A Biomarker and Isotopic Study of the Source Rock Organofacies, Oil Families, Source-oil Correlation and Thermal Maturity of Cenomanian Petroleum Systems in the Offshore Abu Dhabi

Abstract

Crude oils and rock samples from Cenomanian carbonate oil field (Field A) in the offshore Abu Dhabi were investigated in order to define oil families, paleoenvironment, origin of organic materials and thermal maturity because the origin of crude oils and their thermal history in this field has not been understood well. Especially, maturity profile in this area has not been determined yet because the source rocks don't contain enough amount of vitrinite due to the dominance of marine organic matters. Field A has two culminations above OWC: North structure and South structure. Therefore, we investigated oils from both structures to figure out the geochemical features of each culmination. The Cenomanian carbonate rocks of Field A are composed of shallow marine porous limestone (reservoir rocks) and deep marine lime mudstone (seal rocks and source rocks). Saturated biomarkers (n-alkanes, isoprenoids, steranes and triterpanes) of crude oils from shallow marine porous limestone and core samples from deep marine lime mudstone were determined using gas chromatography-mass spectrometry (GC/MS), gas chromatography-mass spectrometry-mass spectrometry (GC/MS/MS) and gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS). Ternary diagrams of C27-C29 steranes of all oil samples and core samples show significant similarity among crude oils and source rocks in Cenomanian age. High amount of C30 sterane and the ratio of C27-C29 steranes suggest that the crude oils were generated from Cenomanian marine source rocks. Biomarker maturity parameters such as Ts to Tm ratio and diasterane (Dia) to regular sterane ratio (Reg) were slightly different among the crude oils. The crude oils from South structure were slightly more maturated than those from North structure. The stable carbon isotopic values of individual n-alkanes in the crude oil would be classified into two crude oil families: dominance of lighter carbon in North structure and relatively heavier carbon in South structure. These results were consistent with the results of biomarker maturity parameters such as Ts/(Ts + Tm) and Dia/(Dia + Reg). Thus it was confirmed that the crude oils of North structure and South structure were from same Cenomanian source rock, however, thermal maturity of the crude oils were slightly different between North structure and South structure.