The Application of Sedimentology and Sequence Stratigraphy to Correlation Challenges in the Wall Creek and Turner Sandstones, Powder River Basin
Presented by Jeff May
.Geologic Consultant and Colorado School of Mines
Abstract
The Upper Turonian Wall Creek Member of the Frontier Formation in the western Powder River Basin is generally time equivalent to the Turner Sandstone Member of the Carlile Shale to the east. The Wall Creek is commonly interpreted as a progradational clastic delta. But reconstructions for the Turner vary wildly: brackish valley fills, tide- to wave-dominated delta fronts, shoreline-detached hyperpycnites, and open-shelf shoals. Direction of sediment input likewise is disputed, ranging from the west, east, and/or north. Sedimentologic and sequence stratigraphic analyses from outcrops, cores, and well logs can help decipher the inconsistent interpretations.
The Wall Creek-Turner package is discontinuously distributed across the basin. In the west, the Wall Creek comprises the reservoir from Snake Charmer Draw to Powell fields. Immediately northeast in the Marys Draw region, this interval is absent. Progressing eastward, productive sandstone then reappears in the Crossbow-Porcupine and Hilight areas, where it is assigned to the Turner. Farther east, the reservoir section disappears again until recurring in Todd, Finn-Shurley, and Boggy Creek fields.
In the western hydrocarbon fields, the basal Wall Creek sandstone is sharp-based and blocky, with laminated and burrowed middle shoreface facies directly above open-shelf mudstone. The overlying Wall Creek section, in contrast, contains stacked cleaning- and coarsening-upward parasequences.
In Hilight Field, similar parasequences characterize the Turner. Heavily bioturbated lower shoreface intervals, with large open-marine trace fossils, grade vertically to horizontally to low-angle cross-laminated middle shoreface sandstones.
In contrast, the Turner in the Finn-Shurley area encompasses sandstone with wavy, lenticular, and flaser bedding and mudstone drapes. Three separate “benches” are identified, with diminutive burrows and low bioturbation index indicating stressed conditions in the basal bench. Each succeeding bench is more clay-rich than the one below, concomitant with an increase in burrow sizes and bioturbation index, signifying an evolution to more open-marine conditions during sea-level rise.
Our model calls upon a sea-level drop first creating a series of falling stage Wall Creek-Turner deltas overlain by a regional unconformity, i.e., sequence boundary, across the western to central Powder River Basin. Eastward prograding, top-truncated deltas were irregularly preserved between areas of sand removal. In contrast, the Turner in the eastern Powder River Basin lies above the sequence boundary and apparently reflects westward backstepping of shelf shoals during the ensuing sea-level rise. This transgression additionally led to coeval retrogradational Wall Creek-Turner shorefaces farther south and west. .
Bio
Jeff received his B.A. in Geology from Earlham College, M.S. in Geology from Duke University, and Ph.D. in Geology from Rice University. He has worked in the oil and gas industry for over 40 years: as a research geologist with Marathon Oil Company (1981-1994); as a geological and geophysical consultant with Enron Oil & Gas (1994-1996) and GeoQuest Reservoir Technologies (1996-1998); as an exploration geoscientist with DDD Energy (1998-2001); and with EOG Resources beginning in 2001, first as Chief Stratigrapher and then as Chief Geologist, until his retirement in 2011. He now is a geologic consultant and an Affiliate Faculty member in the Department of Geology and Geological Engineering at Colorado School of Mines.
Jeff has conducted sedimentologic, sequence stratigraphic, and seismic stratigraphic projects on basins and fields worldwide. Areas of expertise include onshore and offshore Gulf of Mexico; onshore and offshore California; Uinta, Green River, Washakie, Denver, Powder River, Permian, and Williston Basins; northern and eastern Egypt; and Natuna Sea, Indonesia. He also leads a variety of classroom and field seminars on clastic facies, deep-water sandstones, sequence stratigraphy, and mudrock deposition and stratigraphy, most notably for the American Association of Petroleum Geologists, the Petroleum Technology Transfer Council, Nautilus Worldwide, many oil and gas companies, and universities. His publications encompass numerous papers and abstracts on deep-water sandstones, sequence stratigraphy, geophysical interpretation, and mudrock deposition. Jeff has twice been presented the best luncheon speaker award by the Rocky Mountain Association of Geologists, was presented the Frank Kottlowski award for best speaker in the Energy Minerals Division at the AAPG annual convention in 2012, completed an AAPG Distinguished Lecture tour, and received the Outstanding Scientist Award from RMAG in 2017.