Shale oil and gas plays: Indian sedimentary basins

Abstract

Shale oil, shale gas constitute unconventional resource plays and are found in petroleum source rocks. Shale is considered as the conventional prime source facies for the naturally occurring hydrocarbons. These hydrocarbons are generated from organic rich shale under optimal temperature-pressure condition (thermal maturation) due to basin subsidence. A part of the generated hydrocarbons has been migrated and trapped in ideal locales constituting commercial accumulations. In case of shale oil/gas, shale acts as both source and reservoir rocks, Based on the type of organic matter and maturation level (either oil or gas window), the unconventional hydrocarbons could be shale oil or shale gas or a combination of both. The gas in shale are held as free gas within natural fractures, pore spaces and some are stored as adsorbed gas on the organic matter. As such, shale is characterized by low matrix permeability, and it requires hydraulic fracturing for shale gas production. Recent technological advancement in hydraulic fracturing and horizontal drilling has made shale gas exploration and exploitation economically viable. There are number of active shale gas plays (Barnett, Eagleford, Haynesville Marcellus etc.) in US sedimentary basins and other parts of the world including India. Apart from North America (USA and Canada). shale gas exploration activities of varying geological age (Proterozoic to Cenozoic) are in full swing across all the continents such as South America (Argentina, Brazil), Africa (South Africa), Europe (Poland, Romania, Hungary and Germany) and Asia- Pacific (Saudi Arabia, China, Mangolia and Australia). Shale occurs over the broad geological time scale, mainly in Proterozoic. Cambrian. Silurian, Devonian, Permian, Cretaceous and Paleogene-Neogene. Understanding of shale geology, its sedimentological and depositional settings in sedimentary basins merits due importance. The geochemical characteristics of shale reservoirs, mineralogical aspects, organic content, maturation, thermal history of the basin are some of the important parameters for evaluating the shale gas and oil potential. Generally the shale gas is characterized by high gamma value, high resistivity, low density and high porosity. Shale gas plays of the selected onland basins of India, analogous to the other world shale gas plays, have been enumerated.