Geochemical Characterization and Compositional Evolution of the Gomo Member Source Rocks in the Reconcavo Basin, Brazil

摘要

The lacustrine shales of the Gomo Member (Candeias Formation, Lower Cretaceous) are the main source rocks with type I kerogen in the Reconcavo Basin in northeastern Brazil. We investigated variations in source rock fades of the Gomo Member and their respective initial petroleum potentials, as well as the compositional changes of petroleum fluids generated with increasing maturity. To address these objectives, a comprehensive analytical procedure involving Rock-Eval pyrolysis of whole-rocks and kerogens, elemental analyses, and whole-rock and kerogen extracts was adopted on a series of samples covering the range from immature to highly mature levels. Whole-rock and kerogen Rock-Eval data indicated a strong mineral matrix retention effect that led to previous underestimation of the petroleum potential of the Gomo Member source rocks. The organic matter in low-TOC samples was shown to have lower petroleum potential and to be more oxidized than in high-TOC samples. Source rock facies variations were observed from one basin compartment to another, and initial HI values as a function of TOC were proposed for each area. Although the amounts of kerogen extracts, indicative of kerogen retention capacity, do not to vary considerably with depth (20-60 mg/g TOC), the amounts of whole-rock extracts increase considerably until the peak of petroleum generation and accumulation is attained. The trend in whole-rock extracts is accompanied by a substantial increase in absolute amounts and in relative contribution of saturates in the C_(15+) fraction, whereas the amounts of NSOs and aromatics do not vary significantly. Thus, we propose a partial secondary cracking of NSOs and aromatics within the source rocks before petroleum expulsion to explain changes in extract composition. With further maturity and expulsion, whole-rock extracts and the relative contribution of saturates decrease.