Hydrocarbon Habitat in Growth Fault Regime

The follovving text is focused on the rale of growth faults in trapping hydrocai-bons. Various kinds of traps involving growth faults have been discussed along with a brief case study of Niger Delta and a detailed study and interpretation of a block in Mumbai offshore-Shelf Margin. Hydrocarbon in Habitat Growth Fault Regime With case study on Mumbai Offshore and Niger Delta Vaibhav Singhal Hydrocarbon Habitat in Growth Fault Regime Under Mt-. S.N. Mohanty Vaibhav Singhal Summer Train, EEPIL 3rd Year, Int. NA.Sc. Applied Geology IIT Kharagpur Hydrocarbon Habitat in Growth Fault Regime Page 1 Acknowkowleclgement I take this opportunity to express my gratitude towards my mentor Mr. S.N. Mohanty, Vice President, EOL-E&P, Mumbai for his consistent guidance and support. I am grateful to Ms. Urmi Surve and ail the members of ESSAR Mumbai for their constant motivation to work positively and for extending a helping hand whenever in need. Special thanks to Mr. Saurabh Goyal, Dy. Manager-EOL, for his kind support, guidance and encouragement throughout the project work I would also lifte to thank ail those who are knowingiy or unknowingly involved in completion of my project. L-Iydrocarbon Habitat in Growth Fault Regiine Page 2 Table of Contents 1. HYDROCARBON FORMATION AND ACCUMULATION 1.1. Formation 1.2. Accumulation... 1.3. Migration 1.4. Entrapment 2. PETR_OLEUM SYSTEM 2 .1. Introduction... ..... ...... ... ...... ...... ..... ....... ... 2.2. Reservoir Seal. . 2.4. Traps......... 2.4.1. Stratigraphic Traps 2.4.2. Structural Traps 2.5. Trap Evaluation 3. G ROWTH FAULTS 3.1. Introduction 3_2. Formation And Structure 3.3. Geological Settings For Growth Faults .• ........ 3.4. Importance Of Growth Faults 3.5. Limitations 4. INTERPRETATION 4.1, Data Loading 4.2, The Process..... ..... ... 4.2.1. Data Loading MM•b•M ........ ....... Fault Picking 4.2.3. Horizon Picking... Fault Polygon 4.2.5. Contouring 4.3. Interpretation of Growth Faults . 4.4. Geometry of Growth Faults 5. MUMBAI OFFSHORE 5.1. Introduction 5.2. Tectonic Setup .. 5.3. Stratigraphy 5.4. Petroleurn System 5.5, Data Interpretation ....... Mai MM•ba ...... ........ 5.5.1. Seismic Data Interpretation 5.5.2. Horizons and Faults Hydrocarbon Habitat in Growth Fault Regiiiie Page 3 5.5.3. Tinie Structure rviaps 5.5.4. Isochronopach Map 5.6. Prospect Analysis 6, NIGE RIA DELTA 6.1, Introduction... 6 .2. ............... .... .... 6.2.1. Tectonics 6.2.2. Stratigraphy 6.3. Petroleum System 6.3.1_ Location ... 6.3.2. Reservoir 6.3.3. Entrapment. 6.3.4, Cap Rock 6.4. Conclusion.. 7. CONCLUSION Hydrocarbon Habitat in Growth Fault Regime Page 4 INTRODUCTION There has been a lot of study on growth faults w.r.t. subsurface exploration of hydrocarbons. The main reason for special recognition of growth faults in this field is Their excellent hydrocarbon trapping capabilities. A schematic diagram of a growth fault has been shown in figure 1. Gravide' fault (cross section) 1: Diagram of a commun Growth Fouit Growth faults are mostly listric Normal faults generally found in extensronal sedimentary regime. Their dip decreases with depth and fault ends in the decolInnent zone. The thickness of the hanging wall {which is the clown thrown block) is increased resulting in thickening of both reservoir and cap making conditions more favorable for industries, Ambre numbers of Growth faults have been found in both Niger Delta and Mumbai Offshore (discussed in a case study of this report later). The traps involving growth faults are structural traps of both Fault dominated (Fault block) and fold dorninated (Rollover anticline). Hydrocarbon Habitat in Growth Fault Reginie Page S CIMPTER 1: HYDROCARBON FORMATION AND ACCUMULATION 1.1 FORMATION Oïl is formed from diatoms, extremely small sized marine organisms. Diatoms float in the top few meters of the oceans and also happen to be a major source of food for many forms of ocean swimmers. Their skeletons are chemically vert' similar to sand and are also made of the silica. Diatoms produce a kind of ail by themselves to store chernical energy from photosynthesis and ta increase their ability ta float. But this small amount of oil stiil needs to becorne concentrated and mature before it can be taken from the ground and used as fuel. Almost ail cil cornes from rocks that were formed underwater, floating ocean Fife (tint' creatures known as diatoms, foraminifera, and radielarian) that settle to the bottom of the sea eventually turns into oil. It tales many millions of years to form thick deposits of organic-rich siudge at the bottom of the ocean. This siudge afterwards undergoes Secliment Maturation. Given many thousands of years, a stack of mud and organic remains, many kilometers thick may pile up on the sea fluor, especially in nutrient-rich waters. Given enough tire, the overlying sediments that are constantly being deposited will bury these organic remains and mud deeply so that they eventually are turned into solid rock. High heat and intense pressure help Along various chernical reactions, transforming the soft parts of ancient organisms found in the deep-sea siudge into oïl and naturai gas. This onze at the bottorn of the ocean turns into Source Rock. 1.2 ACCUMULATION Reservoirs are the rock in which the oil is actually stored. An effective Reservoir must be of high porosity and permeability and must allow active exchange of fluids so that existing ouater in the trap is exchanged with hydrocarbOnS. It is a place that oil migrates to and is held underground. Sandstone has plenty of room inside itself ta trap oil and acts just like a sponge. lt is fer this reason Hydrocarbon Habitat in Growth Fault Regime Page 6 that sandstones are the mort common reservoir rocks. Limestone and dolostones, some of which are the skeletal remains of ancient coral reefs, are other examples of reservoir rocks. Figure: 1 Reservoir grade rock 1.3 MIGRATION 'Vs the process when oïl migrates from source rock to reservoir rock, where it displaces water that was originally present in the formation. If hyclrocarbons hit a trap they get stored there otherwise continue travelling upwards via different mechanisms e.g. Emulsification with water etc. Migration is rnainly of two types: Primary Migration involves release of petroleum compounds from solid organic particles (Kerogen) in source beds and their transport within and through the capillaries and narrow pores of fine grained source bed. Secondary migration, on the other hand, involves expulsion of the oil from source rock into the reservoir, which has rocks with lager pore spaces and permeability through which it is passed to traps 1,4 ENTRAPMENT Hydrocarbons once formed, being Iighter than surroundings have a strong tendency to rire above until they reach the surface of the earth, after which they are Iost in the environment. This is what happens at oil seeps (once common in Pennsylvania, California, Texas and Louisiana). Therefore, it is mandatory for exploration purposes that the hydrocarbon reserve is contained in some way in the subsurface so that it can be explored and exploited. The geological structures that act as barriers are called Traps. Traps are geological settings that with the help of low porosity and perméable rocks trap the hydrocarbons_ Shale, limestone (with low perrneability), etc. act as good seal. Hydrocarbon Habitat in Growth Fault Reginie Page 7 In Figure 2 the yellow particles are clay particles and Pink areas are pore spaces. As we can notice vert' ciearly that clay particles are packed very tightly, Therefore, due to lack of pore spaces the movement of oll through these rocks becomes extremely difficult, PORE SPACE SAND GRAINS Figure: 2 Seal Peculiar case of lime tore: Limestone can act as either a seal or a source rock depending upon the pressure temperature conditions it has faced. The lirnestone which has undergone higher pressures is likely to have a Iow permeability and is likely to end up as a Trap whereas the one that has seen fewer pressures rnight act as a reservoir. Hydrocarbon Habitat in Growth Fault Reginie Page B Chapter 2: PETROLEUM SYSTEM 2.1 INTRODUCTION Traps are the features that prevent ail from escaping from earth's surface. Ta be a viable trap, a subsurface feature must be capable of receiving hydrocarbons and storing them for some significant length of fine. This requires two fundamental components: a reservoir in which to store the hydrocarbons, and a seal (or set of seals) ta keep the hydrocarbons from rnigrating out of the trap. The presence of hydrocarbons is net a critical component of a trap, although this 15 a requirement for economic success. The absence of hydrocarbons may be the result of failure of other play or prospect parameters, such as the lacé of an active source rock or migratïon conduits, and it may have nothing to do with the ability of an individua[ feature ta act as a trap. 2.2 RESERVOIR The reservoir within a trap provides the storage space for the hydrocarbons. This requires adequate porosity within the reservoir interval. The reservoir must aise be capable of transmitting and exchanging fluids. This requires sufficient effective permeability within the reservoir interval and also along the migration conduit that connects the reservoir with a source rock. Because must traps are initially water filled, the reservoir rock must be capable of exchanging fluids as the original formation water has ta be displaced by hydrocarbons. Trap with only one hornogeneous reservoir rock are rare. lndividual reservoirs commonly include lateral and/or vertical variations in porosity and permeability. Such variations may be caused either by primary depositional processes or by secondary diagenetic or deformational effects and can lead to hydrocarbon saturated but nonproductive waste zones within a trap. Variations in porosity and permeability can also create transitions that occur over sortie distance between the reservoirs and the major seals of a trac'. These intervals may contain a significant amount of hydrocarbons that are difficult to produce effectively. Such intervals are regarded as uneconornic parts of the reservoir and net part of the seal, or otherwise, trap spill points may be mis-identified. Many traps contain several discrete reservoir rocks with interbedded impermeable units that forrn internai seals and segment hydrocarbon accumulations into separate compartments with separate gas-oil-water contacts and différent pressure distributions. Hydrocarbon Habitat in Growth Fault Regiine Page 9 wydrocarbon eccumdation lepration paithway Figure I 2.3 SEM The seal is the most critical component of a trap. Without effective seals, hydrocarbons will migrate out of the reservoir rock with tirne and the trap will lack viability. Effective seals for hydrocarbon accumulations are formed by relatively thick, laterally continuous, ductile rocks with high capillary entry pressures, but other types of seals may be important parts of individuel traps (e.g., fault zone material, volcanic rock, asphalt, and permafrost). Ali traps require some four of top seal. When the base of the top seal is convex upward in three dimensions, no other seal is necessary to form an adequate trap. Many traps are more complicated and require other effective seals. These are called the poly -seal traps. Lateral seals impede hydrocarbon movernent from the sides of a trap and are a common element of successful stratigraphic traps. Fades changes from porous and permeable rocks to rocks with higher capillary entrer pressures can form lateral seals, as can lateral diagenetic changes from reservoir to tight rocks. Other lateral seals are created by the juxtaposition of dissimilar rock types across erosional or clepositiona I bounda ries. Stratigraphie variability in lateral seals poses a risk of leakage and trap iimitation. In thinly interbedded intervals of porous and permeable rock in a potentiel lateral seal can destroy an otherwise viable trap. Base seals are present in many traps and are most cornmonly stratigraphie in nature. The presence or absence of an adequate base seal is rot a general trap requirement, but it can play an important rote in deciding how a field will be developed. Faults can be important in providing seals for a trap, and fault leak is a common trap limitation. Faults can create or modify seals by juxtaposing dissimilar rock types across the fault (Figure 1), by smearing or dragging less permeable material into the fault zone, by forming a less permeable gouge because of differential sorting and/or cataclasis, or by preferential diagenesis along the fault. Fault-induced leakage may result from juxtaposition of porous and permeable rocks across the fault or by formation of a fracture network along the fault itself. Hydrocarbon Habitat in Growth Fault Reghne Page 10 2.4 TRAPS Traps as discussed a bove are nnainly of three types: • • Stratigraphie Traps Structural Traps • Conibination Traps These traps and there types are briefly discussed below. 2.4.1 STRATIGRAPHIC TRAPS Stratigraphic traps are those in which the requisite and reservoir seal(s) combination were formed by any variation in the stratigraphy that 15 independent of structural deformation, except for regional titting They are mainty of 2 types: > Primary Stratigraphie Traps Secondary Stratigraphic Traps 2.4.1.1 PRIMARY STRATIGRAPHIC TRAPS They are formed by syn depositional processes by changes in conternporaneous deformation. Theses traps are generally characterized by lateral depositional change e.g. depositional pinchouts and facies change. 2.4.1.2 SECONDARY STRATIGRAPHIC TRAPS They are formed by post depositional pressure on the underlying strata due to deposition of sedinnents. Hydrocarbon Habitat in Growth Fault Reginie Page 11 Top Sul b &AMRI«. AdiliZZI••••••• 4111111111•11.1111111EM» _ AZ•Mi•MMIZZZBZUM 11111111111111' depcsit_ Bannir • .idna•Maiiiiiiirrie••••••• A•Mannana••••••••• BZUMMUMBRZ•ZZ•Bain •••••••••••••••••111 ZUMBEZZZOMMIIIII•••••• •••••••••••••••••••• • IIMI111111111111•1111111111111113111•MI Figure 2: A primary stratigraphie trop Porositylperrneability enhancement by dolomitization •■•• eM. •■• .■P .mm• É» i■ •■•• •■■• •■•• •••• .■11 Figure 3: Seconciory Stratigraphie trop: Trop formed by post deposit!onal differentiai pressures on the strata 2.4.2 STRUCTURAL TRAPS Structurai traps are created by the syn- to post depositional deforrnation of strata into a geometry (a structure) that permits the accumulation of hydrocarbons in the subsurface. The Hydrocarbon Habitat in Growth Fault Reghne Page 12 resuiting structures involving the reservoir, and usually the seal intervals, are dominated by either folds, faults, piercements, or any combination, The mort important types of structural traps are: • Fold Dominated Fault Dominated Piercement • • Combination of Fault-Fold Subunconformity Fold Dominated Structural traps are those in which are dominated by .— —.- ..-■, .a.-.., .....n .„ —■• L—. .M,. ....... ,... , ma■M. ■ folds at reservoir seal .... ".m. ., .m.. ...,,, .... ...a— Fault dominated traps are those which are dominated by faults at reservoir seal level \\ \ \LN \ \ \ "%...: ,,‘ t‘ \ -■. ...--. ..■. ....... --- + • + • + • N.,. . '-...... N., ,..... •--., \ \ \‘‘ ..... --- . \ '' \ ..' '... _ .‘,..\. \ \ '%... + 4 + + • ..., -"" ...— ..-.. _ -....... Piercement traps are formed hy introduction of sait body (or batholiths or dykes) in the sedirreentary strata resulting in formation of traps + + • + + + 4 • + + • + • • Hydrocarbon Habitat in Growth Fault Regime Page 13 14... 4. ..■ m mm • ... m., , 4... .ml. -4... ■. a. i %... .1,. - .".4. ..n. ""..,, '...,. .. emm %,.. ..----- --•wm. ,m... .■••• ,•-■ . ..„ -....".1% 44. ....: . .a. ■ mM■ .,... M ---...›. .■■• = ■'' ■ ■ aaa , 4. 4, ,.. _a ■F . m.- 4- — — .■-■ ■.. -,. .■ -■ ma, — — —a .■. .=. '.. ,, . „ 4 4■. m.. .. —4 ■- m— 44 • m- .8-m 4-m -m• .■ 4- ■ -• —. i-.. ■■. 4- _a.. 4.M — a. .■- .■- 444. -m - .... ■ -• ..4 -.m m. .. — -■ ,-. .4. 44. .4. 4. am. MM. . 4 m-.. 4 , , 4, .84., .m. •4. .4. .4- Combination trips are formed when both fault and folcling have important role in formation seal. Most successful structural traps are cornbination of bath folti and fault ma -4 am. 44 44 .44 44. .44. .4- .4. ... mm- 44 -4 4, mm. —4 44 4. 4 44 ,_ 4a 44. -4 84.. , -, .m -4 .4 -4 48-.. 44 .4 ma 44. .4 44. 44. 4 44 , 44 -4 44 4, 4. 4, 4 .4. 4, 4. ma , 44 , 44 .4. . 4- 44 .m. — T 44. -4 44 Subunconformity Traps are formed when the unconformity surface arts as top seal of the trap 2.5 TRAP EVALUATION Trap evaluation is concentratecl on placing potentiat traps in the content of the operating petroleum system. Plate tectonic setting, basin type, and structural evolution (sedimentary basin study) is used to predict the possible styles of structural and stratigraphie traps that should be expected in an area. Regional seals and their relation ta potential traps are established early in the evaivation. Timing of trap formation and its relation to the timing of generation, migration, and accumulation of hydrocarbon is the key of trap evaluation. Traps that forrn after hydrocarbon migration has .ceased are not attractive targets unless rernigration out of earlier formed traps has occurred. Trap is then mapped. Ideally, this would be the sealing surface of the trap. Identification of the actual sealing surface requires that bath seal and reservoir characterization. A commori flaw in trap evaluation results from ignoring the transition (or waste) zone, if present, between an economic reservoir and its ultirnate seal. Before drilling, reservoir and seal characteristics can be predictecl by combining regiorial and local paleogeographic information, sequence stratigraphie concepts, and detailed analyses of seismic fades and interval velocities. A detailed log analysis and incorporation of pertinent drill-stem test data is done for improved predictions. The absence of oil or gas in a subsurface feature can be the result of failure or absence of other essential elements or processes of a petroleum system and may have nothing to do with the viability of the trap. Therefore, although we use the geometric arrangement of key elements to define a trap, trap Hydrocarbon Habitat in Growth Fault Reginie Page 14 evaluation must include much more Chan just mapping the configuration of those elements. Reservoir and seal characteristics and their evaluation must be an integral part of any trap study. Timing of trap formation is also critical. No trap should be viewed out of context but rather should be evaluated in concert with all of the other efernents of a petroleurn system. Traps can be classified as structural, stratigraphie, or combination traps. In addition, hydrodynamic flow can modify traps and perhaps lead te hydrocarbon accumulations where no conventional traps exist. The trap classification discussed here is a useful way to consider traps during the early stages of prospect evaluation. An understanding of end-rnember trap types can help guide data acquisition strategy and mapping efforts, but there is an alrnost bewildering array of documented and potentiel hydrocarbon traps, rnany of which may be subtle or unconventional. As more and more of the world's hydrocarbon provinces reach mature stages of exploration, such traps may provide some of the best opportunities for future discoveries. Hydrocarbon Habitat in Growth Fault Reginie Page 15 Chapter 3: G ROVVTH FAULTS 3.1 INTRODUCTION Growth Faults are the type of faults in which there were displacements at the saine time as the sediments, on either ride of the fault, were accurnulating. Main distinguishing feature of a growth fault is the greater thickness of horizons, which is distinctly visible, in the downthrown block as compared to upthrown block. Other important feature of growth fault is the formation of rollover anticline (rot always seen) on the hanging wall of the fault system. In absence of a rollover anticline, a tabuiar dip section is seen having contrasting horizon thickness in upthrown and downthrown blocks, They are important clans of syn sedimentary fault which have been widely recognized in the subsurface studies of hydrocarbon-bearing clastic basin successions. They are Listric Normal faults that affect only a discrete sedimentary interval in which they were active. 3.2 FORMATION AND STRUCTURE Growth Faults are not directly related to basement tectonics rather are triggered hy gravity slidln within the sedimentary pile. Whilst active, faults cause the thickness of the successions in downthrown block to increase. The faults rnostly dip towards basin. This is attributed to the fast that the rate of deposition increases as we go towards basin. Therefore, the increased Ioad towards the basin sicle will have a tendency of going clown so, more sediment is deposited there and therefore we get thickened horizons in basinward direction. Following factors may increase the rate of the formation of growth faults 1. Ductility of underlying sediments 2. Overburden of new sediments Hydrocarbon Habitat in Growth Fault Reginie Page 16 L IMiene = . - L Fig. 3.1 A Growth Fouit, listric and normal in nature and having thicker horizons on the downthrown block(right hand skie) and relatively thinner horizons in the footwali wu/1 A ductile sediment succession floues easily and thus faulting occurs more easily, on other hand, hardi consolidated sediments require much more pressure for faulting. Figure 3.1 shows a typical growth fault. As we can see the fault is listric normal fault. Two horizons are marked in the figure showing the increase in throw as we go deep. The throw of growth fauft increases as we go deep rince the pre deposited successions undergo faulting for a greater period of Lime as compared to the newly accumulated oves. Sometimes formation on an anticline is observed on the hanging wall of the fault as shown in figure 3.2. This roll over anticline aise is caused due to gravity sliding of base sediments ire [ower parts of the fault The formation of growth fault leads to cornpaction of shale present in the footwall due ta the overburden of the sediments in hanging wall sicle. Also, if there is sait dome intrusion below the fault there is arching in the layers (concave downwards) due the uprising body. This arching breaks when the sait body exceeds (or rises above) the layer Hydrocarbon Habitat in Growtli Fault Reghne Page 17 3.3 GEOLOGICAL SETTINGS FOR GROWTH FAULTS Since growth faults are generaily restricted to more recent sedimentary successions rather than basernent they are more commun in extensional sedimentary basins, preferably where the sediments were loosely packed enabling there decollment or slang at later stages e,g. in de[taic environrnents where frequently, Ioad of new sedinients is laid over loose older sediments( Niger Delta) 3.4 IMPORTANCE OF RO TH FAULTS Growth Faults are given special recognition in study of subsurface exploration of hydrocarbons because of unique kinds of conditions they form in trapping hydrocarbons. get thicker across a growth fault. Therefore, it is more likely that The successions in hanging we will get larger reserves on the hanging wal[ rather than on footwall. This thickening is important as it restricts more and more cil on one ride of the fault making the extraction much more economicar. Apart from this, numerous kinds of traps are possible in growth faulted regime Rollover Anticline Fig 3.2 Formation of a roi/over antienne in a growth fouit The hanging wall of the growth fault often forms a roll over anticline can efficiently trap the hydrocarbons in presence of effective seal present on top. Also, the fault itseff can act as a lateral seal Second trap possible in growth fault is a fault block trap, Though this kind of trap might form in any kind of fault, the growth fault geometry might enhance the tir-apis capacity by virtue of its Hydrocarbon Habitat in Growth Fault Reginie Page 18 thickness. Also, the fault leak is significantly hampered due to thinner successions on the other ride of the fault. Thus if the hydrocarbon reserve 15 foilowed by a reservoir grade rock across the fault the migration of oil from reserve will be reduced significantly. Fig 3.3 Growth faults can effective!), foret excellent combination traps as shown 3.5 LIMITATIONS First of ail a trap may or may not have hydrocarbon trappecl in it. One must know thoroughly the geological and tectonic history of the block being explored, alors g with the migration history and patterns of the hydrocarbon from source to reservoir. A trap has a positive chance only if the trap formation pre-dates the migration. Also a trap loses the viability if it is not in the migration path of oil. Also fault plane, due ta sriding of various kinds of layers, may or may not seal the reserve. This may lead to significant Ioss of ail te atnnosph.ere if the fault trace extends te the surface. Similarly, if the rollover anticlirie Js net capped by an effective seal the oil will net be trapped. Nature of the rocks aise plays dominant mile. The reservoir rock must be permeable enough te let active exchange of fluids and keep the hydrocarbons absorbed for long periods of time. Hydrocarbon Habitat in Growth Fault Regime Page 19 CHAPTER 4: INTERPRETATION 4.1 INTRODUCTION 1nterpretation refers to study of acquisition data and mapping of the subsurface geophysical data by studying different seismic sections, marking the faults horizons etc. in the most geologically appropriate way, integrating the well Iog data and then tying the features to get a reasonaWe subsurface map. 4.2 THE PROCESS 4.2.1 DATA LCADING: First of all the seisrnic lines are Ioaded on any interpretation software (SMT, Petrel, OpenDtect etc.). Along with lines well Iog data can also be included at this step. Also a Polygon is defined which essentially marks the arec in which we orant to restrict our analysis 4.2.2 FAULT PICKING: Faults are picked by noting the abrupt change in horizons which is more or Iess sanie throughout the depth of the line (or atleast for a noticeable length). Though, in section fault trace may look perturbed {may be due to multiples, noise or other technical limitations) but we must mark the fault trace in the rnost geologically appropriate way (generally it is preferred to have smooth fault}. Faults are marked on every line available 4.2.3 HORIZON PICKING: To start horizon picking dip section is well .suited as on a dip section faults are visible and we can easily take care of the throw of each fault. At preferred time depth level, a suitable and easily recognizable horizon is chosen and then marked. After we mark one horizon on one section a point is obtained on d the Unes intersecting the marked section. This helps the interpreter to Hydrocarbon Habitat in Growth Fault Reginie Page 20 avoid the misties that might occur, Generally 2 or more horizons are marked for analysis of an area 4.2.4 FAULT POLYGON: The fault traces on seismic section appear as a point on the base map. The points corresponding to a single fault are enclosed in a polygon. The thickness of polygon at a particular place represents the throw of the fauft at that place. It is important to specify type of fault (whether normal or reverse), so that it is shown correctly on the contoured ma 4.2.5 CONTOURING: The Software interpolates the surface from the marked liner for a particular horizon, marked b )' the interpréter, and prepares a contoured surface for the area. 4.2.5 THICKNESS MAP: Thickness map is prepared by subtracting higher horizon contour map from lover horizon contour map. It essentially reflects the thickness of the strata between the selected two layers, throughout the area. 4.3 INTERPRETATION OF GROWTH FAULTS Distinguishing features of a growth fault include varying thickness of horizon across a fault, roll over anticline etc. They are listric normal fault and dip towards the basin though we often see opposite dipping antithetic faults which are consequence of growth faulting itself, While marking growth faults it must be kept in mind that the fault must dip in basin ward direction. Also, throw increases as we do deep Inside the section. Also commonly observed feature in growth fault is rotation of the hanging wall if there is some kind of geological barrier e.g. a ridge etc. 4.4 GEOMETRY OF GROWTH FAULTS The dip of growth fault decreases with depth, therefore fault surface map of a growth fault must show denser contours as we go in the direction opposite to the dip of the fault. Amount of thickening in the horizon is a matter of the rate of sedirnentation of the succession at the time of faulting, but a continuous increase in throw must be seen in growth faulting. Hydrocarbon Habitat in Growth Fault Reginie Page 21 CASE STUDY ON G ROWTH FAULTS • Mumbai Cffshore • Nger Illta Hydrocarbon Habitat in Growth Fault Regime Page 22 Chopter 5: MUMBAI OFFSHORE 5.1 INTRODUCTION Mumbai offshore is located on the western continental sheif of India between Saurashtra basin in NNW and Kerela Konkan in the south. The basin has been proven for commercial production of petroleum and hence falls under category I. It is approx. 116000 Knn2 in arec from coast to 200m isobath. The age of the basin ranges from late Cretaceous to Holocene with thick sedimentary fiil ranging from 1100-5000 meters though possibility of occurrence of Mesozoic synrift sequences in the deep-water basin have been indicated by the recentiy acquired seismic data. The first oil discovery in this basin %/vas made in the Miocene limestone reservoir of Mumbai High field in February 1974, Subsequent intensification in exploration and development activities in this basin have resulted in several significant discoveries including oil and gas fields like Fieera,Panna, Bassein, Neelam,Mukta, Ratna,Soth tapti, Mid Tapti etc. 5.2 TECTONIC SETUP Mumbai offshore is a pericratonic rift basin situated on western continental mare of indic. Towards NIVE it continues into the oniand Cambay basin. It is bounded in the northwest by Saurashtra peninsula, north by Diu Arch. Its southern limit is marked by east west trending Vengurla rch, located ait south of Ratnagiri, and to the easten boundary is marked by Indian craton. The Mumbai Offshore Basin is divided into 5 différent tectonic zones (Figure 2) • • • • Surat Depression (Tapti-Daman Block) in the north Panna-Bassein-Heera Block in the east central part Ratnagiri in the southern part Mumbai High-Platform-Deep Continental Shelf (DCS) in the raid-western sicle • Shelf Margin adjoing DCS and the Ratnagiri Hydrocarbon Habitat in Growth Fault Reginie Page 23 ni er 14°. • Tà$9 . • SUNA T 1:1EPRESN0ft olOPTI mu* MI•OCIp • 6T • te PAN PiA 'LAS SEN r:. BLOCK e:1 ir 010 :.I 7 Figure 2: Different tectanic zones in Mumbai Offshore 5.3 STRATIGRAPHIE Within the Bombay geologic province, the stratigraphic record is incomplete. ln the subsurface, Mesozoic rocks are known from drilling only in the Kutch area. ln the Depressions (Panna Formation) were filled with trap wash overlain by carbonates, shales, and interbedded siitstones from fluvial to transitional environments. In the Kutch area, marine sedimentation occurred oniy in the western part until the end of the Paleogene. Lover to middle Eocene rocks are absent from mort of the offshore area, and an erosional unconformity that extends over rnost of the offshore area truncates the Panna Formation. Eocene marine carbonates and shales of the Belapur, Bassien, and Dui Formations extend over mach of the present -day offshore and Eocene -Oligocene sandstones, siltstones and shales refiect shallow marine taa ll uv i a l environments in the south Cambay Graben. Middle to late Eocene tirne in the shelf margin or outer shelf, Bombay High, and Panna - Bassein areas is represented by sharlow-marine shales and shelf carbonates of the Belapur and Bassein Formations. Shoreward„ and to the northeast, shallow - marine to lagoonal Dui Formation shales domi. nate. Stin farther to the north, in the south Cambay Graben, deltaic and alluvial sediments of the AnIdeswar Formation dominate late Eocene-Oligocene environments. Today, the shelf area continues to receive terrestrial sediments, and the Cambay and Narmada Deltas continue to expand. The Stratigraphic chart of différent zones of Mumbai offshore are shown in Figure 3 Hydrocarbon Habitat in Growth Fault Reginie Page 24 BOMBAY HIGH-DCS BLOCK SHELF-MARIN BLOCK 5 1■4111,r RATNAGIRI BLOCK PANNA.BASSEIN BLOCK 1 .* = 1 1 eC l / = rXere TAPTI.DAMAh .■■• •■•••• e-bi 4■ 11. ,•■• .41.• mi lila .mlm 'O- IS - z M=M Mil C., o x MM IM=M ••= a . 1=. MM M=M 1= i'' *M M=M M1 . ■I1 M= -. MC% = L_J MI=M ' i■M=MM a 1.',MMWAIMI b=1 l■M=ZM IMMII -.7 _rM Ma I=Mi IM L. :".' IM.•M M=M 1■. Kilà1 ' —M — 0=i1 Mn MMI r.1 l■M=Il 11 "PM - all1■111>01 •1 •■ • • 41b t-• . 1.= 3 g s— - •• B=M ■11=M . à I • MILLIOLLIM r- 111111B11111111. ATM. 7 — ) 1± ' 419à DiariARM 5 • —1 —I— ,I •■ • (iRANITIC COMPLEX EXPLANATION Oil production. le show. or seep Gas production Of StIOW 11. Source rock potential Conglomerate • Siltstone. shale. and sandstone Sandstone . Flysch FLILirnestone j 7r1 o Volcanics Shale. si ltstoi)e. ---- and sandstone Figure 3: Stratigraphy of Mumbai offshore arec Hydrocarbon Habitat in Growth Fault Regime Page 25 5.4 PETROLEUM SYSTEM 5.4.1 SOURCE The main source in the Mumbai offshore region is the thick Panna shale facies deposits and Belapur Shale faces deposited between Paleocene and Lower to Mid Eocene 5.4.2 RESERvOIR Mumbai Offshore has botte carbonate and clastic reservoir facies everywhere between Paleocene and middle tvliocene. Reservoir grade rocks are frequentry found interlayered between shales in lower Miocene and Mid Miocene level. Main reservoir successions include Mukta, Ratnagii etc. Reservoir Age Lithology/Location Comments Middle Carbonate sections (Ratnagiri & The uppermost part has been found to be Miocene Bandra formations) hydrocarbon bearing at a few places A sheet like sand deposited over Mumbai High is also proved ta be gas bearing in commercial quantity in Mumbai High Lower Represented by a thick pile of Deposited under cyclic sedimentation with Miocene carbonates hosting quantity of ou and gas huge each cycle represented by lagoonal, algal mound, foraminiferai mound and coastal marsh facies The porosity is mainly intergranular, intragranular, moldic, vuggy and microfissures and the solution cavities interconnected by micro-fissures provided excellent permeability. Oligo— Early Sancis ( Panvel formation ) Miocene Hydrocarbon Habitat in Growth Fault Reginie Deposited under prograding delta conditions Page Xi Proved to be excellent reservoirs Eocene and E.Oligocene clastics (Mahuva Proven hydrocarbon bearing reservoirs in Early Formation) Oligocene Deposition of thicker carbonate Graduai increase of sea level, shielding Ta pti area. fades over the horst blocks from the clastic onslaught from the (Bassein, Mukta & Heera northern part of the basin. formations). The intervening regressive phases have aideci in developing good porosity in these rocks rnaking them excellent reservoir levels in the basin. Paleocene Coarser clastic facies developed The clastics of Panna formation are proved within the upper marine shale to be excellent reservoirs. (Panna Formation) 5.4.3 CAP ROCKS tvlan successions acting as cap are also shales deposited later in Pleistocene, also named Chinchini Shales. It is a thick shale succession that sits over other clastic and carbonate layers. 5.4.4 ENTRAPMENT Mumbai Offshore has various tectonic settings that show a range of traps for example in Structural traps litre tilted fault black type, fault closures. Fold dominated traps include anticlines and also roll over anticlines formed due to growth faulting. A large number of growth faufts also have enhanced the probability of striking econornic prospects, especially in the basinward direction of the zone. This study deals with the growth fault reginne located within the shelf rnargin tectonic block of Mumbai Offshore basin 5.5 DATA INTERPRETATION Interpretation of the study area located within the Shelf-Margin tectonic block of Mumbai offshore was done on SMT KINGDOM on Mid Miocene and Lower Miocene level. The seismic time section of 21 liner was used to interpret these levels and prepare time structure maps and a rnidMiocene isochronopach. Hydrocarbon Habitat in Growth Fault Reginie Page 27 5.5.1 SEISM1C DATA INTERPRETATION Figure 4: Base map shawing the study area Hydrocarbon Habitat in Growtli Fault Regime Page 2 5.5.2 INTERPRETED HORIZONS AND FAULTS Figure 5: Seismic section of line as marked in the figure Figure 6: Seismk section of une as marked in the figure Hydrocarbon Habitat in Growth Fault Regime Page 29 Figure 7: Seismic section of line as morkeci in the figure Figure 8: Seismk section of une os morked in the figure Hydrocarbon Habitat in Growth Fault Reginie Page :30 Numerous listric normal faults are seen on the section. This is because large sediment deposition in the basin has led to formation of growth faults. Especially to be noted is the centrai main growth fault marked in red in the above sections. A roll over anticline is seen on the east of tris fault which has been faulted by antithetic faults (Figure 8}. The Green horizon represents Middle Miocene level and the yellow horizon denotes Lower Miocene level. 5.5.3 TIME STRUCTURE MARS +LIU 1 1 SI ro 1 len 1200 1215 12,0 1241 1350 1275 1290 13)5 1»0 1335 Leti 1365 1380 13■15 14.385 Figure 9: Mid Miocene two way tirne structure !pop Hydrocarbon Habitat in Growtli Fault Regime gage 3 1 n mas les .1 es les t_125 tas 1.525 1 575 1 625 1 SIS I 725 1 375 1112$ I 975 1 925 1 975 CCC Figure 10: Lower Miocene two way time structure mafia The faults trend NNW-SSE on the map. A persistent rire is observed in the western part of the study area. This is because of Laxmi-Laccadive Ridge which is present along the shelf margin in Murnbai offshore. Due to this ridge the horizons have been raised on the western part of study area. Eastern hait of the area has homoclinal slope. Main basinal s'ope is towards ouest because of the presence of growth fault pockets. Sediment depocenters are observed in the eastern half of tha study area. Hydrocarbon Habitat in Growtli Fault Regime Page 32 5.5.4 1SOCHRONOPACH MAP .0 top o 100 oi 0 eiso no O29) 0.2e8 3t$0 e 3e 0.370 411:0 0.43) 4f30 0 490 0 i20 59) 0U ~~ VO 0 640 /100 0700 0 73) 07W 4 3"00 Figure 11: isochronopoch of Mid Miocene succession The thickness is reduced sharply from center to west. As shown in (Fig 5-8) the Middle Miocene level doesn't vary rnuch whereas there is a sud en rire in Louver Miocene Level after main fauFt (Marked in Red in respective figures). 5.6 PROSPECT ANALYSIS Interpretation of the area shows that there is rollover anticline on the hanging wall of the main fault forming an excellent trap (Figure 12). The anticline marked rn figure 12 corresponds ta '1' in figure 13. Along with this, similar but srnaller antiennes are also seen in the area e,g. those rnarked in figure 13, The prospect '3' lies on the same fault as '1'. '2' is another anticline, but the main problem with this region is that it lies on Hydrocarbon Habitat in Growth Fault Reghne Page 33 foot rail of the fault in further east decreasing the thickness considerably which might not yield econornic quantities of oil. Probable Plyclroiumbon Prospects in SheI Mar en Mumbai Offshore Inclined Fault Block Type Rollover Anticline •-r• •i&Ifty. • dMig.1.■....0.;11110.ff - . • • •" • :.. ...i■ IrLeFf. •MILe • 4,41. •, • Figure 12: Prospect analysis of the arec (lare view) Hydrocarbon Habitat in Growth Fault Reginie P age 4 1.155 1 Mi 11 $5 200 1211 1 230 1 24 1.2E0 275 ne 13S 1320 305 ■ 13!0 1 305 no 1 795 .1. ME Possible Hydrocarbon Prospects Figure 13: Prospect andlysis of the study arec (Plan view- Mid Miocene TINT rnap) Hydrocarbon Habitat in Growth Fault Regime Page 35 CHAPTER 6 NIGER DELTA 6.1 INTRODUCTION Mo-cliern Coasilinie •t I t t • UPPer Eofen Pre-Getaceous basernent Cretaceous & younger secllrnen t Lowe Tertlary ha sin èdee axis and 'direction of &lui( supply ee' f Appro:drnate coastline positions a t the designated Lime Pi tocene- Pleistocere Figure 1: The changing coostiine of the Niger delta 100 km (35 Mo History) The Niger Delta is situated in the Gulf of Guinea (fig. 2) and extends throughout the Niger Delta Province. The coastal sedirnentary basin of Nigeria has been the scene of three depositional cycles. The first began with a marine incursion in the middle Cretaceous and vki a s terminated by a mild folding phase in Santonian time. The second included the growth of a proto-Niger delta during the late Cretaceous and ended in a major Paleocene marine transgression. The third cycle, from Recent, marked the continuous growth of the main Niger delta. A ne w threefold lithostratigraphic subdivision is introduced for the Niger delta subsurface, comprising an upper sandy Benin formation, an intervening unit of alternating sandstone and shale named the Agbada Eocene to Hydrocarbon Habitat in Growth Fault Reginie Page 36 formation, and a lower shaly Akata formation. These three units entend across the whole delta and each ranges in age from early Tertiary to Recent. From the Eocene to the present, the delta huas shifted southwestward, forming deposition belts that represent the rnost active portion of the delta at each stage of its development). These depositional belts forrn a regressive delta an area of some 300,000 km2, a sediment volume of 500,000 km', and a sediment thickness of over 10 km in the basin deposition center. The Niger Delta Province contains only one identified petroleum system, the Tertiary Niger Delta (Akata —Agbada) Petroleum System. Figure 2: Index map of Nigeria and Cameroon 6.2 GEOLOGY The onshore portion of the Niger Delta Province is Iined by the geology of southern Nigeria and southwestern Cameroon (fig. 1). The northern boundary is the Benin flank--an east-northeast trending hinge line south of the West Africa basernent massif. The northeastern boundary is defined by outcrops of the Cretaceous on the Abakaliki High and further east-south-east by the Calabar fiank--a hinge line bordering the adjacent Precarnbrian. The offshore boundary of the province is defined by the Cameroon voicanic line to the east, the eastern boundary of the Dahomey basin (the eastern-mort West African transform-fault passive margin) to the west, and the two kilometer sediment thickness contour or the 4000-meter bathymetric contour in arecs where sediment thickness is greater than two kilometers to the south and southwest. The province covers 300,000 km2 and includes the geoiogic entent of the Tertiary Niger Delta (AkataAgbada) Petroleum System. Hydrocarbon Habitat in Growth Fault Reginie Page 37 6.2.1 TECTONICS The tectonic framework of the continental margin along the Niger toast is coritrolled by Cretaceous fracture zones expressed as trenches and ridges in the deep Atlantic. Discal Portion of Depobelt Continental Rise Lower Continental Slope Zone of diapir Toe thnis 2 - 4 - • Late Tertiary 6 • -••.• - _ • • ..■16 8 Late Cretaceous EarlyTertiary ••• • ::F F/0-(/ Oceanic Crue 12 so 100 • SO Distance (kr»i Figure 2: Seismic section from the Niger Delta continental slope irise showing the rescrits of internai gravity tectonics on sediments at the lista} portion of the depobelt. The ridges in the fracture zone livide the margin into different basins and, in Nigeria, foira the boundary fautts of the Cretaceous Benue-Abakaliki trough, which culs far into the West African shield. The trough represents a failed aria of a rift triple junction associated with the opening of the South Atlantic. ln this region, rifting started in the Late Jurassic and persisted into the Middle Cretaceous. In the region of the Niger Delta, rifting diminished altogether in the Late Cretaceous, After rifting ceased, gravit y tectonics became the primary deforrnational process. Shale mobility induced internai deformation and occurred in response to two processes. First, shale diapirs fornied from loading of poorly cornpacted, over-pressured, prodelta and deltaslope clays (Akata Fm.) by the higher density delta-front sands (Agbada Fm.). Second, slope instability occurred due to a lack of lateral, basinward, and support for the under-compacted delta-slope clays (Akata Fm.) (Fig. 2). Shale rnobility induced internai deformation and occurred in response to two processes. First, shale diapirs formed from loading of poorly compacted, overpressured, prodelta and delta-slope clays by the higher density delta-front sands (Agbada Fm.). Second, slope instability occurred due ta a lack of laterat, basinward, and support for the underHydrocarbon Habitat in Growth Fault Reginie Page 38 compacted delta-clope clays (Akata Fm.) (Fig. 2). For any given depobelt, gravity tectonics were completed before deposition of the Benin Formation and are expressed in complex structures, including shale diapirs, roll-over anticlines, collapsed growth fault crests, back-to-back features, and steeply dipping, closely spaced flank faults. These faults mostly offset different parts of the Agbada Formation and flatten into detachment planes near the top of the Akata Formation. 6.2.2 STRATIGRAPHIE The Cretaceous section has not been explored beneath Niger Delta Basin, the youngest and southernmost sub-basin in the Benue-Abakaliki trough. Lithology of Cretaceous rocks deposited in the Niger Delta basin can only be extrapolated from the exposed Cretaceous section in the next basin to the northeast--the Anambra basin. From the Campanian through the Paleocene, the shoreline was concave into the Anambra basin Niger Delta Basin' Anambra Basin B.tonikr, SB L u te dan Agbada Fm -50 Yenesian Thatiet -60 nies n Aka ta Fm Mags-r7 <nuiri -70 ?Amin tg Trri ÇA C arnpae 41 1 11 111116±:11 Sinter s SB) 3 C onda SB Sequence Boundary Channel -- Maximum Ficioding Boundare Tidal structure — Coal -80 n -90 Figure 3: Strutigraphic section of Anan-imbra basin resulting in convergent longshore drift cells that produced tide-dominated deltaic sédimentation during transgressions and river-dominated sedîmentation during regressions. Shallow marine clastics were deposited farther offshore and, in the Anambra basin, are represented by the AlbianCenomanian Asu River shale, Cenomanian-Santonian Eze-Uku and Awgu shales, and CampanianiMaastrichtian Nkporo shale, aniong others (figs. 3 and 6) . The distribution of Late Cretaceous shale beneath the Niger Delta is unknown in the Paleocene, a major transgression (referred to as the Sokoto transgression by Reijers and others, 1997) be an with the Imo shale Hydrocarbon Habitat in Growth Fault Regime Page 39 being deposited in the Anambra Basin to the norlheast and the Akata shale in the Niger Delta Basin area to the southwest (fig. 5). In the Eocene, the coastline chape became convexly curvilinear, the longshore drift cells switched to divergent, and sedirnentation changed to being wave-dominated (Reijers and others, 1997). At this tirne, deposition of paralic sediments began in the Niger Delta Basin proper and, as the sediments prograded south, the coastline became progressively more convex seaward. Today, delta sedimentatiori is stil' wave-dorninated and longshore drift cells divergent Tertiary section of the Niger Delta is divided into three formations, representing prograding depositional facies that are distinguished rnostly on the basic of sand-shale ratios. The Akata Formation at the base of the delta is of marine origin and is composed of thick shale sequences (potential source rock), turbidite sand (potential reservoirs in deep water), and minor amounts of clay and silt (figs. 3, 4, 5 and 6). The Akata Formation formed during [owstands when terrestrial organic matter and clays were transported to deep water arecs in low energy conditions and oxygen deficiency. The formation underlies the entire delta, and is typically over pressured. Turbiciity currents likely deposited deep sea fan sands within the upper Akata Formation during clevelopment of the delta. Deposition of the overlying Agbada Formation, the major petroleumbearing unit, began in the Eocene and continues into the Recent (figs. 3, 4 and 5), The formation consists of paralic siliciclastics over 3700 rneters thick and represents the actual deltaic portion of the sequence. The clastics a ccumulated in delta-front, delta-topset, and fluvio-deltaic environrnents. ln the louver Agbada Formation, shale and sandstone beds were deposited in equal proportions, however, the upper portion is mostly sand with only minor shale interbeds. The Agbada Formation is overlain by the third formation, the Benin Formation, a continental Iatest Eocene ta Recent deposit of alluvial and upper coastal plain sands that are up to 2000 m thick (Avbovbo, 1978). A A' East West Figure 4: AN Section (refer Figure 1) Hydrocarbon Habitat in Growth Fault Reginie Page 40 PROVINCE PRESE NT DAY DELTA FRONT BOUNDARY t ers M10-P 10CENE DELTA FRONT IOCENE DELTA Fele «DIE MITA Ma« AGAOADA FA OB de> MASADA FACIES 0/0/ MATA FADES Out 1 AC1ES itEleNfACIES efe iesu RneERCIP i goe eior. ------- e te.> 11000 AEROMAGNETK r...% pure cepirec BOISEMENT 11000 •11 far ine fartes 11 Conti nental facies Baser* nt compte' (Oceanic and continen Unconformfty -- 1.$ Facies boundamo 90410A 1 2061 0 100 1 I 1 I 3111 PAL ji Figure 5: Bir Section (Refer Figure 1) Hydrocarbon Habitat in Growth Fault Regime Page 41 SOUTHWEST NORTHEAST • Ouaternai ••n I •Moid d • •• ••• .• • Deltaic Facies Pli0(ene .e• I •• • • „ dry +rr: (Ag bada Fm.) mn' • • n...n ' •• r %Me -fr.d...••••••• .•.••• ••• •••+d• dy•• • ! ,.:•••••%;..::•'...:•....:••;•• • • .:«.•■ •••••: • $. Lace " • •• • •.• ••;"•:': nui LA I ly via Sa ncr •■ • • • "-:." •• • • • "• • ••• ••• dr• .1. ••••2•;.•:;: : if)d• • y ..• • j. .$ •• • •• •-• •• • +.,. -. • à+ e dr,• • •• ••:. ••••••:2 _ MM MM MM ! MM • .• • ":!" • MM MW MW ! 1- — — Mickile i_—______ --- -C-ley — ' ••■ • M.. wm. 4..-.. -". . ■•"--.7.2Lriginn a ei aï-_ 4 ■ ■• ,.' MM. ! Earty —— ■■ ■■ 4 ■■ ■■ ! ■■ ■■ Oligocene j.— -- Te _—_-,Morratia lez 'MM MM MM 'MM —_—_À74.1r,a7SErn ) 'MM MM MM. M. ! • MW MF !• . ■ ! ! MM MM 'MM . ! y. M. Laie .. MM M. .. MM MM MM MM ! •■• Mi M. !a MM . 11■ 911 MM MM MM M. ! « Mo M. MM M. ! ■■11 ! ■■ ! ! ! ! ! ! ! dm. dam. ■■ •■• •■• ■■ ■■ ■■ ■■ ■■ ■■ ■■ 4 ■rn ..■ ■■ ■■ ■■ ■■ 4■ ■ • Earty — — MM MM MM MM. MM MM MM ■■ .■■ .. 6." Paleoce ! +MM W. ! a- - - Mi ! MM M. MM Mi Mi MM MM MM ! MM . ■ M. ■■ Mi Mi - m m ■•■ - m. ! . • Cr Laite_— Extert or erosional truncat on Figure 6: Stratigraphic column showing the 3 formations of Niger delta Hydrocarbon Habitat in Growth Fault Reghne Page 42 6.3 PETROLEUM SYSTi M 6.3.1 LOCATION llydrocarbons are present throughout the Agbada Formation of the Niger Delta (Figure 1). Though, oil rich trends have been found having the lowest gas to oil ratio as shown in Fig. 7 Shate proue areas Coastline /Direction of clastic input Figure 7: The oil rich trend line in the Niger delta province As shown above, the oil rich trend ruas close to the shore and roughly corresponds to transition betvveen continental and oceanic trust, and within the axis of maximum thickness. Source Rocks Marine Akata shales and interbedded shales cretaceous shale of Agbada along with Cretaceous shales have been identified. Also Agbada Formation has intervals that contain organic carton contains in sufficiently high quantifies but the thickness of Chose layers is riot thick enough to produce a world dass oil province. Also they are immature in various parts of the deltas. The thick Akata shales, below Agbada, are however are of large volume and produce oil in large quantifies. Hydrocarbon Habitat in Growth Fault Regime Page 43 6.3.2 RESERVOIR The main reservoirs are sandstone and unconsolidated sands in Agbada formation. The reservoirs are often stacked and are show a large variation in thickness ranging from 15 ruts to 10% having thickness of 45 nits. 6.3.3 ENTRAPMENT N Grawthfaults Giowth fault V Strat. trap Roi lover structure Clay filled channe losures au:T c/oit:te — — Rollover çtructure • Sand pinçhout •_ • • nef Pirktioca • Akata - - A katie, g L Simple rollover structure with clay filled c hannel 48. .4 Structure with multiple growth faults N Antithetic fault \\I Growth fault ve Structure with antithetic fault G rowth fa tilts Antithetic Mun Collapseci crestil f ollapsecl crest structure Figure 8: Various traps found in the Niger delta province the Niger delta region are mostly structural mostly involving syn sedirnentary deformations, rnostly growth faults and rollover structures. The shales, acting as a seal trap typically either form clay smears a[ong the faufts or provide interbedded sealing units against which reservoir sands are juxtaposed due to faulting or set a vertical seal and act like lateral seals to prevent horizontal migration of oil. On the flanks of delta (offshore} the clay filled canyons provide top seal and foret excellent traps, The traps in Hydrocarbon Habitat in Growth Fault Reginie Page 44 6.3.4 CAP ROCK The primary seal rock in Niger delta is the interbedded shale within the Agbada formation. As discussed earlier the shale provides three types of seals—clair srnears along faults, interbedded sealing units against which reservoir sands are juxtaposed due tu faulting, and vertical seals 6.4 CONCLUSION kis clear from above discussion that the main traps in the Niger Delta province are based on syn sedimentary structures. The abundance of such traps is increased mainly due to the uncompacted layers of sand stones being overpressured by the heavy sediment loads of the delta. This has recuit in gravit y siiding and fracturing of subsurface on a large scale, Thickness of reservoir is selectively increased at places by growth faults making exploration and extraction very profitable. Hydrocarbon Habitat in Growth Fault Reginie Page 45 CONCLUSION From the study done tilt now following important points have been concluded: • oir occurrence in the trap depends on the Urne of formation of trap and the time of » migration. The Trap should have formed before migration otherwise the oïl will escape. If the underlying sediments are not compact enough their ductility increases and pence theïr tendent y of undergoing gravit y siiding eventually leading to growth faulting (As seen in the case of Niger Delta) ~~ Interpretation shows that, in the stuclied ares of S'Il& Margin of Mumbai Offshore, there is a possibility of good quantifies of hydrocarbons due to existing rollover structures and highly faulted strata. Hydrocarbon Habitat in Growth Fault Reginie Page 46 Works Cited Sheltori, p.801-15, 32 July 1984, Listric Normal Faults; An Illustrated Summary, AAPGBulletin, v.68, Brown,L. Frank Jr,Loucks, Robert G., Trevino, Ramon H.,Hammes, Ursula, 2004, Understanding Growth faulted,intraslope subbasins by applying sequence stratigraphic pririciples: Examples from the south Texas Oligocene formation, AAPG Bulletin, y. 88, no. 11 ,pp. 1501 —1522 Brown,L. Frank Jr,Loucks„ Robert G., Trevino, Ramon H.,Hammes, Ursula, 2004, Understanding growth-faulted, intraslope subbasins by applying sequence-stratigraphic principles: Examples from the south Texas Oligocene Frio Formation: Reply, AAPG Bulletin, U. 90, no. 5 (May 2006), pp. 799-805 Gazes, c_A., December 2004, Oyerlap zones, growth faults, and sedimentation: using high Resolution gravity data, Livingston Parish, LA, Phd. Theisis, Louisiana State University Wandrey, C.J , May 2004, Bombay Geologic Province Eocene to Miocene Composite Total Petroleum System, india, Petroleum Systems and Related Geologic Studies in Region 8, South Asia, U.S. Geological Survey Bulletin 2208-F, v.1 Jackson, M.P,A, Vendeville, B.C.,July 1991, The rire of diapirs during thin-skinned extension, Catuneau, Octavian, 2005, Principles of Sequence and Stratigraphy Tearpock, D.J., BischkeR.E., 1991, Applied Subsurface Gecdogical Mapping wwvv.wikipedia.com www.britannica.rom mAnv.banglapedia.com ihotnevv_ sciencedirect corn wiNvv.cighindier.rirq Hydrocarbon Habitat in Growth Fault Reginie Page 47