Unconventional Shale Gas Prospects in Indian Sedimentary Basins

Natural gas is rapidly substituting fuel to suffice the growing energy requirement of today’s world. The rising oil prices, high input cost in exploration and production of hard oil resources and relative abundance of gas resources have fuelled the interest towards gas resources. As the consumption of natural gas is increasing rapidly, it is essential to identify and develop the available energy resources. India has the immense prospects of unconventional shale gas resources. Commercial exploration of these shale gas resources can effectively make the global natural gas curve more elastic. The commercial shale gas exploration requires exhaustive investigations of all the shale rock properties of hydrocarbon bearing shale beds having significant organic matter and maturity. This paper presents overview of shale gas resources as well as the prospective shale gas horizons in Indian sedimentary basins. Introduction Natural gas plays the key role in increasing energy demand. Its environmental soundness and multiple applications across all sections imply that natural gas will continue to play an important role in meeting the energy demand. But the gap between natural gas demand and supply has been increasing day by day. To bridge the gap between energy demand and supply, it becomes essential to go for the available alternative energy resources in the country. This has spurred the interest towards unconventional resources i.e. Shale gas, Coal Bed Methane, Gas hydrates, tight gas etc. Shale gas is the future energy basket for the mankind which has tremendous prospect worldwide even in India (fig 1) only it requires systematic and proper methodology for its delineation, exploration and development. Particularly USA and Canada have been contributing commercial shale gas production which is 20% of total gas production. India have tremendous prospects of shale gas in different sedimentary basins i.e. Cambay, Assam Arakan, Krishna – Godavary, Gondwana, Cauvery, Vindhyan etc. though all shales are not evaluated yet. It requires exhaustive investigation of the prospectiveshale horizons having significant shale volume, organic matter (> 3%) and maturity (gas window zone). Source rock evaluation can be made by using parameters like Total organic carbon content, rock eval pyrolysis, elemental analysis, vitrinite reflectance, gas chromatography, etc. Petrophysical analysis using SEM, XRD etc can help in detail mineralogical as well as clay mineralogy studies. Seismic attributes help not only to differentiate the shale pay horizon but also to identify the brittle zone i.e. the sweet spots for hydrofracturing. Integrated logs with geological, geophysical, petrophysical, geochemical, geomechanical studies can help to identify the sweet spots for commercial shale gas exploration and exploitation. Unconventional Shale Gas System Shale is the fine grain sedimentary rock exhibits a wide variety of different geological ages, geographical areas and formations (Potter 1980). Unlike the conventional petroleum reservoirs, shale reservoirs are continuous (Jarvie 2007), low porosity ( 1,000 daltons) of component compound. With increasing depth and temperature gradient organic matter get matured and passes through the stages of oil window at 60–160 °C and gas window at 150– 200 °C both depend on time of the source rock is heated. The labile kerogen breaks down to form heavy hydrocarbons (i.e. oils), refractory kerogen breaks down to form light hydrocarbons (i.e. gases). The insitu hydrocarbon gas present within the shale sedimentary rocks is called shale gas which may be in the state of free gas or adsorbed gas or both. Unlike the conventional oil and gas, shale gas does not accumulate in a typical petroleum system (i.e. source rock, reservoir rock, seal or cap rock etc). The shale gas systems can be defined as the continuous type biogenic, thermogenic and combined biogenic-thermogenic gas accumulations characterized by wide spread gas saturation, subtle trapping mechanisms with short hydrocarbon migration distances. In shale rocks, gas can be accumulated in three forms: Gas is stored on the shale as adsorbed gas, within the intergranular porosity as free gas, within the natural fracture system as free gas. Shale gas may vary from area to area even in the same basin.This is due to the local changes in permeability which is highly depends on the both fracture intensity and fracture aperture width. Shale gas is unconventional natural gas that can be produced at neither economic flow rates nor in economic volumes unless the well is stimulated by a large hydraulic fracture treatment, a horizontal well bore, or by using multilateral well bores or some other technique to expose more of the reservoir to the well bore (Kent Perry and John Lee, 2007). Shale gas prospects in Indian Sedimentary Basins The first commercial shale gas production (1821) was from organic rich Devonian shale of Appalachian Basin which was economically marginal. After the great economic success of the Barnett Shale play in Texas the interest had spread in search for other sources of shale gas across the United States, Canada, Europe, Asia and Australia.The U.S. Energy Information Administration projects that the U.S. will produce 50% of its natural gas from unconventional sources by 2030. In 2005, approximately 10 trillion cubic feet (tcf) of conventional gas was produced in the U.S., versus 8 tcf of unconventional gas. Natural gas from shale accounted for about 6% of the gas produced in the U.S. (1.1 tcf). The majority of U.S. gas shale production came from four basins i.e. San Juan Basin, New Mexico, Antrim Shale of Michiga, Appalachian/Ohio shales and Barnett Shale, Fort Worth Basin, Texas. In India ONGC has started pilot project and drill 4 shale gas wells in Damodar Valley Mainly Cambay (in Gujarat), Assam-Arakan (in the North-East), Krishna Goda-

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