--> Evidence Some Oil Accumulations in the Woodford Formation and the Meramec Formation Received an Additional Charge of Dry Thermal Gas

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Evidence Some Oil Accumulations in the Woodford Formation and the Meramec Formation Received an Additional Charge of Dry Thermal Gas

Abstract

Do oil or natural gas migrate into and charge kerogen or matrix porosity in mudrock reservoirs? Those reservoirs typically are so impermeable that most of the oil or gas they contain probably was generated by indigenous kerogen: i.e., they were not generated by other source-rock beds and then migrated into them. This implies that dry gas generated at VR >1.5 will not be found in most mudrock reservoirs containing oil-prone kerogen that only has reached the oil window (VR <1.2). The Woodford Formation is an excellent oil-prone source rock in the SCOOP area of the Anadarko Basin and in the nearby Arkoma Basin, where it is being developed as a shale-oil reservoir or a wet gas-shale reservoir (depending on its level of thermal maturity). Oil produced from the overlying Meramec Formation in the STACK area of the Anadarko Basin probably was generated by Woodford kerogen as well. The C isotopic composition of methane through n-pentane in solution gas samples or free gas samples collected from most wells completed in Woodford or Meramec reservoirs is consistent with the thermal maturity of Woodford source rocks at the well locations. But the C isotopic composition of methane is ∼3-4 per mil heavier than expected in a Meramec solution gas sample collected from one well in the STACK area, a Woodford free gas sample collected from one well in the SCOOP area, and a Woodford free gas sample collected from one well in the Arkoma Basin where crude oil dropped below its bubble point during basin uplift. In addition, the abundance of saturate and aromatic compounds in the C15+ fraction of “condensates” collected from Woodford gas wells where anomalously-heavy methane also occurs indicates the liquid HC actually is crude oil that evaporated into undersaturated natural gas, and the abundance of steranes and diamondoids in the Arkoma Basin sample indicate it is a mixture of native oil and severely-cracked oil. These observations indicate an additional charge of thermal gas containing isotopically-heavy methane migrated into some Woodford and Meramec reservoirs. Natural gas samples with isotopically-heavy methane do not contain isotopically-heavy ethane or propane, indicating the additional gas charge consisted almost entirely of methane. The very dry gas charge containing isotopically-heavy methane probably was generated by very mature Woodford source-rock beds at VR >2.0, or by older source-rock beds in the Anadarko Basin and the Arkoma Basin that are not well characterized.