The Woodford Shale in Oklahoma is one of the most prolific unconventional petroleum reservoirs in the United States. Within the study area, the petroleum produced from this reservoir is self-sourced with a significant component having migrated in from deeper in the basin. It is hypothesized that oils migrated in an up-dip direction, but the extent of this migration has not been well documented. This study provides new insights as to the extent of this self-sourced petroleum and its characteristic phase behavior. Source rock maturity, carbon isotopes, produced oil chemistry, and biomarker ratios were all analyzed and compared to better understand migration and self-sourcing in the study area.
This geochemical work was performed on well cuttings, cores and oils extracted from source rocks; as well as produced oils from the reservoir. Sampled Woodford organic-rich shales contain very little vitrinite; however, they contain abundant solid bitumen. Thermal maturity data from solid bitumen were converted to a vitrinite reflectance equivalent (Jacob, 1989) and compared with pyrolysis data (e.g. Tmax and Hydrogen Index). Both methods were found to be in excellent agreement. Source rock maturities vary across the area of study from early oil (~0.70 % Ro) to gas windows (~>1.35 %) and approximately follow the present-day structural depth of the Woodford source rock.
Produced oil geochemistry data (specifically bulk molecular compositions, isoprenoid distributions, saturate and aromatic carbon isotopes, and biomarker ratios) from several wells suggest a common Woodford source; however, the oil maturities (e.g. Ro equivalent from biomarkers) are significantly higher than the Woodford source rock in many locations. Furthermore, the predicted GOR (gas-to-oil ratio) values from the same oil chemistry data are well matched with the produced GOR and estimated GOR from the recombined fluid samples. These findings support the interpretation that production from several Woodford wells includes migrated hydrocarbons from a deeper source. This study highlights two important ways in which geochemistry can be used to better evaluate source rock reservoirs:
by identifying the existence and extent of hydrocarbon migration, and
by predicting and understanding the quality and type of petroleum fluids stored in tight, unconventional source rocks. The approaches described in this paper can be utilized to predict, understand, and more accurately classify unconventional reservoirs all over the world.