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Unravelling Provenance of Condensates and Associated Natural Gases in the Almond Formation Carbonaceous Shales and Sandstone Reservoirs, Southwestern Wyoming

Abstract

Detailed molecular and isotopic compositions of natural gases and condensates from the Upper Cretaceous Almond Formation carbonaceous shales and sandstone reservoirs in southwestern Wyoming were compared to those of extracts and pyrolysis products from the related Almond carbonaceous shales and other likely source rocks from the Mesaverde and Mowry Formations in the area. The very light oils/condensates were analyzed using a novel whole-oil GC-MS/MS method developed for simultaneous analysis of various types of molecular markers without group-type chromatographic pre-separation and multiple analytical runs of discrete fractions. The new approach avoids loss of light-end components and minimizes co-elution problems. The rocks from the inventory of likely source beds were analyzed with another novel technology involving inline micro-pyrolysis-GC-MS method, which allows simultaneous full-scan and selected-ion monitoring of micro-pyrolysis products of several temperature ranges for a given source rock sample. The biomarker, diamondoid, and isotopic compositions reveal that the natural gas and associated condensates are probably co-generated primary products from the source rocks with advanced thermal maturity (1.5 %Ro). The thermal imbalance between the published thermal maturity of the kerogen in the host rocks (0.6 %Ro) and the inferred maturity of the source-rock (1.5 %Ro) suggests that the natural gases and condensates are migrated from a more mature source section. The results of the geochemical correlation work suggest that the most likely source is the down-dip lower Almond in the Washakie Basin.