Reservoir overprinting of crude oils

Biomarker concentrations are generally low in high API gravity oils and condensates. Admixture of even small amounts of low-maturity, biomarker-rich bitumen can drastically alter the biomarker profile of such fluids. We suggest that a 41° API DST oil from the Sunfish #1 well, Cook Inlet, Alaska, is a mixture of Jurassic-derived oil with approximately 5 wt% Tertiary input derived from coals interbedded with the reservoir sands. Reservoir overprinting has significantly altered the biomarker fingerprint of the Sunfish #1 oil. Therefore, biomarker patterns or parameters for biomarker-lean, high API gravity fluids should be scrutinized closely for internal consistency and conformity with bulk properties.