CHAPTER - 1 INTRODUCTION

Transcription

1 CHAPTER - 1 INTRODUCTION India is having a very special distinction, wherein the setup of various sedimentary basins, very rich in hydrocarbons both on Eastern and Western Offshore India can be compared to a invaluable gold necklace studded with Gems and Precious Stones adored on a Pretty Queen s neck. A careful attempt has been made to integrate the valuable data on various Indian offshore basins of both East and West Coasts. To understand the basin evolution of this type, the author feels that the reader should have a basic knowledge on the Plate Tectonics, especially with reference to Indian Plate Movements, with varied divergent passive continental margins and also convergent margins in geological times in the formations of sedimentary basins. Accordingly, a very comprehensive chapter is included on the Plate Tectonics and also a map on Tectonics of India depicting various tectonic elements. In the process of integrating valuable data in bringing out this volume, an attempt has been made to give an alternative interpretation wherever it is required, for better understanding the tectonics, basin evolution and inturn to focus on the future exploration needs of the country. One special chapter is included on the demanding alternate source of energy which is drawing the attention of the whole world that is the Gas Hydrates. Favourably Indian Offshore Basins are very rich in gas hydrates. Incidentally, the estimated amount of natural gas in the gas hydrate accumulations of the world greatly exceeds the volume of all the known conventional gas resources. Gas hydrate reserves are estimated to be more than double the known reserves of fossil fuel, as per C.N.R.Rao, founder and Honorary President of Jawaharlal Nehru Centre of advance research and Kuznetsov, Director of Organic Chemistry, Russia. Rightly, Government of India is focusing and concentrating its efforts on Gas Hydrate Exploration & Production and a road map is drawn to achieve the goal in the very near future. Since the scope and subject of this book is quite large and open, only a short brief with few details are given on the producing fields and also on oil/gas discoveries. 1

2 2 Geology, Tectonics and Hydrocarbon Prospects Off Shore Sedimentary Basins of India CHAPTER - 2 SEDIMENTARY BASINS OF INDIA The sedimentary basins of India onland and offshore up to 200 m isobaths have an aerial extent of about 1.79 million Sq. Kms. An additional area of about 1.35 million sq. kms. has been estimated for the deep waters beyond 200 m isobaths. The total area thus works out to be 3.14 million Sq. Kms. All together, 26 major sedimentary basins are grouped into four categories based on the geological history, tectonic evolution, accumulated sediments and on their present status of exploration and prospectivity (Fig. 2.1). The categorization was earlier made by ONGC, essentially to prioritise their exploratory efforts. These sedimentary basins have been in age from Proterozoic to Cenozoic. The classification of the four categories / groups is as under. Category I Basins Category I basins are proved petroliferous basins with commercial production: Basins included are Bombay Offshore, Cambay Basin, Cauvery Basin, KrishnaGodavari Basin, Assam Shelf, TripuraCachar Basin and Rajasthan Basin. Category II Basins Category II basins are sedimentary basins with known occurrence of hydrocarbons but not on commercial production. Such as Kutch, Mahanadi, Andaman & Nicobar, Himalayan Foot Hills and Bengal Basins. Category III Basins Category III basins are those, in which significant shows of hydrocarbons have not yet been found, but which on general geological considerations are assumed to be prospective. Himalayan Foreland, Ganga, Vindhyan, Saurashtra and KeralaKonkan fall under this category. Category IV Basins Category IV basins are those, which on analogy with similar hydrocarbon producing basins in the world are deemed to be prospective. The hydrocarbon potential of such basins in India is still uncertain and yet to be established. Karewa, SpitiZanskar, Satpura, Rewa Damodar, Narmada, Deccan Synclise, BhimaKaladgi, Pranhita Godavari, Bastar and Chhattisgarh basins are under this category. Besides the above four categories, Indian deep water basins located beyond 400 m bathymetry are exclusively categorised. The geology and prospectivity of these basins are getting unfolded as exploratory efforts, are intensified in these areas in the last decade and half. In the process, some of the basins which are in the lower category have been upgraded to the next category depending on the exploration efforts and success. For e.g. Cauvery, KrishnaGodavari, TripuraCachar, Rajasthan basins which were originally placed in CategoryIII 2

3 Sedimentary Basins of India 3 were upgraded to CategoryI. The detailed breakup of sedimentary basins as per the categorisation is shown in the Table 2.1. Fig. 2.1 (Courtesy DGH)

4 4 Off hore Sedimentary Basins of India Geology, Tectonics and Hydrocarbon Prospects Table 2.1: Categorisation of Sedimentary Basins Category UPTO 200 M ISOBATH Basin Basinal Area (Sq. Km) Onland Offshore Total I Cambay 51,000 2,500 53,500 Assam Shelf 56,000 56,000 Mumbai Offshore 116, ,000 Krishna Godavari 28,000 24,000 52,000 Cauvery 25,000 30,000 55,000 AssamArakan Fold Belt 60,000 60,000 Rajasthan 126, ,000 SUB TOTAL 346, , ,500 II Kutch 35,000 13,000 48,000 MahanadiNEC 55,000 14,000 69,000 AndamanNicobar 6,000 41,000 47,000 SUB TOTAL 96,000 68, ,000 III Himalayan Foreland 30,000 30,000 Ganga 186, ,000 Vindhyan 162, ,000 Saurashtra 52,000 28,000 80,000 KeralaKonkanLakshadweep 94,000 94,000 Bengal 57,000 32,000 89,000 SUB TOTAL 487, , ,000 IV Karewa 3,700 3,700 SpitiZanskar 22,000 22,000 SatpuraSouth RewaDamodar 46,000 46,000 Narmada 17,000 17,000 Deccan Syneclise 273, ,000 BhimaKaladgi 8,500 8,500 Cuddapah 39,000 39,000 PranhitaGodavari 15,000 15,000 Bastar 5,000 5,000 Chhattisgarh 32,000 32,000 SUB TOTAL 461, ,200 TOTAL 1,390, ,500 1,784,700 Deep waters KoriComorin 85 E Narcodam 1,350,000 GRAND TOTAL 3,134,700

5 Sedimentary Basins of India 5 The total hydrocarbon resources inclusive of deep waters are estimated at 28 billion tons of oil and oil equivalent of gas. As on April 2008, an initial inplace of 9.13 billion tons, of which recoverable reserves of 3.4 billion tons have been established indicating an upgradation of 32% of resources to reserve category. With more and more exploration efforts further upgradation can be achieved. However, the resources as estimated by DGH stands at 32 billion tons (O + OEG) Table 2.2. Table 2.2: Basinwise Hydrocarbon Resources (MMT) Basin Offshore Onland Total Mumbai 9,190 9,910 AssamArakan Fold Belt 3,180 3,180 Cambay 2,050 2,050 Assam 1,860 1,860 KrishnaGodavari ,130 Cauvery Rajasthan Kutch AndamanNicobar KeralaKonkan Saurashtra Offshore Ganga Valley Bengal Himalayan Foreland Mahanadi Deepwater * ,000 GRAND TOTAL 18,815 9,270 28,085 (After DGH) *Hydrocarbon resources in Deepwater off East Coast are likely to increase further by about 400 MMT as per interpretation results of reconnaissance surveys carried out by DGH. These resource potentials do not include the gas hydrate resources. The total prognosticated gas resources from the gas hydrates in the country is placed additionally at 1894 TCM. Indian Stratigraphy The stratigraphy of Indian basins in general as established by different authors is indicated in Table 2.3 & 2.4.

6 6 Off Shore Sedimentary Basins of India Geology, Tectonics and Hydrocarbon Prospects Table 2.3: General Geological Succession in different parts of India and BURMA Prof. M.S. Krishnan, 1943 Standard Scale Northern Himalaya Baluchistan Arc Salt Range & Potwar Recent.. Alluvium Sands, Loess Alluvium, Loess Pleistocene Alluvium, gravels, moraines Pliocene Miocene Middle Miocene Lower Miocene.. sands Siwalik Sy. Manchhar Sy. Mekran Sy. Flysch, Bugti B. Gaj S. Oligocene Nari S., Khojak Shales Eocene Eocene Kirthar S. Laki S. Ranikot S. Lower Chikkim S. Cardita Beaumonti B., Pab Sst. sands Kashmir Hazara Simla Garhwal Alluvium & C Alluvium & C moraines Karewa Siwalik Sy. Lower Karewa Siwalik Sy. Murree S. Murree S. Fatehjang zone moraines Assam Burma Peninsula Alluvium, River gravels gravels Alluvium, River gravels River terraces Siwalik Sy. Dihing S. Irrawaddy Sy. Kasauli B. Dagshai B. Tipam S. Surma S Barali S. (in part). Chharat S. Kirthar S. Laki S. Ranikot S. Giumal Sst. Parh Lower Chharat S. Hill Ranikot S. Giumal S. Orbitolina Subathu B. Barail S. Jaintia S. Sylhet Lst Disang S. Cherra Sst. Disang S. (in part) Pegu S. Lower Pegu S. Yaw and Pondaung Stages. Tabyin, Tilin, and Laungshe stages Axial S. (in part) Negrais S.... Lower Alluvium Khadar Bhangar, Cave deposits, river terraces Coastal Areas Alluvium sands Raised beaches, Porbander stone etc.. Siwalik Sy. Karaikal B. Quilon B. Cuddalore Sst. Etc.. Gaj s... Nari S. Dwaraka B... Deccan Trap and Intertrappeans, Lametas Deccan Trap and Intertrappeans, Lametas Kirthar S. Laki S. Ranikot S. Bagh B. Trichinopoly Umia B.. Umia B. Table 2.3: Contd 6

7 Sedimentary Basins of India Standard Scale Northern Himalaya Jurassic.. Spiti Shales Koto Baluchistan Arc Massive Crinoidal Salt Range & Potwar Kashmir Hazara Jurassic Spiti shale Jurassic Simla Garhwal Triassic.. Triassic.. U. Triassic Triassic Triassic? Shali? Krol S. Permian Productus B. Syringothyris Up. Carboniferous Middle & L. Carboniferous Permo Carboniferous Productus Speckled Sst. Conularia Po Series Fenestella Sh. and Eurydesma B. Boulder Bed Agglomertic Slates, Zewan B. Agglomeratic sh. Gangamopteris B. Tanakki Boulder B. Assam Burma Peninsula? Tal. S... Namyau B.? Loian B.? Infra Krol.?Blaini boulder Bed. Jubbulpore Kota Rajmahal.. Napeng B. Maleri Pachmarhi.. Plateau Moulmein Subansiri B. Fenestella sh. L. Plateau Devonian Muth Qtzt.. Muth Qtzt Devonian (Chitral)? Jaunsar S. Padaukpin Panchet Raniganj S. Barren Measures, Barakar S.? Umaria B. Wetwin Sh. Silurian Silurian.. Silurian Zebingy S. Namshim S. Ordovician Ordovician. Ordovician Naungkangyi Cambrian. Cambrian. Cambrian Cambrian? Deoban Algonkian Haimanta Sy. Attock Slates Archaean Vaikrita Sy.? Salkhala S. Gneisses Dogra Slates Simla Slates Chail S. Jotogh S. S. Mawson S. Bawdwin volcanic?mergui S. Buxa S. Chaung. Daling S. Magyi S. Darjeeling S. Shillong s. Mogok S. Gneisses Gneisses Talchir boulder B. Coastal Areas Katrol Chari Patcham Vindhyan Sy. Cuddapah Sy. Delhi Sy. Dharwar Sy. Aravalli Sy. Etc. Gneisses 7

8 8 Off hore Sedimentary Basins of India Geology, Tectonics and Hydrocarbon Prospects International statigraphic nomenclature, as established by International Commission on Statigraphy is shown in Table 2.5. Table 2.5: International Statigraphic Chart

9 Jagdish Pandey, 1998 Table 2.4