Carbonate Diagenesis in the Kallankurichchi formation, Ariyalur Group, South India and its Implications on Petroleum Prospects

Petrographic observations on diagenesis of the Kallankurichchi Formation carbonates, supplemented by geochemical, mineralogical and stable isotopic data have delineated 5 diagenetic zones viz , sediment-water interface, marine phreatic, meteoric vadose, meteoric phreatic and meteoric ciosed/semi-closed system, that have acted through eo, meso and telodiagenetic stages. Though the depositional conditions could have supported extensive production of organic matter during deposition, biological organisms such as borers during deposition and extensive chemical compaction during marine burial stage diagenesis have destructed the organic matter. Repeated occurrences of dissolution-Precipitation mode of diagenesis under oxygenated conditions have impoverished the rocks as poor source rocks. Prolific cementation made the formation not to be a hydrocarbon reservoir. However, owing to the occurrences of poorly cemented, loosely packed and moderately well sorted siliciclastic deposits over and below the Kallankurichchi Formation, it can serve as stratigraphic trap and/or reservoir seal if those siliciclastic deposits contain hydrocarbon reserves. It is also revealed that the viscous hydrocarbon residues found in dolomitic limestones are a localized phenomena restricted within shell cavities that escaped open system of meteoric diagenesis.