Petroleum production basins of India.pptx

Description of oil producing basins of india

1. Petroliferous basins of India

2. Category-I Category-I: 7 basins are grouped under Category-I covering 30% of total basinal area and holding 85% of the total unrisked conventional hydrocarbon in-place of 41.8-billion-ton oil and oil-equivalent gas. These 7 basins are namely Krishna-Godavari (KG), Mumbai Offshore, Assam Shelf, Rajasthan, Cauvery, Assam-Arakan Fold Belt and Cambay. These basins are fairly appraised to the extent of 47% of country’s total appraised area (1.6 million square kilometre ) with 65% of country’s total active operational area (0.3 million square kilometre ).

3. Category-II Category-II basins cover 23% of the total basinal area, holding 9% of the total hydrocarbon in-place. 3 basins fall in this category and those are namely Saurashtra, Kutch, Mahanadi and Andaman. These basins are moderately appraised to the extent of 22% of country’s total appraised area with 26% of country’s total active operational area.

4. Category-III and Category IV Category-III and Category IV basins cover 47% of total basinal area, holding 6% of the total hydrocarbon in-place. Sixteen basins fall in this category and those are namely Himalayan Foreland Basin, Ganga Basin, Vindhyan Basin, Kerala-Konkan-Lakshadweep Basin, Bengal Basin, Karewa basin, Satpura -South Rewa -Damodar Basin, Saurashtra Basin, Narmada Basin, Panhita -Godavari Basin, Spiti -Zanskar, Deccan Syneclise , Bhima- Kaladgi Basin, Cuddapah , Chhattisgarh Basin and Bastar . These basins are appraised to the extent of 31% of country’s total appraised area with 9% of country’s total active operational area. As on 31 March 2018, India had estimated crude oil reserves of 594.49 million metric tonnes .

5. Cambay Basin The Cambay basin is an Intracratonic rift graben basin in the form of a N-S trending graben flanked in the east by the Aravalli Swell and Deccan Plateau and in the West by Saurashtra Plateau, opening to the south through the Gulf of Cambay into the Arabian sea and extending northward to the Sanchor Basin (Babu, 1977). Cambay basin is essentially of Tertiary age with mainly clastic sediments ranging from Eocene to Recent deposited over Late Cretaceous-Paleocene basalts known as Deccan Traps. Only a few wells drilled in the marginal area of the basin have penetrated to Pre Cambrain basement through intervening sequences of ferruginous sandstone (Biswas, 2012). Hence information on the sequences below the Deccan Trap is very scanty. The oil production averaging 3.5 million tonnes per annum comes from the sandstone reservoir of Eocene and Oligocene age. Fifty-one fields have been discovered in the Cambay basin as on 1.1.1982. Of these, 27 are oil fields, 10 oil and gas and 10 are gas fields. Ankleshwar oil field is noteworthy as the first giant oil field discovered by ONGC (1960) where more than 230 wells have been drilled and oil production of more than 49.35 million tonnes has been obtained. The reservoir rocks of Cambay basin have been attributed to environments ranging from shallow marine to deltaic s (Raju et al., 1971; Das et al., 2006). The prognosticated estimate of hydrocarbon reserves is now considered to be 900 million tonnes . Of this, 554 million tonnes of geological reserves have already been proved. Future exploratory efforts with additional inputs of men and materials along with new concepts are expected to add significant reserves to the already discovered hydrocarbons. The generalised stratigraphy, geological history and petroleum geology of the basin in the figure (Bhandari, 1984).

6. Source Rock Thick Cambay Shale has been the main hydrocarbon source rock in the Cambay Basin. In the northern part of the Ahmedabad-Mehsana Block, coal, which is well developed within the deltaic sequence in Kalol , Sobhasan and Mehsana fields, is also inferred to be an important hydrocarbon source rock (Banerjee et al., 2002). The total organic carbon and maturation studies suggest that shales of the Ankleshwar / Kalol formations also are organically rich, thermally mature and have generated oil and gas in commercial quantities. The same is true for the Tarapur Shale. Shales within the Miocene section in the Broach depression might have also acted as source rocks (DGH, 2010). Reservoir Rock There are a number of the reservoirs within the trapwacke sequence of the Olpad Formation (Banerjee et al., 2000). These consist of sand size basalt fragments. Besides this, localized sandstone reservoirs within the Cambay Shale as in the Unawa , Linch , Mandhali , Mehsana, Sobhasan , fields, etc are also present (DGH, 2010).

7. Trap Rock The most significant factor that controlled the accumulation of hydrocarbons in the Olpad Formation is the favourable lithological change with structural support and short distance migration. The lithological heterogeneity gave rise to permeability barriers, which facilitated entrapment of hydrocarbons. The associated unconformity also helped in the development of secondary porosity. Transgressive shales within deltaic sequences provided a good cap rock. Timing of migration & Trap formation: The peak of oil generation and migration is understood to have taken place during Early to Middle Miocene (DGH, 2010).

8. Assam Shelf Basin: The alluvial plains of Brahmaputra River formed a part of shelf of the Assam-Arakan Basin. The general picture given by the seismic surveys indicates a broad arch at the basement level with its apex in the region of the present Brahmaputra River course and sloping towards the Himalayan foothills in the north and Naga Hills in the south. Similarly, for the slopes northeastward, the deepest part being in front of Mishmi Hills. This broad arch is dissected by a number of faults with a general NE-SW or ENE-WSW trends (Ray et.al., 1983). The Upper Assam Shelf is essentially a Tertiary basin where sediments ranging in age from Eocene to Recent have been encountered in subsurface. The outcrops of Palaeogene and Miocene are limited to SE and SW in the Naga foothills, schupen belt and south of Mikir hills. This basin has a long exploration and production history. Digboi oil field has been producing oil for almost a century from the Miocene Tipam Sandstones. In the Upper Assam Shelf, as a result of extensive exploration work, several structures have been delineated out of which commercial reserves have been established in Nahork atiya , Moran, Rudrasagar , Lakwa-Lakhmani , Geieki , Borholla-Champang , Amguri , Charli, Kusijan etc. The main producing reservoir rocks are sandstone within Barail and Tipam formations of Oligocene and Miocene age respectively. However, seme production from basal sandstone (basement), Sylhet limestone, and interbedded sands in Kopili shales is also reported. The oil accumulations are mainly in structural traps. Some stratigraphic traps in Barail and Tipam formations are also known. The prognostic reserves of the basin are estimated at 1790 million tonnes of these 931 million tonnes belong to the area with Oil India Limited. The geological reserves of oil and gas equivalent of oil discovered so far are 790 million tonnes of which 343 million tonnes belong to oil and gas fields discovered by ONGC in Upper Assam Shelf and Nagaland areas (Bhandari, 1984).

9. Anticlines and faulted anticlinal structures sub parallel to and associated with the northeast trending Naga thrust fault, are the primary traps. Sub thrust traps are probably present below the Naga thrust sheet. There have also been stratigraphic trap discoveries, such as Dholiya gas field, described as an Oligocene Barail clastic depositional lens, and Hapjan and Sarojani oil fields identified as Barail depositional sandstone lenses. Two major tectonic grains are imprinted on the Assam shelf. NE-SW is the older grain whereas east north east- west south west (ENE-WSW) is the younger one. These trends have been reactivated during foreland tectonic phase. ENE-WSW fault show differentiation as well as strike slips movement. The basement inversions in north Assam shelf and south Assam shelf started forming subsequent to Tipam deposition. Traps

10. The major trap formation has happened during the close of Girujan deposition nearly 1.8Ma. Separation and sinistral development across Jorhat fault confined the Mikir forebulge isolating north and south Assam rack. From there on north and south Assam carried on diversely across Jorhat fault during the superposing period of the Himalayan foreland. In north Assam, the slant turns around to NW, which is a causative factor for storm cellar angling, where as in south Assam Mikir upliftment kept causing separation across a large portion of the faults partitioning the zone into fault blocks. The compensating antithetic fault along major faults provided gave trapping mechanism.

11. Mumbai Offshore Basin Mumbai Offshore basin is a seaward extension of Cambay basin. To the east are the Western Ghats and to the north is the Sausashtra platform both comprising of Upper Cretaceous Paleocene Volcanics known as Deccan Traps (Basu et al., 1980). The southern limit is marked by a basement arch of Panjim. Tertiary sediments in excess of 5000 m are deposited over Deccan Trap volcanics in the basin depo- centres (Goswami et al., 2007). However, the thickness of sediments on the major basement highs such as Bombay High is considerably less. Archean inliers are exposed over basement uplifts and the presence of Mesozoic the sediments have not yet been recorded. The sedimentary sequence can be broadly divided into (1) lower basal sand, lignite and clays (2) middle-limestone and shale alternations and (3) upper-shale and claystone. Hydrocarbons occur in the basal sands of the lower unit (Paleocene-Early Eocene), and in the micrite, biomicrite and chalky limestone of the middle unit (Early Eocene-Middle Miocene). The Middle carbonates provide the principal reservoirs in Mumbai High oil field. This is a giant field, discovered in 1974, located 160. kms west-northwest of Mumbai in about 70 m of water depth followed by many small oil/gas fields. Today it provides about 66% of the country's oil production(Goswami et al., 2007).

12. The structures are mostly developed by the draping of sediments over palaeohighs while some are fault bound. The development of porosity in limestone is selective and not uniform. It is mainly intergranular, intra-granular, moldic , vuggy and micro fissured. Prognosticated reserves of 5.5 billion tonnes of oil and gas equivalent in oil have already been established and geological reserves of the order of 2.5 billion tonnes of oil and gas in oil equivalent have already been proved. Thus, vast reserves remain to be discovered which would add substantially to the already proven reserves. The main exploration objectives in the basin are faulted upthrust blocks, sedimentary wedges, carbonate build ups and also stratigraphic traps associated with delta build up in the northern part of the basin. The basin has a NW-SE-trending horst-graben geometry. The grabens are bounded by normal faults, and the horsts/ridges are dissected by NE-SW trending cross faults (Biswas, 1982; Gopala Rao, 1992; Pandey and Pandey, 2015). On the basis of its structural configuration and its nature, as well as the type of sediment fill, the basin is divided into six tectonic blocks: TaptiDaman , Diu, Heera-Panna-Bassein, Bombay high-DCS, Ratnagiri, and Shelf Margin blocks.

13. The main Mumbai high block is surrounded by three depressions: • Surat depression (Daman, Purna and Navsari lows) and its southward extension through Mahim depression in the east. • Saurashtra low in the northwest. • Southern paleosink and Murad depression in the southwest.

14. Source Rock There are three major depocenters in the basin viz. Surat Depression in the north, Shelf Margin in the west Central and Vijayadurg Grabens in the south.

15. Caprocks: Shale encompassing the coarser clastic facies in the Paleocene section, widespread transgressive shale overlying the Middle Eocene Bassein Formation, alternation of shale and tight limestone over early Oligocene Mukta Formation, widespread intervening shale layers within Early Miocene Mumbai formation over Mumbai High and in Platform-Deep Continental Shelf (DCS) area, post Middle Miocene clay/claystone of Chinchini Formation over parts of Heera etc. had provided effective seal for the underlying hydrocarbon accumulations in the Mumbai offshore basin (DGH, 2010).

16. Entrapment Mumbai offshore basin has been endowed with a wide variety of entrapment situations like- structural closures with independent four way closures of very large, large, medium and small sizes, fault closures and faulted closures with effective fault sealing, strati-structural features like Paleogene wedges against rising flanks of paleohighs , mud mounds, carbonate build-ups, unconformity controlled traps, Paleogene and Neogene carbonate wedges against the rising Eastern and Jaygad Homoclines . Mumbai Offshore Basin Introduction Tectonic History Generalized Stratigraphy Petroleum System Petroleum Plays (DGH, 2010).