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Modeling geologic controls on hydrocarbon generation in the Wolfcamp: integrated analysis of the Delaware Basin
Kelleghan, Cahill
Kelleghan, Cahill
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2021
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Abstract
The Permian Basin has been a critical source of oil and gas in the United States since its discovery in the early 1900’s. Recent developments in unconventional resource extraction have driven the United States to reach milestones in its domestic energy industry, including becoming a net energy exporter for the first time in decades, as well as becoming the largest oil producer in world. In 2019, US production exceeded 13 million barrels of oil per day (BPD) with nearly 5 million bpd coming from the Permian Basin alone. Natural gas produced from the Permian also exceeded 20 billion cubic feet per day (BCFD) in 2019, propping up Texas to produce more natural gas than any other state and establishing domestic energy security for the US. The geologic complexity of the prolific hydrocarbon reservoirs require unconventional, integrated geologic and engineering solutions to successfully extract hydrocarbons from the stacked, heterolithic source-reservoir systems. Continued research and development in the Permian Basin will be necessary for the basin to continue to be America’s most prolific petroleum basin. Within the Permian Basin, the Delaware Basin resides as the westernmost structural component of the greater Permian Basin. The stratigraphy of the Delaware Basin contains multiple intervals of organic-rich source rocks with varying degrees of thermal maturity. Multiple publications have identified a trend in the gas-oil ratio in the Delaware Basin, characterizing the western side of the basin with a significantly higher gas production on the shallower, gently dipping flank of the basin. Varying levels of uplift and erosion, as well as variable spatial and temporal geothermal gradient are both leading explanations for the anomalous spatial change in gas-oil-ratios. In order to increase understanding of the geologic controls on hydrocarbon generation in the basin, examination and modeling the geologic history identified critical moments in formation of the petroleum system and illuminated areas that require further investigation. The geohistory integrate thermal and structural history of the basin, eustatic sea-level change, and tectonic subsidence to interrogate timing and magnitude of hydrocarbon generation from the organic-rich intervals. Measured thermal, geochemical, and mechanical well data were used as constraints to calibrate 1D basin models across the basin. Vitrinite reflectance, rock-eval pyrolysis, and bottom-hole temperatures (BHT’s) are all sources for calibration of models by fitting calculated curves to measured well data. Burial history analysis and sensitivity analysis were used to characterize geologic variability across the basin, followed by uncertainty analysis of the results with respect to hydrocarbon generation with emphasis on the Wolfcamp. Results from the models identify that Wolfcamp peak rates of oil and gas generation occurred in the late Permian to Triassic. The Wolfcamp experienced compaction by the end of the Permian due to rapid tectonic subsidence, resulting in maximum porosity and permeability reduction prior to the onset of hydrocarbon generation. Testing the models’ sensitivity with respect to the key geologic and geochemical uncertainties identified paleo heat flow and initial TOC of the Wolfcamp as influential parameters in calculating both thermal maturity and hydrocarbon generation. Results from the thermal models show present day geothermal conditions are not capable creating the thermal and geochemical signatures preserved in the sediments in the western side of the basin. The western edge experienced a higher paleo geothermal gradient following the Permian basin phase that contributed to higher thermal maturity of the Wolfcamp, converting in-situ hydrocarbons to a more gaseous phase. The geologic history of the western basin is a separate heat flow province from the eastern basin, with hydrocarbon generation controlled by Cenozoic tectonism.
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