Stable Isotope Stratigraphy: Correlations and Implications for Hydrocarbon Microseepage and Prospecting

Abstract

The stable isotope systematics documented in the stratigraphically controlled sedimentary rocks represents the isotopic differences established in response to earth processes in the past. Relative enrichments and depletions of the oxygen (O) and carbon (C) ratios provide insights onto the paleoenvironmental conditions that existed during sediment deposition and diagenesis. Oxygen isotopes (18O/16O) in sediments have been used, mainly for the paleoclimatic reconstructions on regional and global scale, whereas the variations in the carbon isotopes (13C/12C) serve as a proxy for the amount of buried organic carbon, which in geological time under given conditions of sufficient thermal exposure and overburden, leads to the development of hydrocarbon source beds. Marked increase in carbonate carbon (Ccarb) and organic carbon (Corg) budget during Proterozoic and Mesozoic time periods is well known to coincide with the development of enormous and prolific Infracambrian and Mesozoic petroleum systems around the globe.In the present paper, we present an overview on stable carbon and oxygen isotope stratigraphy, and an observation from Proterozoic Vindhyan Basin of Central India, wherein the isotopic enrichments in carbon ratios are coincident with the near surface light gaseous hydrocarbon manifestations that indicate the occurrence of thermally generated hydrocarbon in the basin. The carbon isotopic excursions because of increased organic carbon burial and preservation, as evident from the stratigraphically controlled isotopic signatures of the carbonate rocks from the Vindhyan Basin (Kumar et al., 2002), possibly signify the occurrence of potential source rocks at depth, albeit noncommercial as of present, which in turn form the source for the light gaseous hydrocarbons observed in the near-surface soils. Such associations of stable isotope events with the microseepage of hydrocarbons from subsurface have been observed in other sedimentary basins like Cuddapah and Kutch and are extremely useful in the qualitative and quantitative evaluation of the potential hydrocarbon generative source rocks, particularly in the frontier basins.