Gas potential of Proterozoic and Phanerozoic shales from the NW Himalaya, India: Inferences from pyrolysis

Abstract

Organic richness and kerogen properties of sixty-seven shales, obtained from the outcrops and underground mines of Jammu, Kashmir and Ladakh regions of Northwest Himalaya, India, have been studied to evaluate their gas generation potential using Rock Eval pyrolysis. Ranging in age from the Proterozoic to Tertiary, organic matter content and characteristics of the carbonaceous and coaly shales vary widely, indicating that sedimentary and burial history significantly affected the preservation and maturation of organic components in rocks.The total organic carbon (TOC) content ranges from 0.01 to 1.2% in the Permian–Jurassic and Paleozoic–Tertiary shales of Ladakh to as high as 32.5% in the Eocene shales/coaly shales from Jammu. The thermo-labile hydrocarbons (S1) and those from cracking of kerogen (S2), released from the pyrolysis of Eocene Subathu shales, are observed to be high (from 0.1 to 2.6 and from 0.5 to 15.5mgHC/g rock, respectively). Rock Eval thermal maturity parameters, indicated by Tmax (temperature at highest yield of S2; >490°C) and calculated vitrinite reflectance (1.5 to 3.7 Ro %), suggest a post-mature, dry gas stage for the hydrocarbon generation. Based on hydrogen index (HI) and Tmax correlations, organic matter in the Subathu shales is characterized by a late metagenetic gas prone Type III kerogen and a fair to excellent gas potential is exhibited by these shales.The interbedded shale units in the Proterozoic Sirban Limestone Formation, occurring as an isolated inlier in Jammu, show TOC values from 0.1 to 1.4% with quite low S1, S2 and HI values. Thermal maturity of shales within the stromatolitic Sirban succession indicates a post-mature and/or already spent hydrocarbon stage. The Plio-Pleistocene carbonaceous clays, lignites and mudstones from the Lower Karewa Group of Kashmir basin are organically rich with TOC content ranging from 5.86 to 29.4%. A thermally immature, Type II/Type III kerogen is indicated by the HI (109 to 278mgHC/g TOC) and Tmax (399 to 427°C) plots of the Karewa samples. The exposed Lower Triassic Black shales from the Permian–Triassic boundary sections in Kashmir are comparatively lean in organic matter, with TOC values ranging from 0.18 to 0.93%, whereas the Permian–Jurassic and Paleozoic–Tertiary shales from Ladakh have a TOC content ranging from 0.01 to 1.22%. The pyrolyzable organic carbon is <0.1% and the residual organic carbon contributes significantly to the TOC content of these shales. The other Rock Eval parameters (S1, S2, HI) are quite low and indicate a poor source potential. The majority of shales from the Tethys and Trans Himalayan regions appear to have undergone metamorphism and exhumation of organic carbon associated with the Himalayan orogeny.