Scientific Underpinning of Concepts of Geochemical Inversion: Developing and Improving Methods with Application to Petroleum System Analysis

Geochemical inversion is a process for inferring source facies and thermal maturity of a petroleum from its molecular and isotopic characteristics. Comprehensive analysis for diverse molecular markers, including saturated and aromatic biomarkers, organosulfur compounds, and diamondoids in petroleum, are important for unravelling the petroleum’s origin. However, routine approaches are burdened with numerous uncertainties, ambiguities, and inconsistencies due to use of multiple procedures, loss of light-end components during sample processing, and co-elution of components in the GC-MS analysis. A single-step GC–MS/MS method was developed for simultaneous determination of various types of molecular markers in whole crudes without group-type pre-separation and discrete analyses of the diverse fractions. The method was optimized, calibrated, and validated using pure compounds and different types of crudes. The single-step GC–MS/MS method achieved higher resolution and sensitivity than the routine multi-step approaches, precluded the loss of light-end components during sample preparation, and minimized co-elution problems in molecular analyses. The newly developed analytical methods were applied to unravel the origins of light crudes in the Upper-Cretaceous Almond-Formation reservoirs in southwestern Wyoming. Unraveling the origins of light crudes is usually a challenge because they are exceptionally volatile and commonly deficient in biomarkers. The improved methods were particularly suitable to overcome these challenges. Together with modification of some of the prevailing geochemical interpretation schemes, the improvements allowed deconvolution of the complexities in understanding the origin of the Almond-formation condensate and natural gas accumulations. The modifications entailed: (i) a basin-specific calibration of diamondoid-based thermal-maturity parameters, through correlation of diamondoid indices in rock extracts versus measured reflectance of vitrinites from the same rocks; and (ii) modified interpretation schemes based on C7-hydrocarbon distributions to infer source-facies and thermal-maturity. Integration of data from molecular- and isotopic-compositions of the condensates and natural gases, with geological perspectives, and basin subsidence and thermal history modeling constraints, suggest that the petroleum in the Upper Almond-Formation reservoirs has originated in and migrated from the distal downdip, highly mature (1.3 to 1.7% Ro), non-marine transitional/paralic source rocks of the Lower Almond Formation/Upper Mesaverde Group in the Washakie Basin and the Great Divide Basin of southwestern Wyoming.