Organic geochemistry and basin modeling of Late Cretaceous Harshiyat Formation in the onshore and offshore basins in Yemen: Implications for effective source rock potential and hydrocarbon generation

Abstract The purpose of this study is to present the source rock characteristics of Late Cretaceous Harshiyat Formation source rock in the onshore and offshore basins of Yemen (i.e. Mukalla-Sayhut, Sayun–Masila, and Jeza-Qamar). Source rock geochemical characterizations were completed and integrated to perform 1-D basin modeling to back-predict source rock thermal maturation, oil generation and expulsion. The geochemical findings revealed that mudstone intervals within the Harshiyat Formation comprised favorable source rocks, with total organic carbon (TOC) content between 0.50 and 35.10 wt %. The organic matter intervals in the Harshiyat mudstones are mainly Types II/III and III kerogens, with certain amount of Type II and I kerogens based on Rock-Eval HI values, ranging from 40 to 923 mg HC/g TOC. Mixed kerogen types were consistent with vitrinite and sapropelic organic matters provided by terrigenous plant inputs, as observed by visual kerogen examination and the distribution of biomarker fingerprints. The existence of mixed types I, II, and III organic matter suggests that the Harshiyat mudstones could contribute to oil and gas at sufficient thermal maturity levels. The biomarker data of the organic matter in the Harshiyat mudstones show the source rock contains a mix marine phytoplanktonic-bacteria/terrestrial land plant enthronement and were deposited in suboxic-anoxic conditions. Results of chemical and optical maturity indicators led to the conclusion of varying maturity stages from immature to late oil window levels for the mudstones of the Harshiyat Formation. Thermal maturation distributions inferred from the analyzed samples indicate that mudstone intervals in the offshore Mukalla-Sayhut Basin are extremely mature, compatible with the late-mature oil window and may have generated significant amounts of oil. This interpretation was supported by strong geochemical similarity between the organic matter in the mudstones and those of the oils found in the offshore Mukalla-Sayhut Basin and demonstrated by the concentrations of n-alkanes and isoprenoids as well as carbon isotopes. In the offshore Ras-Ghashwah-1X well, burial and thermal models were constructed and used to address the effect of tectonic events in triggering oil generation and expulsion from mature Harshiyat source rock in the offshore area. Kinetic database models for Type II and III kerogen mixtures indicate that the initial conversion of kerogen to oil occurred since the Late Oligocene with transformation ratios (TR) that span between 10% and 50%. Furthermore, the oil was expelled from organic matter intervals in the Harshiyat source rock starting from Late Oligocene and continuing until Late Miocene, with TRs of 50%–72%. From that period up to the present, the retained oil was cracked into wet gas in the gas window with an Easy %Ro greater than 1.30, and with peak TR ratios greater than 72%.