--> Re-Evaluation of Thermal Maturity and Stages of Petroleum Formation of the Barnett Shale, Fort Worth Basin, Texas

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Re-Evaluation of Thermal Maturity and Stages of Petroleum Formation of the Barnett Shale, Fort Worth Basin, Texas

Abstract

The Mississippian Barnett Shale in the Fort Worth Basin is generally considered the type “tight-gas shale” play, and has proven to be a significant energy resource. Although this unconventional gas play may commonly be used as an analog for exploration on a global scale, it is not completely understood with respect to thermal maturity and stages of petroleum formation. Through a collaborative effort with Chesapeake Energy, EOG Resources, Devon Energy, Quicksilver Resources, and the U.S. Geological Survey, 104 samples of the Barnett Shale were collected from 102 wells and one outcrop within the Fort Worth Basin. Rock-Eval hydrogen index (HI; mg S2/g TOC) and Tmax (temperature at maximum S2 yield; °C), and measured reflectance (%Ro) were compared and evaluated for their overall utility to determine extent, stage, and type of petroleum generated within the Barnett Shale at different thermal maturities. Tmax showed the poorest correlation with HI and measured %Ro, and showed no similarity to previously prescribed Tmax versus %Ro relationships. However, a good correlation between HI and measured %Ro was observed and compared favorably with other data sets and published relationships. Hydrogen indices and their calibration with the atomic H/C ratio of isolated kerogen from selected samples proved to be the most useful parameter for evaluating thermal maturity and various stages of petroleum formation. The resulting HI and %Ro maps show that 1.1 %Ro was not the start of major gas generation as previously suggested, but rather 1.6 %Ro was the start of major gas generation at an equivalent HI value of 37 mg S2/g TOC. The 1.6 %Ro is typically considered the start of secondary gas generation when oil cracks to gas. This higher %Ro value is more consistent with other thermogenic unconventional shale gas plays involving oil-prone kerogen (e.g., Marcellus Shale of the Appalachian Basin) and limits the likelihood of shale-gas plays in less thermally mature basins (e.g., New Albany Shale in the Illinois Basin).