Unconventional oil tight carbonate reservoir of the Shuqra Formation in the Say’un–Masilah Basin, Yemen as evidenced from combined organic geochemical, mineralogical and basin modeling results

An integrated approach combining organic-geochemical and mineralogical data, along with microscopic examinations was employed to investigate the shale intervals of the Shuqra Formation from the Say’un–Masilah Basin. Geochemical, geological, and maturity datasets were incorporated into 1-D basin modelling to evaluate the unconventional tight-oil reservoir prospectivity of the Shuqra shale intervals. The Shuqra shales are primarily composed of clay-rich carbonate mudstone lithofacies, characterized by a high concentration of total organic carbon (TOC), ranging from > 1 wt.% to as high as 5.94 wt.%. The Shuqra shale intervals predominantly contain hydrogen-enriched kerogen of Types II and II/III, exhibiting a hydrogen index (HI) ranges of 210 to 554 mg HC/g TOC, and indicating strong oil-generating potential. This finding is corroborated by the observed predominance of alginite (i.e., lamalginite and telalginite) and bituminite, with minor vitrinitic materials. The deeper sections of the Shuqra shale Formation are inferred to represent oil-charged, tight reservoir rock with potential for hydrocarbon exploitation. This assessment is supported by high oil saturation index (OSI) values above 30 mg HC/g TOC and peaking at approximately 181.82 mg HC/g TOC. The elevated OSI values are linked to high kerogen conversion, with a transformation ratio (TR) exceeding 40% (46%-65%) that resulted in a higher retaining hosting capacity. Consequently, the Shuqra shale Formation can be considered an oil-bearing tight reservoir rock and a suitable candidate for unconventional production purposes. This interpretation is substantiated by a good correlation between the oil obtained in the limestone facies of the Shuqra Formation and the Shuqra shale source rock, as evidenced by molecular biomarker measurements.