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Gas Geochemistry of the Spraberry and Wolfcamp Formations in the Midland Basin

Abstract

Integration of well cuttings and mud-gas data analysis can be a cost-effective approach for identifying source and reservoir intervals during petroleum exploration in stacked unconventional plays. A vertical well located in the Midland Basin was selected to evaluate this approach. The objectives of this study were to: (1) identify petroleum sources and reservoirs by direct comparison of the geochemistry of mud gas, headspace gas, and gas released from rock crushing, and (2) quantify the impact of natural (thermal maturation, organic matter type) and post-drilling processes (gas release during transportation and storage) on changes in gas chemistry. Changes in i-C4/n-C4 in the mud gas and headspace gas with depth are well correlated, indicating that the ratios are mainly related to hydrocarbon source and thermal maturity rather than the formation mechanism (e.g., desorption) or alteration processes affecting the gases. The Spraberry and Wolfcamp petroleum systems can be identified by using this ratio. Normalization of the individual C1-C5 compositions to the total hydrocarbon gas concentration allows for comparison between the mud gas and headspace gas. In general, methane concentrations are much higher and propane, butane, and pentane are much lower in the mud gases compared to the headspace gases. This is thought to be due to the preferential release of methane relative to heavier gas hydrocarbons occurring during degassing from source rock intervals. However, for a few depth intervals, compositional differences between mud gas and headspace gas is minimal to non-existent. The C1/C2+C3 ratio of the mud gas has a constant value throughout the sampled well interval. The C1/C2+C3 ratio of the headspace gases is generally lower (i.e., wetter) than the mud gas, but more variable. The following relationships in mud gas and headspace gas chemistry for source rock intervals and reservoir facies were interpreted: (1) intervals where the mud gas is drier and the headspace gas is wetter are interpreted to be more organic-rich, fine-grained source rock intervals with low porosity and permeability; and (2) intervals where both gases are methane-rich, the dry gases are interpreted to be reservoir facies with higher porosity and permeability. Results from the Midland Basin confirm that integration of well cuttings and mud gas data can be an effective approach for identifying source and reservoir intervals in unconventional petroleum systems.