Thermal maturity calculation in Type II-S source rocks using the alkyl dibenzothiophenes

Abstract

Distributions of the alkyl dibenzothiophenes (DBT) are proven as effective proxies for the thermal maturity of bitumen extracts and oils expelled from marine source rocks containing Type II-S kerogen. Here we report on an expanded sample suite and DBT ratios to further refine these relationships. This study investigates the methyldibenzothiophenes (MDBT), dimethyldibenzothiophenes (DMDBT), and trimethyldibenzothiophenes (TMDBT) in 52 source rocks from formations in the United States and the Middle East to substantiate the reliability of these sulfur-bearing compounds to estimate maturity in Type II-S samples. The isomerization of MDBTs, DMDBTs, and TMDBTs shows strong correlations with pyrolysis data (Tmax, hydrogen index, and equivalent %VRo), enabling a direct calculation of thermal maturity from the ratios of the MDBT homologous series. Maturity indicators from biomarkers and other ancillary compounds are also analyzed to highlight the limitations and inconsistencies that can be observed when applying such methods to assess the maturity of Type II-S samples. The derived MDBT, DMDBT, and TMDBT maturity equations provide a reliable assessment of maturity for Type II-S samples, ranging from the immature window to the end of the condensate/wet gas window. Moreover, evaluation of 18 sulfur-rich oils validates the application of MDR maturity equations to fluid samples. Such flexibility allows for inclusive, yet consistent, maturity assessment in sulfur-rich formations when compared to other molecular maturity proxies.