GEOCHEMISTRY OF SHALES FROM PERM IAN BARAKAR FORMATION, WEST BOKARO BASIN AND ITS IMPLICATIONS

Abstract

Carbonaceous shale of Barakar Formation (Permian) is commonly associated with bituminous to sub-bituminous thick coal seams. In recent years, these shale units are considered as important - reservoirs of hydrocarbons (shale gas). Analysis of the textural, mineralogical and organic/inorganic geochemical parameters determines the hydrocarbon potentiality of different shale horizons. The present study attempts integrated mineralogical, sedimentological, geochemical analysis of Permian carbonaceous shale members within Barakar Formation, West Bokaro Basin. This study further interprets the prevailed depositional environment. Petrographic study reveals red colored oxidized clay fabrication and flocculation near the transition from Upper Barakar Formation to overlying Barren Measures Formation. Kaolinite is the dominant clay mineral present in most of the samples with minor illite, chlorite, vaterite, ursillite. Absence of smectite is significant as it indicates deep burial condition where most of the smectite is converted into illite. Presence of illite gives evidence of considerable marine influence in the depositional area. Presence of folded, long mica flakes indicates deposition from suspension fallout. The studied shales show significant enrichment in organic matter. Total organic carbon (TOC) content of the shales varies in between 3.46-6.20 wt%, whereas this value is significantly higher for coal seams (> 45 wt%). The organic carbon content increases up section to Barren Measures Formation. TOC vs. oxygen diagram reveals an oxidizing condition with generation of type-IV kerogen within the shales, imparting the shales immature for hydrocarbon generation. Red coloration of the clay fabrics and the TOC vs S pyritic) diagram also suggests an oxidizing, shallow and normal marine environment.