REGIONAL STRATIGRAPHY AND DEPOSITIONAL CHARACTERISTICS OF THE PENNSYLVANIAN PRUE SANDSTONE, LINCOLN COUNTY, OKLAHOMA

Abstract

The Pennsylvanian Prue Sandstone in northeastern Oklahoma represents fluvial and deltaic deposits that formed across a gently dipping shelf margin. The Prue Sandstone is predominantly well-sorted, fine to medium-grain sandstones that are light grey to tan in color within a stratigraphic interval of sandstone and shale. To evaluate the Prue Sandstone regional stratigraphy and depositional characteristics for the 990 mi2 (1593 km2) study area of Lincoln County, Oklahoma, cross sections and subsurface maps were generated to illustrate the spatial variability of the Prue Sandstone. Raster logs from 1724 wells were used for subsurface correlation and to generate geological maps. LAS logs from 95 wells were used to generate a 3D lithology model. Two wells totaling 126 ft (38.4 m) of Prue Sandstone core spanning the selected interval were examined, along with the petrographic analysis of ten thin sections. Subsurface maps combined with well-log signatures, core descriptions, and literature review were used to further define the Prue Sandstone depositional environment, stratigraphy, and reservoir characteristics. The Prue Sandstone is bounded at the top by the Excello Shale and at the base by the Verdigris Limestone. The Excello and Verdigris are easily recognized on well-logs and form excellent stratigraphic markers given their lateral continuity. Iron Post Coal and Breezy Hill Limestone deposits are observed intermittently throughout the study area. Top and base Prue Sandstone structure maps illustrate the gradual southwesterly dip across the study area. Isopach and net sandstone maps reveal thickness trends that are oriented from northeast to southwest and are consistent with a sediment source from the northeast. In central Lincoln County, interval thickness increases toward the southeast and ranges from 20 – 160 ft (6.1 – 48.8 m) across the county. Prue net sandstone thickness ranges from 0 – 116 ft (35.4 m) and displays similar trends as the isopach. Porosity ranges from 0 – 0.24 within the main depositional trend. By incorporating core, log-signature analysis, and subsurface maps, three key environments are interpreted: fluvial channels, distributary channels, and floodplain environments. Fluvial and distributary channel fill are both characterized by well-sorted, medium to fine-grain sandstones. Using gamma-ray logs, fluvial channels were distinguished from distributaries by their blocky sandstone appearance. Distributary channel signatures displayed stacking, thinner sandstone packages, and an increased amount of interbedded shale. Interpreted floodplain deposits are predominantly shale with a non-distinct or serrated well-log signature. Lithology proportion maps indicate a central-southeastern trend of sandstone deposition with isolated pockets of coal occurring intermittently throughout the study area. Modeled sandstone trends are representative of fluvial and distributary channel-fill deposits. Channel geometry and heterogeneity affect vertical sandstone variability, while changes in the depositional environment are the primary control for spatial variability.