phanerozoic petrol basin

Phanerozoic Petroliferous Basins of India 253

The term Phanerozoic or “revealed life”, which isapplied to Paleozoic, Mesozoic and Cenozoic eras, representsa relatively brief geological period of half-a-billion yearsthat constitutes the age of multi-cellular animal life onEarth, and which left a detailed fossil record and built upcomplex and diverse ecosystems. Though there was complexLate Precambrian (Vendian/Ediacaran) biota, thePhanerozoic period defines the period of development andevolution of higher groups of organisms like arthropods,mollusks, vertebrates, etc.

The sedimentary basins of India have received attentionof geoscientists due to increased activities for petroleumexploration since 1950’s. Sophisticated geophysical techniquetogether with drilling made it possible to obtain vast amountof subsurface data, and tied wherever possible with surfacegeology. These gave new insight into the mechanisms ofvarious types of basin formation and their relationship withthe different petroleum occurrences.

In India, hydrocarbons have been struck in differentbasins ranging in age throughout the Phanerozoic. The mostrelevant feature has been the occurrence of most of thecommercial hydrocarbons within the Cenozoic successionsboth on land and offshore. The Mesozoic, comprising ashallow marine Jurassic succession in Kutch-Rajasthan andpoorly developed shallow shelf facies of the Cretaceous inmost parts of the eastern coastal basins between Assam andsouthern tip of India, have had relatively lesser hydrocarbonfinds till date. We have, however, the Gondwana successionsin several parts of the country which contains rich coaldeposits where exploration for coal bed methane (CBM) hastaken off in a big way.

Not all the Phanerozoic sedimentary basins in Indiaare equally prospective, as borne out by the oil explorationprogramme and a small number of basins produce mostof the hydrocarbons. Proven petroliferous sedimentarybasins with commercial production include the Mumbaioffshore, Cambay, Assam-Arakan, Cauvery, Krishna-Godavari and Tripura-Cachar basins. Sedimentary basinswith known occurrence of hydrocarbons but lackingcommercial production include Andaman-Nicobar, Bengal,Mahanadi, Himalayan Foothills and Rajasthan basins. Basinswhich on general geological grounds appear fairlyprospective but where significant amounts of hydrocarbonhave not yet been found, the Kutch-Saurashtra and Kerala-Konkan basins. Lastly, basins like Arunachal Foothills,Deccan syneclise, Ganga Valley, Karewa (Kashmir Valley),Mizoram-Manipur and Narmada basins, could still beconsidered in the initial phase of exploration. The geo-scientific studies by ONGC in the petroliferous basins ofIndia from are summarized in the present contribution, and

Phanerozoic Petroliferous Basins of IndiaP.K. BHOWMICK*

Keshav Deo Malviya Institute for Petroleum Exploration,Oil and Natural Gas Commission, 9, Kaulagarh Road, Dehradun - 248 195

* E-mail: [email protected]

takes into account, the interpretations based on real welldata, the seismic and other geophysical information, multi-microfossil bio-stratigraphy, sedimentology andgeochemistry. More specifically, we deal with,1. Sedimentary petroliferous basins along the Western

margins: viz. Rajasthan, Cambay, Kutch, MumbaiOffshore and Kerala- Konkan

2. East Coast Basins: Cauvery, Krishna-Godavari,Mahanadi and Bengal basins

3. Northeast Basins: Assam and Assam-Arakan basin4. Central Indian Basins: Ganga and Purnea basins

Each basin has been discussed with reference to itsbasin architecture, sedimentary fill, major unconformities,sequences and petroleum system, in light of latestunderstanding of these basins and their hydrocarbonprospectivity.

SEDIMENTARY PETROLIFEROUS BASINS ALONGTHE WESTERN MARGINS

Two important phases of basin evolution are recordedalong the western continental margin. There is an initialrecord of the Mesozoic basins in Jaisalmer, Bikaner-Nagaur,Barmer (Rajasthan Basins), Kutch, and Cambay that couldbe traced to the Mesozoic basins occurring along the westernmargin of Madagascar. (Fig. 1). The first stage of separationof the Western Gondwanaland (South America and Africa)from the Eastern Gondwanaland (Madagascar, India-Seychelles, Antarctica and Australia) is recorded duringLate Triassic/Jurassic (~196-203Ma) and is closely associatedwith Karoo volcanism in South Africa, the conjugate ofwhich is seen in Antarctica. The second stage was theseparation of Seychelles-India from Madagascar in LateCretaceous (~93Ma), associated with minor volcanism foundon conjugate margins of southwestern India and southeastMadagascar. The final breakup of Seychelles at KTB (~65Ma)contiguous with the Deccan volcanism is associated witha series of rift basins along the western continental marginsin which the Cenozoic basins evolved viz. Barmer basin,Cambay Basin and its southern continuation in MumbaiOffshore. A description of these basins is given from thenorth to the south.

Basins in Rajasthan

The western Rajasthan shelf located to the west ofAravalli ranges, possesses three important basins viz.,Jaisalmer, Bikaner-Nagaur and Barmer, stretching over anarea of about 1,20,000 sq. km.

The Jaisalmer Basin

This is the westernmost is separated from the Bikaner-Nagaur basin (Fig. 2) by the Pokaran-Nachana high to thenorthwest and from the Barmer basin by the Barmer-Devikot-Nachana high in the south. A pronounced NW-SE-trending

Glimpses of Geoscience Research in India254

regional step-faulted Jaisalmer-Mari high zone, marked bythe Kanoi and Ramgarh faults that traverse the centre ofthe basin and divides it into the Shahgarh sub-basin, theMiajalar sub-basin and the Kishangarh sub-basin. This basinin the northwestern Indian shield extends as far as the Mariregion of Pakistan, and is tectonically related to the IndusBasin from the beginning of the Triassic. The aerial extentis over 30,000 km2.This basin is controlled by wrench-faulttectonics.

The outcrop and the well-data indicate sedimentationfrom the Cambrian to the Tertiary (Fig. 3). Three distinctsequences are identified, based on the basin forming tectonicevents namely, the Proterozoic to early Cambrian riftsequence comprising mainly the thickly bedded sandstonewith shale intercalations in the lower part and dolomiticand cherty limestone with shale and sandstone interbedsin the upper part, Permian to Eocene shelf–sag, andPleistocene to Recent sequences. The Proterozoic-earlyCambrian succession unconformably overlies thePrecambrian basement rocks, while the unconformableupper contact of the sequence with Permian, has a hiatusspan of about 190 Ma.

Exploration by ONGC & OIL in Jaisalmer basin hasresulted in discoveries of several gas fields, namely MaheraTibba from the Cenozoic and Cinnewala Tibba from Cenozoicand Cretaceous reservoirs, respectively. The gas from thesefields is methane-rich and commercially viable. Gas fromthe other fields viz. Ghotaru, Bankia, Bhakari Tibba, Khartarand Sadewala is nitrogen-rich and is commercially notviable. Oil shows have also been reported in wellsChinnawala Tibba-1 and Ghotaru-2 from early-lateCretaceous sequence, though as of now there is no

commercially viable discovery of liquid hydrocarbons. Thepotential source-rock layers are from Late Jurassic and earlyCretaceous. Genetic correlation of known accumulation ofgases in Cenozoic and Cretaceous reservoir suggests thatthey have been generated from sediments at higher maturityand have been trapped at their present locale after long-distance migration.

The Bikaner-Nagaur Basin is mostly a Paleozoic basinwith a considerable thickness of Paleozoic sediments,overlain by a thin veneer of Tertiary/Mesozoic sediments.The Paleozoic sediments include evaporites and carbonates,and are similar to more NW Pakistan. Potential source andreservoir rocks have been identified in the Paleozoicsediments. The Cenozoic sedimentation in the Bikaner-Nagaur basin began with the coal-bearing Palana Formationthat was deposited during the Paleocene in subtropicalswampy conditions on the continental part. Marinesedimentation indicates encroachment of the sea during theUpper Paleocene to Lower Eocene. The exploration drillingby ONGC, Oil India and a Joint Venture Private Companyhas indicated presence of heavy oil in haline/carbonates.Baghewala, Nanuwala and Binjybala areas have indicatedhydrocarbon occurrences.

The Barmer Basin is interpreted as a narrow, N-S-trending graben, a northern extension of Cambay rift. Thefaults exposed at Fatehgarh, on Barmer Hill near Barmerand at Sarnu constitute the peripheries of the Barmer Basin.The pre-rift sediments deposited on the Late ProterozoicMalani Igneous Suite, represented by Randha, and BirmaniaFormations (a siliceous facies-shales, sandstone,orthoquartzite) and calcareous facies-limestone,phosphorites and dolomudstone) respectively, are exposedon the western margin of the basin. The Sarnu Formation(co-relatable with the sandstones of Jodhpur Group) exposedon the eastern margin of the basin, comprises thin andfining-upward sand bodies with intervening red siltstone.

Fig. 1: Paleo-tectonic reconstruction of the Madagascar-Seychelles– India during the Mesozoic, showing the structural trends, theJurassic outcrops (in grey) and the Mesozoic rifts (in red)

Fig. 2: Structural map of western Rajasthan


The Lathi Formation exposed at the northern periphery thebasin, comprises medium to coarse, fining upward fluvialsands with fossils.

The syn-rift sediments: Barmer Hill and FatehgarhFormations are exposed at Barmer and Fatehgarh. TheBarmer Hill Formation comprises sandstone and clastsupported conglomerates, exposed along the westernboundary of the basin and rest unconformably on thebasement. These represent rapid deposition in an alluvial-fan environment with source from Malani Rhyolite. TheFatehgarh Formation exposed at the northern boundary ofthe basin shows a mixed sand and mud tidal-flatenvironment. It comprises conglomerate at the base, overlainby sandstone. This in turn, is overlain by ferruginousphosphatic sandstone. Seismic data reveals that in subsurfaceFatehgarh Formation continues as Barmer Hill Formation.The Fatehgarh Formation is overlain by siliceous earth ofBariyara Member (base of Mataji-ka-Dunger Formation).

The post-rift sediments are constituted by the Mataji-ka-Dunger and Akli Formations. The Mataji-ka-DungerFormation is exposed at the northern and western marginsof the basin and consists of cyclically arranged claystone,siltstone, sandstone, and is interpreted as a shallowing-upwards fluvio-deltaic complex. The sandstone shows 15%visible porosity with no secondary infill except authigenicquartz overgrowth, signifying itself to be a good reservoirrock. The base of the sequence exposed at the south ofFatehgarh comprises sand-poor claystone. This is interpretedas pro-deltaic, delta-slope/delta-mouth deposition. The AkliFormation exposed at the central part of the basin, overlyingthe Mataji-ka-Dunger Formation comprises bentoniticclaystone, grey bituminous clay-stone, lignites and lightyellow claystone. Widespread exploration work by variousoil companies notably Shell and then Cairn Energy, haveresulted in a number of oil and gas discoveries mainlywithin the Paleocene sediments. These are given inFigure 4.

Fig. 3: Stratigraphy of the Barmer, Bikaner-Nagaur and Jaisalmer Basins


CAMBAY BASIN

The Cambay Basin, the southern continuation of theBarmer-Sanchor Graben is a narrow elongated (NNW-SSEtrending) intra-cratonic rift basin (area 59,000 sq.km), situatedbetween Saurashtra craton to the west, Aravalli swell onthe northeast and Deccan craton to the southeast. In thesouth, it extends into Cambay Gulf and ultimately into theArabian Sea. A large part of the basin is covered byQuaternary sediments. Cenozoic outcrops are rare andoccur only on the fringes of the basin. The extensionalarchitecture of the basin is defined by three majorPrecambrian trends viz., NNW-SSE trend related toDharwarian orogeny NE-SW trend related to Aravalliorogeny, and ENE-WSW trend related to Satpura orogeny.

The basin is sub-divided into four major tectonic blocksSanchor-Patan, Ahmedabad-Mehsana, Tarapur-Cambayand Jambusar-Broach, by cross-trending NE-SW. trendinghighs (Fig. 5). Each of these blocks is characterized by locallows which form the source areas. The pre-rift LateCretaceous sediments are deposited in continental, fluvialsetup and comprise sandstones with minor claystone, basalconglomerate and thin inter-beds of shale. The syn-riftPaleocene sequence, bounded by unconformities below andabove, is either absent or thinly developed over paleohighsand restricted only to fault grabens and fault scarps. Thesesediments extend as piedmont fanglomerate representedby trapwash, claystone, shale and sand with thin coal bedstoward north representing initiation of basins fill. The LatePaleocene also witnessed the first marine transgression,followed by regression, during which Proto-Saraswati river

from northwest brought arenaceous sediments in thenorthern areas of Mehsana and deposited these under tidalregime on the downthrown blocks. The top of Paleocenemarks the end of rifting followed by an unconformity overwhich Early Eocene transgression is observed in the southernpart of Cambay Basin. Eocene to Recent post-rift succession,overlying the Paleocene with an unconformity is encounteredthroughout the Cambay Basin. Early Eocene facies is markedwith the deposition of grey shale, silts and coals. The topof early Eocene witnessed regression during which proto-Saraswati river from NW deposited deltaic sands and silt,shale and thick coal bands. Early/Middle Eocene boundaryis marked by an unconformity followed by a renewedtransgression in Middle Eocene during which depositionof sandstone, siltstone, shale and coals took place in tidalto estuarine setup. Upper part of Middle Eocene and lateEocene witnessed regression represented by a mixed faciesof laminated sandstone, with limestone nodules andoccasional carbonaceous shale. Early Oligocene successionoverlies the Eocene strata with a brief hiatus. Early Oligocenetransgressive sediments include grey shale with claystone,argillaceous sandstone. This is followed by a major regressionand unconformity spanning the entire duration of lateOligocene. The Miocene succession in the basin is representedby current-bedded sandstone, conglomerates, clays andshale. This is followed by an unconformity above whichclaystone, sandstone, clay, sand and kankar of Pliocene toRecent age were deposited. Extensive exploration in thisbasin by ONGC since 1959 has resulted in numerous oiland gas discoveries, notably Ankleswar, Gandhar,Nawagam, Dholka, Kalol, Sanand, Jhalora, Viraj, Kadi andS. Kadi, N. Kadi, Jotana, Sobhasan, Balol, Santhal, etc.

Fig. 4: New Discoveries in Barmer-Sanchor. Review of geologicalmodel time to time and its implementation in exploration in oneof the largest onland discovery of the country.

Fig. 5: Location and tectonic map Cambay Basin


The source rocks in Cambay basin are identified inPaleocene-Eocene levels. The southern part of the basin hasmultiplicity of source rock, as the layers from Paleocene toMiddle Eocene attained peak maturation and contributedto the oil accumulation. In the northern part of the basin,however, only the Paleocene sediments attained peakmaturity. The reservoirs are distributed in the passive marginsequence from Paleocene to Miocene, with maximumaccumulation in Middle Eocene. The Eocene shale is theregional cap for the hydrocarbon accumulation in the basin.

THE WESTERN CONTINENTAL OFFSHORE BASINS

The western continental margin (WCM) of India hoststhree major basins viz., Kutch, Mumbai offshore and Kerala– Konkan, out of which the Mumbai offshore is the majorpetroliferous basin, with the other two having oil and gasindications. The continental margin is featured by parallelto sub parallel ridges and intervening depressions. Thecontinental shelf on the western margin is wide and tapering,300 km wide off Kutch-Saurashtra in the north, narrowingdown progressively southward to 60 km in Kerala offshore.The WCM comprises shelfal horst/graben complex, Kori-Comorin ridge (KCR) and Laxmi-Laccadive Ridge (LLR)from east to the west with sediment fill in the basinaldepressions between them. Three basins are recognized inthe offshore, which from the north to south are the KutchBasin, the Mumbai Offshore Basin, and the Kerala-Konkanbasin. A series of ENE-WSW ridges separate each of thesebasins (Fig. 6).

KUTCH (KACHCHH) BASIN

The Kutch basin, located roughly west of the CambayBasins extending from land to offshore, exposes classicJurassic and Cretaceous succession amidst the vast alluviumcovered Rann. The Mesozoic basin, is a south-westerncontinuation of the Rajasthan basins, extending into theoffshore with a wide shelf platform. The northern limit ofthe basin continues beyond the Indo-Pakistan border. Insouth, the basin is contiguous with Mumbai offshore basin.

The surface outcrop mapping and stratigraphy hasbeen worked in detail in Mesozoic-Cenozoic succession andit has been possible to extend these units in the offshorethrough excellent biostratigraphic correlation especially inthe Cenozoic. Extensive development of Deccan Trap coversthe highest Mesozoic succession in the outcrops andcontinues in the subsurface of the Kutch shelf. The infra-trappean succession comprises clastic sediments of MiddleJurassic to Late Cretaceous. In the Gulf, however, the LateCretaceous is mainly a carbonate facies.

Two distinct sequences have been identified: the earlyMiddle Jurassic to Middle Cretaceous, and the upper lateCretaceous to Recent sedimentary succession (Fig. 7).Hiatuses between Middle/late Jurassic and Jurassic/Cretaceous boundary, have been recognized. MiddleJurassic, late Jurassic and later successions have beenencountered in several domes in mainland Kutch, as alsoin wells drilled on land and offshore. Middle Jurassic periodconsists of non-marine-estuarine sandy shale of Aalenianage, followed by shallow- marine sand, shale, and carbonatesof Bajocian marking the first major marine event. Theoverlying golden oolite, shale and minor carbonates are ofBathonian-Callovian age. The succeeding sequence isbounded by limited by the Middle/late Jurassicunconformity at the base and by the overlying Cretaceousboundary comprising carbonates, shale and minor sandsin lower part (Oxfordian) and dominantly shale and finerclastics in the upper part (Kimmeridgian-Tithonian). Alarge thickness of non-marine sandstones (Bhuj sandstones)of uniform character constitutes the uppermost unit of theMesozoic sequence. This, unconformity bounded successionis encountered both on land and in subsurface. In theoutcrops, the early Cretaceous succession is unconformablyoverlain by Deccan Traps but in the subsurface, it is overlainby late Cretaceous carbonate dominated succession. Thespan of hiatuses in the outcrops is maximum (coveringentire late Cretaceous) and decreases in the subsurface, tobe between early/late Cretaceous showing highlydiachronous nature of the unconformity. The late Cretaceous

Fig. 6: Map of the Western offshore basins


succession is encountered only in the subsurface of wellsdrilled in Kutch offshore, characterized dominantly by shelfcarbonates. In the wells drilled on land, like the outcrops,Deccan trap directly overlies the Early Cretaceous and theentire late Cretaceous is absent. The late Cretaceoustransgression is most extensive during Campanian-Maastrichtian, marked by increase in bathymetry upto100 m.

Deccan basalts, dated to 65-67 Ma, extend from Lakhpatin west to Anjar in the east, and occur in more or less linearoutcrops extending across mainland with a maximum widthof 10 km in the east and gradually tapering westward. Thethickness of these basalts encountered in offshore wellsincreases from south to southwest.

Cenozoic sedimentation in the basin commenced in astable shallow marine setup and had remarkable lateralcontinuity from onland to offshore, enabling biostratigraphiccorrelation across the basin. Paleocene rocks in the on landKutch are exposed consistently bordering the outcrops ofyounger Cenozoic rocks and Deccan Trap. The upperboundary is demarcated by biostratigraphically definedPaleocene/Eocene unconformity. In the offshore, thesuccession is present both in shelf and deep water. Paleoceneoutcrops near Matanomadh village consist of basalt derivedsediments, variegated clay, tuffaceous shale and sandstone.Limestone interbedded with shale is observed in the upperpart. Overlying early Eocene succession is recorded in severalwells drilled in Kutch offshore, besides the outcrops in themainland. In the wells of shelf and deeper areas, thecarbonates of this sequence are dolomitic. In the outcrops,the succession is represented by claystone, shale, limoniteand bands of sideritic concretions with limestone and shalein upper part.

The Middle – late Eocene succession is recordedextensively from outcrops and several wells in Kutchoffshore, characterized dominantly by shallow inner shelflimestone and argillaceous limestone with thin interbedsof shale. The Middle Eocene witnessed major transgressionand northeastern extension of the shelf, with coast lineshifting north to lie close to Kutch mainland fault. Extensiveoutcrops of Middle Eocene carbonates extend as homoclinal

shelf over an extensive area in offshore. The overlyingOligo-Miocene succession is represented by limestone,argillaceous limestone with thin shale in lower part andsandstone, shale, claystone in the upper part. A minorhiatus is inferred between early and late Oligocene. Theearly Oligocene succession with claystone and corallinelimestone is well exposed in the outcrops and extends overthe entire offshore. In the offshore, however, the carbonatesare associated with finer clastics deposited in an inner shelfsetup. The late Oligocene-Miocene succession is relativelyconformable over the outcrops and in offshore. It dominantlycomprises limestone with thin shale in the lower part andsandstone, shale and claystone in upper part. The Mio/Pliocene correlative conformity defines the upper limit ofthe succession that is overlain by Pliocene to Recentsedimentary succession. The Middle Jurassic and earlyCretaceous sediments have organic-rich interval with TOCvalues within good source rock interval. During Paleoceneorganic-rich shale and coal were deposited in coastal andlagoonal swamps are have a good source potential.Hydrocarbons have been encountered in the early and lateCretaceous succession in the offshore Kutch indicatingpresence of Mesozoic petroleum system with sedimentsdeposited in rift grabens acting as source. Hydrocarbonstrikes are also recorded from early-Middle Eocenecarbonates.

MUMBAI OFFSHORE BASIN

The Mumbai offshore basin is the most importantproducer among the Cenozoic hydrocarbon basins of India,mainly from the carbonate reservoirs stretching betweenthe Deep continental shelf (DCS) structure in west to shallowwaters in the east. The basin is bounded by the Deccan Trapoutcrops to its north and east, Kori-Comorin ridge to itswest and Vengurla arch to its south and covers an area ofabout 1,48,000 sq km up to 200 m isobath. Tectonically,Mumbai basin has evolved from a rift basin, with the mainrift (Central Graben) and is a continuation of the Cambayrift to the south with a lateral shift. The basin is dissectedby ENE-WSW grabens forming two additional very largedepo-centres, apart from that of the Central graben, one tothe north (Tapti-Daman Low), and one to the south (SouthBombay Low). Post Oligocene there has been a westwardtilt in the basin with a Miocene Hinge line developing.

Based on the tectonic evolution, two sequences havebeen identified within the Cenozoic succession. The LowerPaleocene sediments deposited as synrift and initiating theCenozoic sedimentation. The upper post-rift sedimentarysuccession of late Paleocene to Recent have been furthersubdivided into successions based on unconformities andtheir magnitude on the basis of excellent bio-stratigraphiccontrol.

The early Paleocene succession is dominantly fluvialintertidal to shallow inner-shelf clastic (siltstone, claystone,shale and sandstone with coal shale) with subordinatecarbonates, trapwash and trap derivatives in the lower partrepresent and post-trappean sediments as graben-fills duringsynrift. As the rifting continued, these sediments filled theinitial morphotectonic depressions during Paleocene in thelows and grabens. Late Paleocene-early Eocene representd

Fig. 7: A north-south section across Kutch Offshore Basinshowing the Mesozoic and post-Mesozoic sections


the first widespread Cenozoic marine transgression in thebasin. The sequence represented by fluvial-intertidal shale,coal shale and finer clastics in the lower part and, shallowinner-shelf carbonates in the upper parts. This is recordedall over the Mumbai offshore basin except over the MumbaiHigh platform. The lower boundary is an unconformitybetween the rift and the passive margin sequence while theupper boundary is early/Middle Eocene boundary, witha hiatus of ~2 million years.

The succeeding Middle – late Eocene succession(Bassein Formation) has a large areal extent over thebasin, except Mumbai High platform. During this period,a large part of the basin witnessed widespread carbonatesedimentation in a shallow inner-shelf setup. The upperboundary between Eocene and Oligocene is highlydichronous covering a span of 3-6 million years in Middleand late Eocene chronospans. Wide variation in lithofaciesis observed. In the northern areas of Tapti-Daman, finerclastics dominate while in other parts of the basin, limestoneand shale with minor clay are the dominant facies. Theuppermost sequence is recorded between Eocene topunconformity and Mio-Pliocene unconformity/correctiveconformity, covering the chronospan of early Oligocene toMiocene. The sequence is divided into two units viz. earlyOligocene and Late Oligocene-Miocene succession,separated by an unconformity between early/late Oligocene.The lower unit covering the chronospan of early Oligocene(Mukta Formation) is present over the entire Mumbaioffshore basin except Mumbai High platform. Lithologically,the sequence shows a facies variation across the basin. Inmany areas, it is characterized by carbonates and shaledeposited in shallow inner-shelf setup, while in the northernareas of Tapti – Daman, intertidal to shallow-marine clasticscharacterize the sequence (Mahuva Formation). Theoverlying, Late Oligocene-Miocene epochs representdeposition of shallow-shelf carbonates and shale over mostof the basin, except northern parts, where fluvial to shallow-marine clastics characterize the late Oligocene. Thissuccession is encountered over all areas of Mumbai offshoreand overlain by the finer clastics of late Miocene to Recentage.

Hydrocarbon accumulations, in this basin occur inreservoirs ranging in age from Paleocene to Middle Miocene,with early Miocene carbonates of Mumbai High being thebiggest producers. Paleocene and early Eocene sedimentsare the source rocks (Figs. 8 and 9). The synrift Paleocenesediments, deposited in supratidal to intertidal setupcomprise coal shale, shale, siltstone and sandstone. Theorganic matter is dominantly terrestrial with maturationlevel at the early phase of oil window. Early Eocene sedimentsare also characterized by terrestrial organic matter in theorganic rich shale and coal. The timing of beginning ofhydrocarbon expulsion in these source rocks is around 30Ma, with critical moment around 5 Ma.

KERALA – KONKAN BASIN

This basin lying south of Mumbai offshore basin isbounded by Vengurla arch in north and extends beyondCape Comorin into the Indian ocean to the south. It coversan area of ~77,000 sq km up to 200m isobath and divided

Fig. 8: Geographic extent of Cenozoic Petroleum System shownon Isopach of Paleocene Sequence of Mumbai Offshore Basin- theblue and dark blue patches on the map indicate thickersedimentation.

Fig. 9: Map showing fields of Mumbai Offshore basin


into two subbasins the Konkan basin between Vengurla andTellichery arch and Kerala basin between Tellichery andTrivandrum arch. The basement arches control thearchitecture and north- south limits of the basin. Thenorthernmost Vangurla arch separates the shelfal horst-graben complex of this basin from that of Mumbai offshorebasin, and is differentiated into three shallow depressionsby transverse basement arches. These are Konkan depression,Cochin depression and Cape Comorin depression. TheCretaceous-Recent sedimentary succession in the basin isdivided into two sequences. The lower, corresponding torifting and separation of Madagascar from India-Seychelles(~90-110Ma in early Cretaceous and earliest part of LateCretaceous) is referred to as the rift sequence and overlyinglate Cretaceous to Recent is referred to as the passive marginsequence. The event of separation of Madagascar (³ 90 Ma)is represented by ‘older’ traps dated 90-110 Ma in the basinrepresenting the rift sequence. The sedimentary succession(Late Cretaceous and younger) overlying the older trapsrepresent the passive margin setup. The lowest sequence(late Cretaceous), limited between older and younger basalts(Deccan trap equivalent) is encountered in wells drilled inoffshore. The succession comprises sand, shale and siltstonedeposited in an inter-middle shelf setup.

The succeeding sedimentary succession overlies theDeccan trap (younger basalts) and represents the Paleocenechronospan. Its upper boundary with early Eocene is anunconformity. The gross lithology is sandstone interbeddedwith clay/claystone and siltstone with a few streaks oflimestone. Incidentally, several flows have been observedin the upper part of this sequence in some wells whichindicated that the youngest traps in this part of the basinare of late Paleocene age and evolved within a shallowmarine setup.

The overlying unconformity-bounded succession,represents the early Eocene epoch and this is recorded inseveral offshore wells. The gross lithology varies fromdominantly carbonates succession to carbonates and clasticsdeposited in supratidal environment to shallow inner-shelfsetup. Middle-Late Eocene succession is also recorded fromseveral wells and is represented by limestone with minorshale beds. The hiatus at the upper boundary is diachronousand in several wells, late Eocene is absent.

Overlying Early Oligocene sequence is bounded byhiatuses. The dominant lithology of the sequence is limestonewith minor sandstone beds. With gradual sediment loading,the basement tectonics become more pronounced duringOligocene and the Tellichery arch became more prominentseparating the northern areas of Konkan and southern areasof Kerala as two separate depositional regimes.

Late Oligocene-Miocene chronospan is dominated bycarbonates with thin beds of claystone in some parts, whilein other areas well-sorted sands with interbedded shaledominate the sequence. Differentiation of sedimentaryregimes in Konkan and Kerala basin continued during thisperiod with the former receiving dominantly carbonatesand the latter more clastics. There is no commercial discoveryof hydrocarbons in this basin till date. The synrift sedimentsof Early Cretaceous or older age are expected to be source

rocks but not yet penetrated in the wells drilled. In theabsence of hydrocarbons, the identification of petroleumsystem in the basin is at the best speculative.

EAST COAST BASINS

The Jurassic fragmentation of eastern Gondwanalandinitiated with the dismembering of Antarctica and Australiafrom India and, concomitant formation of NE-SW trendingMesozoic rift basins on the eastern continental margin ofthe latter including Assam, Bengal, Mahanadi, Palar,Krishna- Godavari and Cauvery basins. These evolved froma composite of rifted graben in late Jurassic, and laterformed a part of the divergent passive margin. Numerousdown-to-basin extensional faulting took place in the basindue to rifting. Active subsidence along these normal faultsparallel to Precambrian Eastern Ghat trend gave rise tohorst-graben setting. Several stages of reactivation of synriftextensional faults are noticeable.

Cauvery Basin

Cauvery Basin, on the east coast of India, extends fromPondicherry in the north to Tuticorin in the south, stretchinginto offshore Bay of Bengal and spans over an area of 62,500sq.km. upto 200 m isobath (Fig. 10). In the exploratory wellsin onland and offshore, the sediments range in age fromOxfordian (late Jurassic) to Recent. Outcrops are patchy.Five major unconformities late Albian, Turonian,Campanian, Maastrichtian and Miocene are observed in

Fig. 10: Tectonic map of Cauvery Basin


outcrops, and excepting the Campanian unconformity, otherunconformities are also recorded in the subsurface. Thehiatus at Turonian was probably caused by Marian mantleplume and consequent mild basin exhumation. At the KTBunconformity, development of continental facies has beenobserved in outcrops. During the late Cretaceous, directionof sea floor spreading changed parallel to Ninety-East ridge.The north ward movement of Indian plate was taken upon the Chagos-Laccadive transform on the west and NinetyEast Ridge transform at eastern plate edge.

The stratigraphic record of the basin comprises variousdepositional sequences representing synrift and post-riftpassive margin.

Initial syn-rift deposits were the fresh water/shallowmarine late Jurassic sediments followed by the earlyCretaceous deposits. In the outcrops, the successioncomprises of basal boulder beds, overlain by fluvio-lacustrineclastics with gritty sandstone and clayey plant fossil-richbeds. In the subsurface, it is represented by sands and shalewith subordinate algal limestone and claystone in the lowerpart. Post-rift, late Cretaceous to Recent sediments arerecorded in many wells drilled in the basin. These sequencesare interrupted by four well defined hiatuses at the top ofCretaceous, late Eocene, Oligocene and Miocene,respectively. The basal part of the succession comprises ofcarbonaceous shale and sandstone of Albian– Cenomanianage, overlain by Cenomanian-Turonian sandstones andthick shale of Coniacian- Santonian. The sandstones representdeposition by debris flow or high density turbidity current.The Coniacian-Maastrichtian, in its upper part representsregressive phase with high percentage of arenaceousdeposits. The horst–graben morphology almost disappearedat this period, with peneplanation of most of the ridges,resulting in emergence of a unified basin margin at the endof Cretaceous. Further in the outcropping areas, LateMaastrichtian sediments are represented by continentalfacies, with coeval sediments in northeast in Pondicherrybeing marine and fossiliferous. Paleocene clastics arecharacterized by ferruginised chamosite oolite depositedin shallow-marine setup. In outcrops, thickPaleocene carbonate succession is encountered inseveral peaces. Eocene strata are well developedin subsurface and are represented dominantly bysandstone and clays. The succession is characterizedby continuous Eocene planktonic foraminiferalassemblage. The Middle Eocene succession isrepresented by pebbly sandstone deposited mainlyas debris floor. The late Eocene sediments aredominantly sandstone and limestone.

The Oligocene succession is characterized bythin, transgressive sediments overlain by thicknormal regressive package. These sediments aredominantly calcareous and argillaceous sandstonedeposited by mixed processes of slumps and debrisflow. Some areas have intercalated red shales atthe base. In the outcrops, the Miocene sedimentsare mainly non-marine sandstone, while insubsurface the limestone are well developed, withalternation of siltstone and calcareous sandstone

in the upper part. The Pliocene transgression resulted indeepening of the basin and deposition of clay-stone overthe Miocene sediments.

Numerous oil and gas discoveries have been made inthe Cauvery Basin, concentrated mainly on the flanks ofthe NE-SW-trending ridges. Based on the maturity of organicmatter, the effective mature source rocks in the basin arelargely limited to the synrift succession belonging to Albianand older periods. The source rock type is generally type-III and type-II. The reservoirs in the basin are distributedin sediments of all ages from late Jurassic – Albian toOligocene, with maximum accumulation in the Cretaceoussuccession. The natural gases in the basin are thermogenicgenerated in the catagenetic phase of maturation of source.The oil is generated from a mixed source input depositedin peat swamp to sub-oxic environment in the rifted setting.

The Krishna-Godavari Basin has a polycyclic (dual-riftprovince) evolution history. It comprises a wide array ofsedimentary facies from early Permian through Cenozoicwith analogous outcrops in the northwestern part. To thenortheast and southwest of this basin, along the coast, liethe Mahanadi and Cauvery Basins (Fig. 11). Broad tectonicexpression of the basin comprises linear NE-SW-trendinghorst-graben system, growth fault/rollover and block tiltingalong synthetic fault over intra shelf regime followed bytoe thrust, thus exhibiting a manifestation of typical passivemargin.

The tectono-stratigraphic framework reveals threedistinct stages of evolution of the basin. Till early Jurassic,the northeastern area of the basin was part of intracratonicrift setup constituting the southeastern extension of NW-SE-trending Pranhita-Godavari Basin. The succeeding lateJurassic to early Cretaceous synrift sediments were depositedduring early tectonic subsidence accentuated by basement-rifted fault system. The Late Cretaceous and younger periodrepresents the passive margin (drift stage) setup. Exponentialdecay in thermal subsidence with rapid subsidencecontinued during the drift phase with open marinetransgression during Cenomanian. The basin witnessed

Fig. 11: Tectonic and the oil and gas map of KG Basin


widespread volcanism during Paleocene. During lateCretaceous, Paleogene and Neogene major sedimentpathways (KG fluvial system) resulted in further spreadingof delta progradation to southeast. Major sea-level dropsare observed in late Oligocene and Late Miocene. The presentdeep-water regime is characterized by shelf, slope andbasin-floor corridors. The continental slope is deformedwith contraction towards mid-down slope and more ofextensional in basinal part, characterized by mud rich debrisflow, turbidity flow, channel-levee complex, marine onlapwedge, suspended sediment-rich onlapping packs, etc. Thebasin plain is manifested as undeformed continuous regime.

The lowermost intracratonic Gondwana sequence wasdeposited in NW-SE trending Pranhita-Godavari grabenbefore Gondwana breakup. The sequence is underlain byArchean basement and topped by sub aerial unconformity.Based on the sub-aerial unconformity at Permian top, twosedimentary successions have been identified. The lowerPermian sequence in the subsurface has its equivalents inoutcrops as Talchirs and Barakars. It comprises argillites,sandstone, coal-shale deposited in fluvial-lacustrineenvironment. The Upper Triassic sequence comprisesmonotonous vertically stacked predominant fluvialsandstone.

The late Jurassic – early Cretaceous rift sequence isrepresented by sandstone, shale with occasional coal bandsand red claystone, deposited in fluvial-marginal marinesetup. The sequence is further subdivided into lower LateJurassic – early Cretaceous and Upper Aptian-Albiansuccession. The lithology of the former is dominantlysandstone with interbedded shale with minor coal andcarbonaceous shale. The Aptian-Albian sequence is mainlyargillaceous with highly carbonaceous black shale in lowerpart and sand in the upper part. The passive margin sequence

comprises late Cretaceous clastics, Paleocene volcanics andCenozoic clastics and carbonates. It is divided into foursuccessions based on unconformities with early Paleocenevolcanics occupying the time gap between the first twosequences. The lowermost late Cretaceous (Cenomanian-Maastrichtian) comprises lower argillaceous section(claystone, shale and minor sandstone) marking the regionalmarine transgression and the upper arenaceous (dominantlysandstone) unit representing delta progradation. Theoverlying volcanics are primarily restricted to onland part,though part of them constitutes subaequeous extrusion.Overlying late Paleocene-Oligocene succession compriseslate Paleocene marine shale, early Eocene near shore toshallow-marine sands, Middle Eocene shallow marinecarbonates, late Eocene-Oligocene sands and their equivalentshale. The Miocene section is characterized by sandstoneand claystone deposited in middle-outer shelf setup.Uppermost Pliocene to Recent succession is represented bytransgressive shale with minor sandstone and silt bands.Numerous oil and gas fields have been found both inlandand offshore. Many of the large offshore structure- relatedfields are associated with roll-overs. The oil-sourcecorrelation studies indicate presence of effective sourcerocks in Permian, late Jurassic, early Cretaceous, Aptian-Albian, late Paleocene and Eocene ages. The reservoirs aredistributed in sediments of late Jurassic-early Cretaceous,Aptian-Albian and Pliocene ages. The unique feature ofPliocene hydrocarbons is that they are charged by morethan one vertically stacked source rock.

Mahanadi Basin

Mahanadi basin is located along northeastern part ofeast coast of India and extending to the Bay of Bengal. Thearea covers roughly 1, 40,000 sq km up to 2500 m iso-bathin offshore region.(Fig. 12).

Fig. 12: Geological map with the offshore acreage and the tectonic map


Mahanadi basin has a polycyclic history. The oldestPermo-Triassic Gondwana sediments are deposited in thecontinental grabens. The next phase of basin developmentin end-Jurassic leads to the development of horst grabenfeatures along pre-existing weak zones, which is superposedon the existing NW-SE-trending Gondwana tectonicelements. The synrift sedimentation began with depositionof non-marine sediments, and was followed by developmentof a delta system and a subsequent main transgression. Therift phase ended with the flow of Rajmahal lavas. Subsequentto rifting and volcanism during early Cretaceous, Mahanadishelf received continental-deltaic sediments in LateCretaceous-Paleocene times. Eocene period saw the firstcarbonates sedimentation and Oligocene is the period ofnon-deposition/erosion in the basin. Miocene witnessedregional subsidence and marine transgression both inoffshore and onshore, with very high subsidence rate inMiddle Miocene and younger periods. At that time, thebasin experienced tilting with west experiencing uplift andeast undergoing subsidence. This led to renewed influx ofclastics and progradation of deltaic sediments overtectonically altered Miocene section This continues till thepresent day.

The oldest Permian to Triassic Gondwana sequence isrecorded only from the outcrop. The overlying rift sequenceof early Cretaceous is divided into two units, separated byRajmahal basalt. The lower unit is represented by coal shale,while the upper one dominantly comprises sandstone(Athgarh Sandstone). Late Cretaceous to Recent intervalrepresents the passive-margin setup. The sequence is wellrepresented in the basin and based on unconformities, sixdistinct successions are identified viz. late Cretaceous,Paleocene, Eocene, Oligocene, Miocene and Pliocene.

In the late Cretaceous, the first marine transgressionis witnessed in Campanian. However, in some areas ofoffshore, the Campanian- Maastrichtian sandstone and shalesuccession is non-marine. Early Paleocene- Danian is absent,and the Late Paleocene marine sediments are encounteredin the subsurface of several wells. The Eocene is representedby inner-shelf facies. In some wells late Eocene is absent.The succession comprises dominantly of carbonates withthin sandstone in the upper part.

In most parts of the basin especially onland and shallow-water areas, Oligocene has not been encountered. In thedeeper offshore, however, outer shelf facies is prevalent.During Oligocene, due to the collision of Indian and Tibetanplates, the basin experienced tilting towards southeast withconcomitant sea-level fall and subaerial erosion in the shelfregion. The Miocene succession is present throughout thebasin and depicts alternation of sandstone and shale overmost of the areas. In some wells, however, localizedcarbonates are also seen. The Pliocene succession is met inall the wells drilled in the basin and represented mainlyby finer clastics in the inner-to-middle shelf setup.Hydrocarbons of possible biogenic origin have so far beenencountered in the Plio-Pleistocene channels within sandreservoirs. In the shelf of Mahanadi Basin, no thermogenichydrocarbons has been encountered. Thermal maturitymodeling suggests the threshold of maturity in the south-

southeastern part in the passive margin setup, especiallythe Eocene-Oligocene successions.

Bengal Basin

The Bengal Basin lies within the state of West Bengaland covers Bangladesh and northern part of Bay of Bengal.It encompasses an area of 90,700 sq km. It is demarcatedby Indian shield in north and west and Surma Basin to theeast. Singhbhum-Chotanagpur massifs with isolated lowerGondwana as outliers are exposed in the western part(Fig. 13). The Rajmahal Traps are exposed in the northwesternpart.

It is a divergent margin basin, resting orthogonallyover intracratonic Damodar graben with Permo-Triassicsediments and Rajmahal Traps. A thick succession of lateCretaceous and younger succession is deposited over theeroded Gondwanas with several intervening non-depositional hiatuses. The rift and post-trappean phase oftectonic development is recognised. During the initialextensional tectonics, continental-fluvial and lacustrinesediments were deposited in a graben setup. Large amountof basic lava erupted through the fractures as a consequenceof crustal distension accompanying the rifting. The majoreruptive centre was in Rajmahal hills.

Subsequently, the basin was exposed to a passive marginsetup, depositing huge thickness of post-trappean sediments.During this relatively tectonically stable period, four distinct

Fig. 13: Geologic map of the Bengal Basin with the principaltectonics superimposed


tectonic zones viz. basin margin, wide shelf, shelf-slopebreak and deep basin are prominent during Eocene, thoughthese elements are not prominent in late Cretaceous andPaleocene. The oldest graben-fill Gondwana sedimentsrepresent deposition in fluvial to glacio-lacustrine setupduring Permian. Excellent biostratigraphic control hasenabled identification of unconformities of variousmagnitudes to divide the sequence into four units. Thelower Sakmarian (early Permian) succession comprises theTalchir unit, the overlying fine-grained sandstone, shaleand coals of Barakar represented the Artinskian age. Thethird Late Permian (=Raniganj) succession is representedby sandstone, shale and coal seam and the uppermostMiddle Triassic, supra- Panchet succession is characterizedby reddish brown siltstone and sandstone.

The rift-fill sequence is represented by Rajmahal Traps.In the subsurface, presence of rift-fill sediments is yet tobe confirmed. The Athgarh sandstone (late rift fills ofMahanadi) has no homotaxial sediments recognized here.The late Cretaceous period represents the first marinetransgression in the basin and is represented by silty shalein shelf and deeper areas. In the basin margin however, thepresence of coarse clastics suggests continental environment.Paleocene is represented by shale, siltstone, sandstone andimpure limestone. Regional marine transgression and lesserclastics characterize the Eocene thick carbonates in thebasin. The interbedded clastics suggest fluctuating sea levelduring the deposition. Oligocene sequence is representedby sandstone with minor shale, which becomes more shalyin the basinal part. The thick Miocene succession representsdeposition in a fluvial setup and shallow-marine set up uptoshelf edge. The Pliocene and younger succession representsprogradational sequence with thick delta sands depositedby large influx of sediments combined with relative sea-level fluctuation during this period. Numerous hydrocarbonshows, majority gases, are distributed in all sequences fromCretaceous to Pliocene. The source-rock studies indicate thePermian, Paleocene and Eocene succession to be maturewith dominantly type-III organic matter.

NORTHEAST BASINS

Assam and Assam-Arakan Basin

This basin extends over a large area of NE India,Myanmar and Bangladesh, covering an area of over 0.1 Msq km. The basin in its deepest parts has accumulated morethan 15-20 km of Mesozoic and younger sediments. It hasa polycyclic sedimentation history with distinct episodesof tectono-sedimentary evolution, and with phases ofsuperposing tectonics presenting a complex picture(Fig. 14).

Earliest Permo-carboniferous sedimentation record waswithin intra-cratonic grabens along Precambrian weak zoneswith sediments derived from highland as alluvial fans anddelta settings during early Permian. At this time, India wasa part of Gondwanaland continent. This was followed bylater rifting, northward movement, and emergence of passivemargin setting and collision of Indian and Eurasian platesleading to rise of Himalayas. During the second phase ofrifting in early Cretaceous preceding the India-Antarctica

separation, the sedimentation was largely over partiallyexhumed older sediments of Gondwana grabens. A limitedthickness and extent of rift fill sediments suggestsedimentation in distal grabens away from main rift. Endof rift was marked by lava flows and discrete sedimentationin early Albian, when the Indian plate detached completelyfrom Antarctica and entered drift phase. The drift phasefrom late Cretaceous to Oligocene represents passive marginsetup that continued until the collision of Indian plate withEurasian and Burmese plates. The fourth and final phaserepresents deposition of foredeep sequence of Miocene toRecent age in the foreland tectonic setting, and developedas a pro-arc basin in front of Himalayas and Indo-Burmaisland-arc system, overriding the passive margin of Assam.This is overlain by the ongoing sequence from late Pleistocene(~1.81Ma) deposited during the superposing phase ofHimalayan foreland system over Naga foreland. High-energy sedimentation from Himalaya has covered the Kopilivalley with an approx.imate thickness of 3200 m.

Early Permian sediments outcrops in Garo Hills andin the subsurface from the logs of wells in the south Assamshelf, comprise coarse pebbly sediments, conglomerates,granitic or feldspathic washes in lower part and sandstone,shale in upper part. Early Cretaceous sediments comprisingmainly of clastics deposited in continental to shallow-marinesetup, are often covered by basaltic traps – the Sylhet andMikir Traps (equivalent of Rajmahal traps). These exposuresare encountered in south Shillong plateau, while insubsurface it is encountered in several wells of south AssamShelf. The sequence consists mainly of fine-to coarsesandstone, dark shale with occasional coal. The outcropsof late Cretaceous are reported from Garo hills, south Shillongplateau and met only in a well drilled in Shillong plateau.It is characterized by alternation of conglomerate andsandstone at the base and massive sandstone towards the

Fig. 14: Tectonic map of Assam and Assam-Arakan Basin


top. The Paleocene-Oligocene accretionary prism fromBengal to Assam are met in south Shillong plateau, Cacharhills, Mikir hill flanks, Schuppen belt outcrops, and in thesub-surface of Assam shelf, and consists of sandstone andshale with occasional coal in proximal part and shale andcarbonates in the distal part with thin conglomerates at thebase.

The youngest foredeep, Miocene to Recent sequenceis widely recorded from outcrops and subsurface.Lithologically, it comprises dominantly argillaceous facieswith distinct intervals of sandstone/siltstone in lower partoverlain by immature coarse sands and arenaceous facieswith subequal proportion of claystone, clay and silts. TheAssam geological province is endowed with hydrocarbonaccumulations in multiple reservoirs of different ages. Mostof the oil reservoirs are within Mio-Pliocene, Oligocene,Eocene and Paleocene and even fractured basement. Fromthe source-rock perspective, the Gondwana graben withlimited aerial extent has no source rock potential. The EarlyCretaceous rift fill although has good quality organic matterbut is found to be immature to marginally mature. Thesediments within passive margin possess fair to good maturesource- rock characteristics.

CENTRAL INDIAN BASINS

Ganga Basin

The vast alluvial sediments of Ganga River and itstributaries occur between rugged upland of PeninsularIndia and the rising mountain province of Himalaya. ThisIndo-Gangetic plain represents a deep (>8 km thicksediments). In this still deepening foreland basin, sedimentsrange in age from Mesoproterozoic to Recent (Fig. 15). TheBasin extends from Delhi-Kalka ridge in the west, to theMonghyr-Saharsa ridge in east, and covers an area of~ 0.3 M sq km.

Four main phases of geological evolution are recognized,and basement tectonics plays an important role during thesedimentation and deformation of the basin. The basinevolved through multiphase geological history initiatedwith deposition of Mesoproterozoic and Late Neoproterozoicsediments, followed by deposition of shallow-marinesediments in Ediacaran-Early Devonian, to passive marginsedimentation during late Eocene- Oligocene and finally tothe foreland-basin deposition since Middle Miocene. TheBundelkhand massif (granite) constitutes the basement forsedimentation in Ganga Basin. The tectonic setup representsa Proterozoic to Lower Paleozoic basin, superimposed byCenozoic sedimentation. Four tectono-sedimentation cyclesare recognized. These are Mesoproterozoic, lateNeoproterozoic-Lower Paleozoic, Paleogene and Neogenesequences. The marked feature of the basin is the presenceof several depressions in the northern part, separated byprobable Precambrian promontory extending from Indianshield. The Mesoproterozoic sequence represents epigrademetamorphic rocks characterized by phyllitic shale andorthoquartzite. Ediacaran Lower Paleozoic sequenceuncoformably overlies this and represents deposition intypical platform limestone- orthoquartzite-shale facies. Endof this sequence is marked by a phase of compression during

Hercynian orogeny (?). This was followed by a period ofextensive erosion and deposition of Paleogene sedimentsin passive-margin continental setup. This is followed bythick blanket of overfilled siliciclastics sediments of Mioceneand younger age followed by alluvium. This sequenceforms a gentle homocline dipping towards the foothillswithout much structural deformation. The southern limitof this deposition shifted progressively southward withtime so that along the southern margin of Ganga Basin,where the youngest part of this sequence and even alluvium,rest directly over metamorphic basement. No commercialhydrocarbon has been discovered in Ganga Basin so far,even though hydrocarbon shows have been encounteredin few of the wells. Potential structural and stratigraphictraps are well developed, and good quality, highly fracturedreservoirs have also been identified. The extent and burialhistory of potential source rocks remain unclear. TheEdiacaran, Lower Ordovician sequence has good TOC, withthin organic-rich beds with good to fair hydrocarbongeneration potential.

Purnea Basin

The Purnea Basin is located in eastern part of Bihar andnorth Bengal, and bordered in south by hills of Rajmahalvolcanics and to the north by the Siwaliks along sub-Himalayan foothills. Further, it is bounded by Bhawanipurfault in west and Kishanganj fault zone in east. The southernlimit of the basin, covering ~18000 sq. km. remains undefinedin view of its possible continuity with Bengal basin. Thesedimentary package in the basin, comprising Gondwanaand Cenozoic sediments, directly overlies the crystallinebasement. The basin located between northwest Gangabasin and southeastern Bengal basin, shares its geologicalhistory with both (Fig. 16).

The geological history of the basin initiated with earlyPermian Gondwana fragmentation. The absence of basalticmagmatism suggests that the depression had not yetprogressed to the stage of rifting. Based on all availableevidences, Purnea basin is inferred to be a polyhistory basin,an initial rift setting followed by a drift setting culminatingin a foreland setting. Precambrian gneisses andmetamorphics of Indo-Australian plate constitute thebasement for deposition of Paleozoic-Mesozoic graben-fillrift sequence, referred as the Gondwana sequence. LateJurassic-early Cretaceous breakup of Gondwana land,resulted in cessation of Gondwana sedimentation, whichwere later folded, uplifted and eroded. Subsequently, as theIndian plate began its northern movement across Tethyanocean (~127 Ma), this basin was perched on the northernedge of Indian plate. At some stage, during collision withKohistan/Ladakh Island arc, the basin was downthrownalong a E-W fault and developed accommodation space fordeposition of early Paleogene in the Tethyan realm, at theleading northern edge of drifting Indian plate. With theonset of Himalayan orogeny, the north of the basinexperienced development of flexural foreland depressiondue to tectonic overloading of sediments. It was inthis setup, that the third sedimentary sequence withprovenance in the rising Himalayas to the north, has beendeposited.


Fig. 15: Top: Geological map around Ganga Basin. Bottom: Tectonic map


The Gondwana rift sequence spans the duration ofAsselian to Ladinian (Permian to Triassic) and presentthroughout the basin. The Gondwanas do not extend to thewest in Ganga Basin and the lower Paleozoics of GangaBasin does not extend in Purnea Basin. It comprises initialglacio -marine depositional system followed by essentiallyfluvial and lacustrine sediments.The Paleogene driftsequence is a thin sedimentary package of Eocene,compressed between underlying Gondwana and overlyingNeogene foreland sequence. This sedimentary sequence,yet to be encountered in any well in the basin, is discerniblein some seismic sections in northwestern part of the basinadjoining Nepal. An essentially shallow-marine depositionalsystem is conceptualized for this sequence. This sedimentarypackage is deposited in the foreland depression duringMiddle Miocene to Sub-Recent times, in response to therising Himalayas. The sequence is present throughout thebasin and the fluvial deposition is the dominant mechanismof sedimentation for the sequence. Geochemical studiesindicate source potential in the Permian sequence. Highsubsidence during the early phase of rift, along withoccurrence of anoxic zones in a lacustrine system is postulatedas the possible causes for the observed good-quality sourcefacies in the basin. The reservoirs are dominantly clasticsbut are of poor quality due to low porosities.

Fig. 16: Geology and the tectonic setup of Purnea Basin

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phanerozoic petrol basin

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