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Evaluation of geologic controls on geothermal anomalies in the Arkoma Basin, Oklahoma
Vitrinite-reflectance techniques were used to determine if there is a relationship between present geothermal gradient and coal rank in the Arkoma Basin. Three coal seams from high geothermal-gradient areas were compared with the same coal seams, respectively, from low geothermal-gradient areas. Samples were obtained from three core holes that were drilled in the high geothermal-gradient areas in Pittsburg and Haskell Counties, and three core holes that were drilled in the low geothermal-gradient areas in Latimer and Muskogee Counties. Nine additional coal samples were collected from active coal mines and one from an outcrop to supplement the core samples. The vitrinite-reflectance data indicates the present geothermal gradient did not produce the coal rank in the Arkoma Basin of Oklahoma. The coal rank is believed to have developed during the late Paleozoic, possibly in connection with the Ouachita orogeny. The coal isocarb maps suggest that the present geothermal-gradient pattern reflects the paleogeothermal gradient that produced the coal rank. Perhaps the intense folding and faulting associated with the Ouachita orogeny combined to transmit heat from the basement along an east-west thermal-anomaly zone through Haskell and Pittsburg Counties, Oklahoma. 60 refs., 16 figs., 3 tabs
Testing reproducibility of vitrinite and solid bitumen reflectance measurements in North American unconventional source-rock reservoir petroleum systems
An interlaboratory study (ILS) was conducted to test reproducibility of vitrinite and solid bitumen reflectance measurements in six mudrock samples from United States unconventional source-rock reservoir petroleum systems. Samples selected from the Marcellus, Haynesville, Eagle Ford, Barnett, Bakken and Woodford are representative of resource plays currently under exploitation in North America. All samples are from marine depositional environments, are thermally mature (T >445 °C) and have moderate to high organic matter content (2.9–11.6 wt% TOC). Their organic matter is dominated by solid bitumen, which contains intraparticle nano-porosity. Visual evaluation of organic nano-porosity (pore sizes 1.0 produced lowest R values, generally ≤0.5% (absolute reflectance), similar to a prior ILS for similar samples. Other traditional approaches to outlier removal (outside mean ± 1.5*interquartile range and outside F10 to F90 percentile range) also produced similar R values. Standard deviation values < 0.15*(VR or BR) reduce R and should be a requirement of dispersed organic matter reflectance analysis. After outlier removal, R values were 0.1%–0.2% for peak oil thermal maturity, about 0.3% for wet gas/condensate maturity and 0.4%–0.5% for dry gas maturity. That is, these R values represent the uncertainty (in absolute reflectance) that users of vitrinite and solid bitumen reflectance data should assign to any one individual reported mean reflectance value from a similar thermal maturity mudrock sample. R values of this magnitude indicate a need for further standardization of reflectance measurement of dispersed organic matter. Furthermore, these R values quantify realistic interlaboratory measurement dispersion for a difficult but critically important analytical technique necessary for thermal maturity determination in the source-rock reservoirs of unconventional petroleum systems.This research was funded by the USGS Energy Resources Program