Geophysical methods for investigating source rocks

The estimation of kerogen volume and quality in source rocks is fundamental for evaluating a basin's hydrocarbon potential. Methods for kerogen estimation are based upon chemical and mechanical analysis of rock samples from outcrops or cores, thus limiting the spatial accuracy of petroleum system models far from hard data. Recent works demonstrated the possibility of using acoustic impedance inversion and amplitude versus angle analysis from surface seismic data for qualitative and quantitative estimation of kerogen. These methods are discussed in detail, and practical applications to real data are presented. Since kerogen is a considerably anelastic material, a frequency-dependent seismic attribute (sweetness) was tested, and its sensitivity to kerogen presence was investigated. A method for impedance inversion without wells, based upon local impedance constraints is demonstrated as a tool to delineate the distribution of kerogen in undrilled areas. Comparison between rock physics models and rock core samples showed that source rock impedance depends not only on kerogen percentage but also on its origin, with marine algal kerogen showing a lower impedance than terrestrial kerogen. This adds ambiguity to the interpretation of results, which can be solved with help of a priori assumption from sedimentary information related to organofacies accumulation.