Organic matter characteristics and gas generation prospect of the Late Cretaceous fluvial deltaic organic-rich shale in the offshore Jiza-Qamar Basin, Yemen based on organic geochemical, macerals composition, and biomarker results combined with 1-D basin modelling

Abstract

Organic-rich shale samples of the Late Cretaceous Mukalla Formation from the Al-Fatk1 well in the offshore Jiza-Qamar Basin, eastern Yemen, were examined using bulk organic geochemistry, microscopic examination, and biomarker measurements combined with 1-D basin modelling, to evaluate the organic matter characteristics and their ability to generate gas. The studied Mukalla shale samples are characterized by their high organic matter content with total values of up to 14.50 wt.%, and contain mainly hydrogen-poor Type III kerogen, indicating elevated gas generation potential. The hydrogen-poor kerogen was also demonstrated by biomarker measurements combined with an abundance of land-derived organic matter (i.e., vitrinite and inertinite) as established by microscopic investigation. The biomarkers of the studied organic-rich shales were characterized by a high Pr/Ph ratio, abundance of C29 regular steranes, high C27 17a (H)-22, 29, 30-trisnorhopane compared with C27 18a (H)-22, 29, 30-trisnorneohopane, all providing evidence for the organic matter being derived primarily from higher land plant organic matter, deposited in fluvial deltaic environments under highly oxic conditions. The bulk carbon isotope data agree with the findings of terrigenous organic matter. The maturity indicators show that the examined organic-rich shale samples have entered the main oil stage and are at the beginning of wet gas generation windows, exhibiting vitrinite reflectance values that range between 0.73% and 1.20%. Kerogen to gas conversion has been simulated by the basin models and shows that between 10% and 15% of kerogen has transformed into gas during the late Miocene until the present-day, consistent with the wet gas generation window (1.00–1.20 %Ro). Substantial gas exploration potential may exist in the fluvial deltaic Mukalla source rock system in the deeper structural units of the Qamar-Jiza Basin, where the Mukalla Formation has reached the high maturity level of the gas generation window.