Thermal maturity history and petroleum generation modelling for the Upper Jurassic Madbi source rocks in the Marib-Shabowah Basin, western Yemen

Widely exposed in the Marib-Shabowah Basin, the early Upper Jurassic Madbi Formation is divided into two units (Lam and Meem, youngest to oldest) believed to be major source rocks. The Madbi source rocks are characterized by relatively high total organic carbon contents (TOC) ranging from 0.5 to 10.0 wt%, meeting the standard as a fair to excellent source rock generative potential. They contain Type II kerogen grading to mixed Type II/III kerogens and Type III kerogen with HI values between 40 and 620 mg HC/g TOC. Vitrinite reflectance values range from 0.41 to 2.61% Ro, indicating sufficient thermal maturity for both oil and gas generation. /thermal history models indicate that the Upper Jurassic Madbi source rocks passed the peak of oil generation stage and convert the oil to dry gas in the Late Jurassic to Late Cretaceous time. Oil generation began immediately after deposition in the Late Jurassic time (150–145 Ma) and maximum rates of oil with significant gas have been generated during Late Cretaceous time (110–65 Ma). The peak gas generation occurred during the early Tertiary. The modelled hydrocarbon expulsion evolution suggests that the timing of hydrocarbon expulsion from the Madbi source rocks began in the early Cretaceous (140 Ma) persisted to present-day. The modelling results also indicate that the peak hydrocarbon generation and expulsion from Madbi source rocks in the studied area occurs well after deposition of the seal rock and formation of traps, ensuring entrapment and preservation of migrated hydrocarbons.