Petrophysical characterization of bioturbated reservoir facies in the Turner sandstone, Porcupine and Tuit Draw fields, southern Powder River Basin, Wyoming

Abstract

The Late Turonian Turner Sandy Member (Turner) of the Carlile Shale is a low porosity (5-14%), low permeability (0.01-0.4 mD) sandstone reservoir and one of the fastest growing unconventional targets in Wyoming’s Powder River Basin (PRB). Recent drilling activity occurs in southern Campbell and northern Converse Counties, with more than 4,600 horizontal wells drilled and over 116 MMBOE produced since 2010. This study is focused in southern Campbell County, where hydrocarbon production is predominantly from a cryptically bioturbated medium-grained sandstone and a heavily burrowed and bioturbated very fine-to fine-grained sandstone. A prior study completed on the Turner determined that bioturbation is a critical diagenetic process, where burrow type and the extent of biological reworking are the two principle factors that either destroy or enhance reservoir quality (Heger, 2017). Part 1 of this study, was to evaluate the geological controls associated with variations in structure and thickness trends, and their influence on conventional (vertical) and unconventional (horizontal) oil and gas production across the Porcupine, Tuit Draw, K Bar, & Crossbow Fields. After establishing these controls on field scale production, part 2 utilized a multi-scale approach and a novel petrophysical technique called spot permeametry that, before this study has not been used to evaluate the Turner. The main objective of part 2 was to determine the effects of bioturbation on the petrophysical characteristics of the identified reservoir facies, specifically in the Porcupine & Tuit Draw Fields. In part 1, it was determined that the conventional limits of production are restricted to shallower (9,600-9,000 ft.), thinner (75-90 ft.), and shalier intervals in the Porcupine and Tuit Draw Fields, where production is predominantly dry gas. Unconventional oil production and mixed oil and gas production was determined to occur from deeper (9,600-11,000 ft.) and thicker (90-105 ft.) intervals in the K Bar and Crossbow Fields, respectfully. An increase in gas production in the Porcupine and Tuit Draw Fields indicates that there is a stratigraphic component to the hydrocarbon accumulations that exist in the study area, associated with an updip increase in the shaliness of the Turner as it thins to the northeast, away from the sediment source to the west. In part 2 of this research, high grading the two reservoir facies has allowed us to identify the heavily burrowed and bioturbated very fine-to fine-grained sandstone as the primary target for future development based on higher effective permeabilities due to porous interconnected burrow networks, and increased secondary intragranular porosity. Whereas, the cryptically bioturbated medium-grained sandstone, which is considered the traditional target interval, may in some cases, be a secondary contributor to hydrocarbon production. Implications of this research include: 1) a refined prediction of resource estimates, and 2) understanding of the effects of bioturbation on electric log response, which if corrected properly, can enhance our assessment of the geological controls on conventional and unconventional oil and gas production, and provide insight into the most favorable intervals and locations for future production.