Biomarker and Carbon Isotope Composition of the Oil Stains from the North Hadramaut High Area of Eastern Yemen: Implications on the Nature of Organic Matter Input and Their Characteristics

Five core sandstone and carbonate rock samples from the Cretaceous Harshiyat Formation containing oil stains were collected from two wells in the North Hadramaut High area (eastern Yemen) and geochemically analyzed by their sterane and terpane biomarker and stable carbon isotope (delta C-13) compositions. This paper studied the geochemical characteristics of the oil stains and their relation to probable potential source rocks. The source rock characteristics include the origin of organic matter input; redox depositional conditions, lithology, and degree of thermal maturity were investigated. The saturated and aromatic hydrocarbon fractions of the analyzed oil stain samples have delta C-13 values in the range of - 26.5 parts per thousand - - 28.6 parts per thousand and - 26.3 parts per thousand - - 26.7 parts per thousand, respectively. These delta C-13 values indicate that the oil stain samples originated from a mixed organic matter input in their source rock. The presence of mixed marine and terrestrial organic matter in the source rock is further concluded from biomarker distributions of n-alkane, isoprenoid, terpane, and sterane. These biomarkers and their ratio parameters suggest that the analyzed oil stain samples were sourced from clay-rich rock deposited in marine-reducing environmental conditions and likely contained mixed planktonic/bacterial organic matter and land plant inputs. Both saturated and aromatic maturity biomarkers suggest that the analyzed oil stains were from a mature source rock, equivalent to a moderate to peak-mature oil window. The environmental biomarker indicators of the studied oil stain samples correlated well with the shales in the Late Cretaceous Harshiyat Formation in the onshore Jiza-Qamar Basin (eastern Yemen), indicating that the analyzed oil stains were generated from a mature Harshiyat source rock in the onshore Jiza-Qamar Basin and subsequently migrated into the Cretaceous Harshiyat sandstone and limestone reservoirs in the North Hadramaut High area (eastern Yemen) through normal faults system.

Automated summary

This study analyzed the geochemical characteristics of oil stains from the Cretaceous Harshiyat Formation in eastern Yemen, indicating a mixed organic matter input from a mature source rock. Biomarkers suggested the presence of marine and terrestrial organic matter in the source rock, with indicators correlating well with shales in the Late Cretaceous Harshiyat Formation. The analyzed oil stains likely originated from a mature source rock and migrated into sandstone and limestone reservoirs in the North Hadramaut High area through normal faults.