Deep thermal regime, temperature induced over-pressured zone and implications for hydrocarbon potential in the Ankleshwar oil field, Cambay basin, India

Ankleshwar oil field situated in the southern part of the Cambay basin, forms one of the prominent onshore Tertiary hydrocarbons bearing regions of western India. A detailed multiparametric lithological analysis of the well logs obtained from 16 deep wells, indicate conspicuous trend of inversion in density and transit time below a depth of about 1250 m, which coincides with the presence of moderately high overpressure zone within the reservoir and appear temperature induced. Estimated temperature gradient based on the corrected Bottom Hole Temperatures, recorded at different depths in the boreholes, are found to be quite high (similar to 53 degrees C/km) at deeper levels, especially in the overpressured zone, that contains low conductive Cambay shales. Estimated heat flow of 75.2 mW/m(2) at Ankleshwar, together with high Moho temperatures of about 880 degrees C and an extremely shallow depth to subcrustal melting (similar to 50 km), would suggest that not only the northern part, the entire hydrocarbon bearing Cambay basin, along with the Mumbai offshore region, is anomalously warm. In view of the presence of high thermal anomaly, low conductive shales of the Cambay basin might be a good target for shale gas investigation, as the prevailing in situ temperature is one of the crucial factors in oil-to-gas cracking.