Using SARA Data to Reduce Uncertainty About the Type of Petroleum Fluid in the Woodford Formation and the Meramec Formation in the Anadarko Basin and the Arkoma Basin

Abstract

The type of petroleum fluid in liquid-rich shale reservoirs that contain oil-prone kerogen typically changes from volatile oil to wet gas over a short distance. The production GOR provides insight about what kind of fluid is present in a shale reservoir. PVT analyses of shale reservoir fluids commonly are performed on recombined natural gas samples and the oil or condensate samples collected at a separator. Recombining separator samples properly requires knowing the GOR when they were collected – a value that may be difficult to measure accurately due to slugging or other processes. Geochemical properties of HC liquids can reduce uncertainty about the nature of the reservoir fluid in a shale reservoir. The abundance of saturate compounds relative to aromatic compounds and NSO compounds (resins) in the C15+ fraction of oil or condensate generally increases with increasing thermal maturity of the kerogen that generated it. Those trends are observed in oil and condensate samples produced from the Woodford Formation in the SCOOP area of the Anadarko Basin, and in the Arkoma Basin where oil in the Woodford reservoir dropped below its bubble point during basin uplift. Oil and condensate samples generated by Woodford source rock beds that subsequently migrated into the Meramec Formation in the STACK area of the Anadarko Basin exhibit similar trends. Black oil or volatile oil samples contain 62-75 wt% saturate compounds, 16-32 wt% aromatic compounds, and 2.8-7.6 wt% resins. Condensates from the SCOOP area contain 84-96 wt% saturate compounds, 3-14 wt% aromatic compounds, and 0.25-2.5 wt% resins. SARA data demonstrate that very light (40-45°API) liquid petroleum produced from Woodford gas wells in the Arkoma Basin actually is crude oil that dropped below its bubble point (not condensate that precipitated from wet gas generated at VR >1.2). The relatively low abundance of saturate compounds (∼52 wt%) and the anomalously high abundance of aromatic compounds (∼45 wt%) in “condensate” produced from a Woodford gas well in the SCOOP area indicate that liquid probably is crude oil that was converted into wet gas when an additional charge of dry gas migrated into the reservoir (a model supported by the C isotopic composition of methane). SARA data also were used to conclude that the Meramec Formation contains wet gas – rather than volatile oil – at a well location in the STACK area where ambiguous GOR data made it difficult to decide how to recombine separator samples.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016