Phanerozoic Petrol Basin

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<p>Phanerozoic Petroliferous Basins of India</p> <p>253</p> <p>Phanerozoic Petroliferous Basins of IndiaP.K. BHOWMICK*</p> <p>Keshav Deo Malviya Institute for Petroleum Exploration, Oil and Natural Gas Commission, 9, Kaulagarh Road, Dehradun - 248 195 The term Phanerozoic or revealed life, which is applied to Paleozoic, Mesozoic and Cenozoic eras, represents a relatively brief geological period of half-a-billion years that constitutes the age of multi-cellular animal life on Earth, and which left a detailed fossil record and built up complex and diverse ecosystems. Though there was complex Late Precambrian (Vendian/Ediacaran) biota, the Phanerozoic period defines the period of development and evolution of higher groups of organisms like arthropods, mollusks, vertebrates, etc. The sedimentary basins of India have received attention of geoscientists due to increased activities for petroleum exploration since 1950s. Sophisticated geophysical technique together with drilling made it possible to obtain vast amount of subsurface data, and tied wherever possible with surface geology. These gave new insight into the mechanisms of various types of basin formation and their relationship with the different petroleum occurrences. In India, hydrocarbons have been struck in different basins ranging in age throughout the Phanerozoic. The most relevant feature has been the occurrence of most of the commercial hydrocarbons within the Cenozoic successions both on land and offshore. The Mesozoic, comprising a shallow marine Jurassic succession in Kutch-Rajasthan and poorly developed shallow shelf facies of the Cretaceous in most parts of the eastern coastal basins between Assam and southern tip of India, have had relatively lesser hydrocarbon finds till date. We have, however, the Gondwana successions in several parts of the country which contains rich coal deposits where exploration for coal bed methane (CBM) has taken off in a big way. Not all the Phanerozoic sedimentary basins in India are equally prospective, as borne out by the oil exploration programme and a small number of basins produce most of the hydrocarbons. Proven petroliferous sedimentary basins with commercial production include the Mumbai offshore, Cambay, Assam-Arakan, Cauvery, KrishnaGodavari and Tripura-Cachar basins. Sedimentary basins with known occurrence of hydrocarbons but lacking commercial production include Andaman-Nicobar, Bengal, Mahanadi, Himalayan Foothills and Rajasthan basins. Basins which on general geological grounds appear fairly prospective but where significant amounts of hydrocarbon have not yet been found, the Kutch-Saurashtra and KeralaKonkan basins. Lastly, basins like Arunachal Foothills, Deccan syneclise, Ganga Valley, Karewa (Kashmir Valley), Mizoram-Manipur and Narmada basins, could still be considered in the initial phase of exploration. The geoscientific studies by ONGC in the petroliferous basins of India from are summarized in the present contribution, and* E-mail:</p> <p>takes into account, the interpretations based on real well data, the seismic and other geophysical information, multimicrofossil bio-stratigraphy, sedimentology and geochemistry. More specifically, we deal with, 1. Sedimentary petroliferous basins along the Western margins: viz. Rajasthan, Cambay, Kutch, Mumbai Offshore and Kerala- Konkan 2. East Coast Basins: Cauvery, Krishna-Godavari, Mahanadi and Bengal basins 3. Northeast Basins: Assam and Assam-Arakan basin 4. Central Indian Basins: Ganga and Purnea basins Each basin has been discussed with reference to its basin architecture, sedimentary fill, major unconformities, sequences and petroleum system, in light of latest understanding of these basins and their hydrocarbon prospectivity. SEDIMENTARY PETROLIFEROUS BASINS ALONG THE WESTERN MARGINS Two important phases of basin evolution are recorded along the western continental margin. There is an initial record of the Mesozoic basins in Jaisalmer, Bikaner-Nagaur, Barmer (Rajasthan Basins), Kutch, and Cambay that could be traced to the Mesozoic basins occurring along the western margin of Madagascar. (Fig. 1). The first stage of separation of the Western Gondwanaland (South America and Africa) from the Eastern Gondwanaland (Madagascar, IndiaSeychelles, Antarctica and Australia) is recorded during Late Triassic/Jurassic (~196-203Ma) and is closely associated with Karoo volcanism in South Africa, the conjugate of which is seen in Antarctica. The second stage was the separation of Seychelles-India from Madagascar in Late Cretaceous (~93Ma), associated with minor volcanism found on conjugate margins of southwestern India and southeast Madagascar. The final breakup of Seychelles at KTB (~65Ma) contiguous with the Deccan volcanism is associated with a series of rift basins along the western continental margins in which the Cenozoic basins evolved viz. Barmer basin, Cambay Basin and its southern continuation in Mumbai Offshore. A description of these basins is given from the north to the south. Basins in Rajasthan The western Rajasthan shelf located to the west of Aravalli ranges, possesses three important basins viz., Jaisalmer, Bikaner-Nagaur and Barmer, stretching over an area of about 1,20,000 sq. km. The Jaisalmer Basin This is the westernmost is separated from the BikanerNagaur basin (Fig. 2) by the Pokaran-Nachana high to the northwest and from the Barmer basin by the Barmer-DevikotNachana high in the south. A pronounced NW-SE-trending</p> <p>254</p> <p>Glimpses of Geoscience Research in India</p> <p>Fig. 2: Structural map of western Rajasthan Fig. 1: Paleo-tectonic reconstruction of the Madagascar-Seychelles India during the Mesozoic, showing the structural trends, the Jurassic outcrops (in grey) and the Mesozoic rifts (in red) regional step-faulted Jaisalmer-Mari high zone, marked by the Kanoi and Ramgarh faults that traverse the centre of the basin and divides it into the Shahgarh sub-basin, the Miajalar sub-basin and the Kishangarh sub-basin. This basin in the northwestern Indian shield extends as far as the Mari region of Pakistan, and is tectonically related to the Indus Basin from the beginning of the Triassic. The aerial extent is over 30,000 km2.This basin is controlled by wrench-fault tectonics. The outcrop and the well-data indicate sedimentation from the Cambrian to the Tertiary (Fig. 3). Three distinct sequences are identified, based on the basin forming tectonic events namely, the Proterozoic to early Cambrian rift sequence comprising mainly the thickly bedded sandstone with shale intercalations in the lower part and dolomitic and cherty limestone with shale and sandstone interbeds in the upper part, Permian to Eocene shelfsag, and Pleistocene to Recent sequences. The Proterozoic-early Cambrian succession unconformably overlies the Precambrian basement rocks, while the unconformable upper contact of the sequence with Permian, has a hiatus span of about 190 Ma. Exploration by ONGC &amp; OIL in Jaisalmer basin has resulted in discoveries of several gas fields, namely Mahera Tibba from the Cenozoic and Cinnewala Tibba from Cenozoic and Cretaceous reservoirs, respectively. The gas from these fields is methane-rich and commercially viable. Gas from the other fields viz. Ghotaru, Bankia, Bhakari Tibba, Khartar and Sadewala is nitrogen-rich and is commercially not viable. Oil shows have also been reported in wells Chinnawala Tibba-1 and Ghotaru-2 from early-late Cretaceous sequence, though as of now there is no commercially viable discovery of liquid hydrocarbons. The potential source-rock layers are from Late Jurassic and early Cretaceous. Genetic correlation of known accumulation of gases in Cenozoic and Cretaceous reservoir suggests that they have been generated from sediments at higher maturity and have been trapped at their present locale after longdistance migration. The Bikaner-Nagaur Basin is mostly a Paleozoic basin with 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 and reservoir rocks have been identified in the Paleozoic sediments. The Cenozoic sedimentation in the BikanerNagaur basin began with the coal-bearing Palana Formation that was deposited during the Paleocene in subtropical swampy conditions on the continental part. Marine sedimentation indicates encroachment of the sea during the Upper Paleocene to Lower Eocene. The exploration drilling by ONGC, Oil India and a Joint Venture Private Company has indicated presence of heavy oil in haline/carbonates. Baghewala, Nanuwala and Binjybala areas have indicated hydrocarbon occurrences. The Barmer Basin is interpreted as a narrow, N-Strending graben, a northern extension of Cambay rift. The faults exposed at Fatehgarh, on Barmer Hill near Barmer and at Sarnu constitute the peripheries of the Barmer Basin. The pre-rift sediments deposited on the Late Proterozoic Malani Igneous Suite, represented by Randha, and Birmania Formations (a siliceous facies-shales, sandstone, orthoquartzite) and calcareous facies-limestone, phosphorites and dolomudstone) respectively, are exposed on the western margin of the basin. The Sarnu Formation (co-relatable with the sandstones of Jodhpur Group) exposed on the eastern margin of the basin, comprises thin and fining-upward sand bodies with intervening red siltstone.</p> <p>Phanerozoic Petroliferous Basins of India</p> <p>255</p> <p>Fig. 3: Stratigraphy of the Barmer, Bikaner-Nagaur and Jaisalmer Basins The Lathi Formation exposed at the northern periphery the basin, comprises medium to coarse, fining upward fluvial sands with fossils. The syn-rift sediments: Barmer Hill and Fatehgarh Formations are exposed at Barmer and Fatehgarh. The Barmer Hill Formation comprises sandstone and clast supported conglomerates, exposed along the western boundary of the basin and rest unconformably on the basement. These represent rapid deposition in an alluvialfan environment with source from Malani Rhyolite. The Fatehgarh Formation exposed at the northern boundary of the basin shows a mixed sand and mud tidal-flat environment. It comprises conglomerate at the base, overlain by sandstone. This in turn, is overlain by ferruginous phosphatic sandstone. Seismic data reveals that in subsurface Fatehgarh Formation continues as Barmer Hill Formation. The Fatehgarh Formation is overlain by siliceous earth of Bariyara Member (base of Mataji-ka-Dunger Formation). The post-rift sediments are constituted by the Mataji-kaDunger and Akli Formations. The Mataji-ka-Dunger Formation is exposed at the northern and western margins of the basin and consists of cyclically arranged claystone, siltstone, sandstone, and is interpreted as a shallowingupwards fluvio-deltaic complex. The sandstone shows 15% visible porosity with no secondary infill except authigenic quartz overgrowth, signifying itself to be a good reservoir rock. The base of the sequence exposed at the south of Fatehgarh comprises sand-poor claystone. This is interpreted as pro-deltaic, delta-slope/delta-mouth deposition. The Akli Formation exposed at the central part of the basin, overlying the Mataji-ka-Dunger Formation comprises bentonitic claystone, grey bituminous clay-stone, lignites and light yellow claystone. Widespread exploration work by various oil companies notably Shell and then Cairn Energy, have resulted in a number of oil and gas discoveries mainly within the Paleocene sediments. These are given in Figure 4.</p> <p>256</p> <p>Glimpses of Geoscience Research in India from northwest brought arenaceous sediments in the northern areas of Mehsana and deposited these under tidal regime on the downthrown blocks. The top of Paleocene marks the end of rifting followed by an unconformity over which Early Eocene transgression is observed in the southern part of Cambay Basin. Eocene to Recent post-rift succession, overlying the Paleocene with an unconformity is encountered throughout the Cambay Basin. Early Eocene facies is marked with the deposition of grey shale, silts and coals. The top of early Eocene witnessed regression during which protoSaraswati river from NW deposited deltaic sands and silt, shale and thick coal bands. Early/Middle Eocene boundary is marked by an unconformity followed by a renewed transgression in Middle Eocene during which deposition of sandstone, siltstone, shale and coals took place in tidal to estuarine setup. Upper part of Middle Eocene and late Eocene witnessed regression represented by a mixed facies of laminated sandstone, with limestone nodules and occasional carbonaceous shale. Early Oligocene succession overlies the Eocene strata with a brief hiatus. Early Oligocene transgressive sediments include grey shale with claystone, argillaceous sandstone. This is followed by a major regression and unconformity spanning the entire duration of late Oligocene. The Miocene succession in the basin is represented by current-bedded sandstone, conglomerates, clays and shale. This is followed by an unconformity above which claystone, sandstone, clay, sand and kankar of Pliocene to Recent age were deposited. Extensive exploration in this basin by ONGC since 1959 has resulted in numerous oil and gas discoveries, notably Ankleswar, Gandhar, Nawagam, Dholka, Kalol, Sanand, Jhalora, Viraj, Kadi and S. Kadi, N. Kadi, Jotana, Sobhasan, Balol, Santhal, etc.</p> <p>Fig. 4: New Discoveries in Barmer-Sanchor. Review of geological model time to time and its implementation in exploration in one of the largest onland discovery of the country. CAMBAY BASIN The Cambay Basin, the southern continuation of the Barmer-Sanchor Graben is a narrow elongated (NNW-SSE trending) intra-cratonic rift basin (area 59,000, situated between Saurashtra craton to the west, Aravalli swell on the northeast and Deccan craton to the southeast. In the south, it extends into Cambay Gulf and ultimately into the Arabian Sea. A large part of the basin is covered by Quaternary sediments. Cenozoic outcrops are rare and occur only on the fringes of the basin. The extensional architecture of the basin is defined by three major Precambrian trends viz., NNW-SSE trend related to Dharwarian orogeny NE-SW trend related to Aravalli orogeny, and ENE-WSW trend related to Satpura orogeny. The basin is sub-divided into four major tectonic blocks Sanchor-Patan, Ahmedabad-Mehsana, Tarapur-Cambay and Jambusar-Broach, by cross-trending NE-SW. trending highs (Fig. 5). Each of these blocks is characterized by local lows which form the source areas. The pre-rift Late Cretaceous sediments are deposited in continental, fluvial setup and comprise sandstones with minor claystone, basal conglomerate and thin inter-beds of shale. The syn-rift Paleocene sequence, bounded by unconformities below and above, is either absent or thinly developed over paleohighs and restricted only to fault grabens and fault scarps. These sediments extend as piedmont f...</p>