The de- interbedded with marls (Fig. 10), and effective porosity rarely livery of high volumes of hydrocarbons along fractures is amply exceeds 6e8%. The limestone intervals are crossed extensively by demonstrated by Alborz-5. The types of fracture networks bed-parallel stylolites, and also tectonic stylolites that lie at a required to produce the performance seen at Alborz-5 are present

Figure 15. Examples of fracture networks at different scales within the Qom Formation. A) Satellite image of the Mil-Zangar Anticline in the Qom Formation, thrust over Upper Red Formation. B) Detail of image A) (see A for location) showing conjugate fault network. C) Detail of image B (see B for location) showing very closely spaced network of faults spaced in the order of 10’s metres. D) Limestone breccia within fault zone, E) Main fracture types affecting the Qom Formation on sub-vertical bedding surface, conjugate fractures, tectonic stylolites (bedding parallel stylolites are also present) and fault zone (D is a detailed picture of breccia in the fault zone). F) Widespread, intense Type 1 fracturing on a sub-vertical bedding surface. 224 C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229

Formation section was buried under the 400 m thickness of halite- dominant evaporites. During this period, fluids within the formation would have been trapped and isolated from those above. About 50% of the volume of the formation would have followed the carbonate diagenetic pathway of porosity reduction (e.g. precipitation of carbonates in pore spaces, recrystallization, compaction by pressure solution), whereas about 50% of the formation volume would have followed the clastic pathway, with mechanical compaction followed by later diagenetic porosity loss. Today the marls are highly indurated, and are stiff enough to contain the fracture network that fed the Alborz-5 blow out. The Upper Red evaporite seal is very important to developing and preserving overpressures. The Aran anticline is missing the Upper Red halite-dominant evaporite seal, and the Aran-1 well encountered moderate overpressures in the Qom Formation, but much less than the Alborz wells (Fig. 13). The other three drilled anticlines in the area (Alborz, Sarajeh, Silak) are all sealed by thick sections of the Upper Red halite-dominant evaporites and exhibit high overpressures (Figs. 13 and 14). There are two likely causes of the overpressure operating at present: 1) hydrocarbon generation, and cracking of oil to gas (see Tingay et al., 2013 for an example of this mechanism), and 2) tectonic-disequilibrium compaction arising from shortening and compression of the rock during Late Miocene-Pliocene folding and thrusting. The abundant orthogonal/ high-angle to bedding stylolites present in the Qom Formation are evidence for this process (Fig. 15E). Figure 16 summarizes the likely development of overpressures in the Qom Formation. Prior to deposition of the Upper Red Evaporites the >1 km thick Qom Formation would have undergone fluid loss due to compaction, but following evaporite deposition the fluids in the Qom Formation were sealed off and probably could not escape. Overpressures, particularly in the smectite-rich marls would have been developed due to burial-disequilibrium compaction and later due to load-transfer. The underlying Lower Red Formation under- went a similar diagenetic development to the Upper Red Formation, and during the smectite-illite transition probably became overpressured, lost fluid into the Qom Formation and caused some vertical transfer of overpressure. During the Late Miocene folding, Figure 16. Summary of the likely development of overpressures for marls and shales source intervals within the Qom Formation approached maximum within the Qom Formation. a ¼ fluid loss from early compaction, fluid loss is halted by deposition of Upper Red Formation evaporites; b ¼ fluid loss by breach of Upper Red burial and oil and gas generation and gas cracking would have also Evaporites (by hydraulic fracturing, development of a salt weld, movement along contributed to the overpressures (Morley et al., 2013). faults). It is uncertain whether hydraulic fractures were able to temporarily breach the salt seal during periods of extreme over- in the very extensive outcrops of the Zangar-Mil Anticline, which pressure. For example, one explanation why the Sarajeh and Alborz is the next fold south of the Alborz Anticline (Fig. 15B). Fracture Anticlines are underfilled with hydrocarbons is that a phase of networks within the Qom Formation comprises closely spaced hydraulic fracturing naturally depleted the reservoirs, and the salt fractures of mostly Type-1 (Stearns and Friedman, 1972) orien- seal subsequent annealed itself. Such a system has operated in the tations with respect to folds (Fig. 15, E, F). There are also closely Ara Salt in the South Oman Salt Basin, where breached halokinetic spaced (w10e100 m) arrays of small, extensional conjugate faults, salt is indicated by bitumen-defined polyhedral edges to biaxially- clearly seen on satellite images (Fig. 15AeC) that form brecciated flattened halite crystals and is locally known as “black salt” (Kukla zones 1e2mwide(Fig. 15D and E). Some of these faults appear to et al., 2011). have been later reactivated as strike-slip faults. The characteristics What is clear in central Iran is that at some stage in the burial of the fault breccias in the subsurface are not know, but may well history oil has leaked in places into the Upper Red Formation, in the be areas of enhanced porosity and important areas for storage of case of the Alborz anticline leakage was via a thrust (Fig. 17), while hydrocarbons. in Sialk-1 it was probably via a salt weld in the local area where a The limestones within the Qom Formation exhibit very low downbuilding syncline is developed in the lower Upper Red For- porosities and permeabilities, further reduced by the development mation (Fig. 12). The final stage in the overpressure history is pore of intersecting tectonic and burial stylolites. There is no porosity space reduction due to tectonic stylolite development during Late anomaly or density reversal associated with the limestones or the Miocene-Pliocene folding and thrusting. marls of the Qom Formation. Most limestones do not have the same mechanical compaction history as typical clastics, and are not 4.2. Vergence and timing of fluid migration in the Alborz and amenable to the pore pressure prediction techniques used for Sarajeh anticlines clastic sequences. However, the interbedded marls would have exhibited a more typically clastic response. The defining time The Alborz and Sarajeh anticlines, lie en echelon to each other during burial occurred when the approximately 1 km thick Qom and the Qom Formation is present at similar depths in both C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229 225 structures. Superficially there would be every reason to expect (compressional) ¼ maximum horizontal stress sub-parallel to the similar fluid histories for each anticline, yet the Alborz anticline has fold axis). These patterns have been widely recognized in folds, trapped oil, while the Sarajeh anticline has trapped predominantly although additional complexities can be identified (e.g. Stephenson gas and condensate (Abaie et al., 1963). Hence the Sarajeh anticline et al., 2007), and fracture patterns tend to evolve through time and trapped fluids formed at higher temperatures, which migrated later become superimposed, or reactivated with different sense of motion than those in the Alborz anticline. Both anticlines are underfilled by (e.g. Brita et al., 2006; Ahmadhadi et al., 2008). These fractures are hydrocarbons, which indicates that fluid leak off is probably part of important for fluid flow, and particularly in carbonates become the history. Morley et al. (2013) presented a structural and partially or completely cemented with time. geochemical study that concluded the SW dip of the detachment Crestal normal faults are higher displacement features that tend which was also a key to understand the trapping behaviour. The to correspond with Type 2 conjugate fractures both in orientation Sarajeh anticline is an SW verging structure while the Alborz and an outer arc location. However, crestal normal faults have more anticline is a NE verging structure (Morley et al., 2013). This dif- diverse origins than Type 2 fractures. Some, such as the convergent ference in vergence was sufficient for the Alborz anticline to close crestal normal faults of the Silak Anticline (Fig. 12) can be explained relatively early and trap oil, while the Sarajeh anticline was closer to as arising from stresses related to folding (e.g. outer arc bending the main source basin depocentre, attained closure later than the stresses, or a component of strike-slip motion; e.g. Ramsay, 1967; Alborz anticline, and so trapped gas. Greater downbuilding of the Strayer et al., 2004)). Other arrays of crestal normal faults are salt depocentre south of the Sarajeh Anticline enabled the Qom caused by collapse related to a salt or shale diapir, or, in the case of Formation source rock to be buried into the gas window, and also deepwater Brunei, as gravity-driven features related to sediment influenced the SW vergence direction of the Sarajeh Anticline. This instability on the flanks of growing anticlines (see review in Morley, example highlights how sometimes minor changes in fold charac- 2007a.b). teristics can exert an important control on fluid distribution. While In one SE Asia deepwater fold and thrust belt crestal normal vergence is argued to be a significant factor, loss of hydrocarbons by fault are modified polygonal faults. The polygonal geometry rep- leakage along thrusts, normal faults, salt welds, and/or by hydraulic resents deformation in an isotropic stress field, probably in fracturing is clearly another important mechanism operating in the response to diagenetically-driven shear (Shin et al., 2008). Figure 18 basin and is likely to have contributed to the underfilled nature of shows that approaching the deformation front of the fold and some traps. thrust belt the polygonal faults respond to the stress field creating the folds and thrusts, and cease to be polygonal, grading into faults 5. Comparison of crestal normal faults in different fold sub-orthogonal to the maximum horizontal stress direction. As settings folds propagate into this stress front they incorporate the strongly oriented normal faults of polygonal fault origin (Fig. 18). These According to Stearns and Friedman (1972) conjugate and exten- faults lie at a high angle to the fold axis and impose permeability on sional fracture patterns form in three main orientations controlled by the folds of very different orientation from typical normal faults local stress orientations within folds (Type 1 ¼ maximum horizontal that lie sub-parallel to the fold axis. They are also a novel stress stress sub-orthogonal to the fold axis; Types 2 (extensional) and 3 orientation indictor.

Figure 17. Cross-sections based on 2D seismic lines. A) Alborz Anticline, B) Detailed section of the Alborz Anticline showing wells and location of leaking hydrocarbons. C) Sarajeh Anticline. (Modiﬁed from Morley et al., 2013). See Fig. 11 for location. 226 C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229

Figure 18. Coherency time slice (about 1 s two way travel time below the sea ﬂoor) showing a deepwater fold and thrust belt impinging polygonal faults. A) Overview map, B) detail of NW corner of map A. The minor normal faults (dark lineations) that affect the folds lie at a high angle to the NeS to NNWeSSE trending fold axes. They might be interpreted as typical crestal normal faults, but for their foreland-wards transition into polygonal faults. They are interpreted as related to polygonal faults, but they have developed in an anisotropic, not isotropic stress ﬁeld, related to the stresses associated with the fold and thrust belt. a ¼ area of polygonal faults and b area where aligned faults of ‘polygonal type’ are present. I, II, III, IV are anticlines.

Crestal normal fault in Brunei and Iran have played an important basinal fluids from >4 km depth. These fluids could have leaked role in the transmission of fluids. In Iran the crestal normal faults across breaches in the Upper Red evaporite seal either at welds have a convergent conjugate geometry (i.e. the faults dip towards associated with downbuilding structures or at deep normal faults each other) and fault sets extend to 3 km deep (Fig. 12), while in (Fig. 12). Brunei the faults are divergent and tend to be confined to the upper 1 km. Consequently the potential for crestal faults in Iran to 6. Discussion and conclusions transport deeper fluids is greater than those in Brunei. The presence of oil, gas, Ca, Cl erich overpressured fluids along the crestal The general similarities and differences in structural develop- faults of the Silak Anticline, is probably related to expulsion of ment, overpressure generation and fluid type/migration between C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229 227

Table 1 Comparison of the differences and similarities in factors affecting ﬂuid type, pressure and migration in deepwater Brunei and Central Iran.

Similarities Differences

Structures Detachment folds and fault propagation folds present. Absence of polygonal faults and mud pipes and presence of salt Affected by crestal normal faults welds in Central Iran. Crestal normal faults have different origins. Shale detachment (Brunei), salt detachment (Iran). Presence of joints, brecciated faults and stylolites in Central Iran Overpressure development Presence of inflationary and hydrocarbon-related Density-depth, lithostatic pressure gradient curves different. overpressures. Compaction a more gradual process offshore Brunei than Central Iran. Important effects from tectonic- disequilibrium Density/porosity anomaly evidence for burial-disequilibrium compaction compaction lacking in Central Iran. Onset and importance of diagentic changes on overpressure greater, at shallower depths, in Iran (smectite-rich clays) compared with Brunei (smectite-poor clays). Fluid type and migration Migration of fluids along thrusts. No effects of gas hydrates in non-deepwater fold and thrust belts. Anticlines act to focus fluid flow. Salt control on overpressure distribution and leak points in Iran. Few, Crestal normal faults act as fluid conduits. infrequent leakage events, breaching the evaporite seal. Both areas experienced spectacular blow-outs Opportunity for relatively frequent, multiple leakage events in during drilling deepwater Brunei. Variety of deep and shallow fluid escape features visible on seismic offshore Brunei, absent in Central Iran. Limited (fracture) connectivity in Qom Fm. reservoir central Iran. Longer distance connectivity in permeable sandstone related in offshore Brunei (limited connectivity in shales).

the Brunei and Iran examples are summarized in Table. 1. There are stress gradients, for Brunei and for the Silak-1 well, Iran, show many different factors in the two areas that contribute to fluid considerably higher, and more rapidly increasing vertical stress distribution, type, overpressure distribution and interaction with gradients in Iran (reflecting a much higher degree of shallower structures, these include: smectite rich vs smectite poor clays chemical diagenesis-related compaction in Iran compared with (affecting depth of onset of temperature-controlled overpressure Brunei). In both Brunei and Iran, crestal normal faults have played processes), water-rich vs water poor surface environment (affecting an important role in the transmission of fluids towards the surface. volume of trapped pore waters present, and available for expulsion The crestal normal faults in the anticlines of Central Iran penetrate during burial), multiple clay/shale seals vs single evaporite seal deeper in the section (to about 3 km in the case of the Silak Anti- (affecting trapping of overpressure and overpressure magnitude), cline) than deepwater Brunei (generally present in the upper 1 km overpressure development in carbonates vs clastics (type of over- only). Consequently their potential for transporting deep fluids is pressure generating mechanism), type of crestal normal fault (fluid greater. The presence of oil, gas, Ca- and Cl erich overpressured pathways), detachment type, and structural style at depth. fluids along the crestal faults of the Silak Anticline, is probably in In Brunei classic burial (mechanical-compaction) disequilibrium part related to expulsion of basinal fluids from >4 km depth. compaction and inflationary overpressures are important causes of The presence of thick evaporites at two levels in the Central overpressure in the smectite-poor upper 3e4 km of section. The Basin has exerted a considerable influence on fluid type and dis- basal detachment lies below about 150 C, where chemical pro- tribution in the basin by: 1) Providing a seal to highly over- cesses and metagenesis are inferred to drive overpressure pressured fluids in the Qom Formation, 2) influencing the pore development. water chemistry, 3) providing areas of fluid leakage where thrusts In the Central Basin of Iran the dense Upper Red Formation is a detach within the Upper Red Formation evaporites, and where smectite-rich sequence. Below about 1900 m the smectite to illite downbuilding has occurred to create a weld, 4) downbuilding in the transformation begins, causing moderate overpressures related to Lower Red Formation evaporites to create local depocentres where load transfer-disequilibrium compaction, plus local occurrences of the Qom Formation is mature for gas, and 5) influencing the ver- inflationary overpressures. Near-lithostatic overpressures only gence direction of folds (due to downbuilding), which in turn occur below an evaporite seal in the carbonate-rich Qom Forma- caused differences in timing of closure between the Alborz and tion, and are probably related to a succession of overpressure Sarajeh Anticlines, which resulted in the former being oil filled, and processes during burial and different structural stages, the latest the latter gas filled. inferred to be tectonic-disequilibrium compaction and hydrocar- Overall the deepwater Brunei system is very water rich, and bon generation/cracking volume-expansion. multiple opportunities for overpressure generation and fluid Predominantly deepwater, clastic settings like offshore Brunei leakage have occurred throughout the growth of the anticlines. are very fluid rich resulting in numerous shallow water escape The result is a wide variety of fluid migration pathways and features during burial that include mud dykes, sills, laccoliths, structures from deep to shallow levels, and widespread volcanoes and pipes, and fluid escape pipes (polygonal faults and inflationary-type overpressure. In the Central Basin the near sur- sand injectites occur in other deepwater settings). Deeper over- face environment is water limited. Mechanical and chemical pressured fluids migrate along thrust faults until stress conditions compaction led to moderate overpressure development above the trigger vertical pipe formation, typically in anticline crestal areas. Upper Red Formation evaporites, it is the trapping of fluids below This focus of fluids including thermogenic and biogenic gases the evaporites where the large overpressures have developed. commonly leads to the development of a shallow gas hydrate layer, Fluid leakage episodes across the evaporites have either been very which episodically traps gas. few or absent in most areas. Locations where episodes of leakage The Central Basin of Iran is a water-poor environment and lacks can occur (e.g. detaching thrusts, deep normal faults, salt welds) well-developed fluid escape features. A comparison of the vertical are sparse. 228 C.K. Morley et al. / Marine and Petroleum Geology 51 (2014) 210e229

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