Geochemical characteristics of some crude oils from Alif Field in the Marib-Shabowah Basin, and source-related types

•The Alif crude oils are classified into two main genetic families.•The family I oils were derived from marine alga deposited under reducing conditions.•The family II comprises oils generated from mixed organic matter deposited in sub-oxic conditions.•The Alif oil samples have entered early mature to peak mature of oil window.

The Marib-Shabowah Basin is an important hydrocarbon province in western Yemen, but the origin of hydrocarbons is not fully understood. In this regard, geochemical characteristics are used to provide information on source organic matter input, depositional environment and the correlation between crude oils from different pay zones. Two oil families are present within the study area and classified based on biomarker and non-biomarker parameters. The family I oils are characterized by low API gravity, high sulfur and trace metal (Ni, V) contents and low Ph/Ph ratio <1.0. These oils were derived from an alga organic matter that was deposited in a highly anoxic, hypersaline marine depositional environment and generated at low maturity.Family II oils have medium to high API gravity, low sulfur and trace metal contents and relatively high Pr/Ph ratios (1.09-1.59). The family II oils were derived from mixed marine and terrigenous organic matter and deposited under sub-oxic conditions. These oils were generated from source rocks with a wide range of thermal maturity ranging from early to peak oil window. The oil characteristics suggest that family I oils may be derived from the Tithonian age Safer calcareous shales and family II oils from the deeper Kimmeridgian Madbi shales.