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Petroleum geology of the Turner sandstone member of the Carlile shale, Weston County, Wyoming
Bone, Courtney R.
Bone, Courtney R.
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2020
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2020-12-04
Abstract
The Upper Cretaceous Turner Sandstone Member (Turner) of the Carlile Formation in the eastern Powder River Basin is a proven economic hydrocarbon producer with extensive potential for long term development and exploration. The Turner consists of shallow marine Turonian-aged deposits composed of interstratified sandstones, mudstones, and siltstones that were deposited on the western margin of the Western Interior Cretaceous Seaway and likely sourced from the Sevier Orogenic belt to the west. Hydrocarbon accumulation for the Turner is believed to be a result of long distance migration of the hydrocarbons from the Mowry Formation below with secondary sourcing from the Niobrara Formation above. The Turner oil and gas play at Finn-Shurley Field in Weston County, Wyoming, is an unconventional stratigraphic trap characterized by low porosity and permeability on the eastern margin of the Powder River Basin. The field is a continuous accumulation of hydrocarbons that have migrated updip from mature source rock within the region, as the Mowry and the Niobrara Formations are not mature within Weston County. These hydrocarbons saturate the extremely heterolithic lower Turner sandstone reservoir, which is the primary target for operators in the field. Finn-Shurley Field, discovered in 1965 and producing since the 1980’s, is a shallow, under-pressured, reservoir, where production seems to be limited by the facies, reservoir quality, such as porosity and permeability, and stage of diagenetic alteration. In recent years, Finn-Shurley has become a horizontal target because of its low porosity, low permeability, and tight oil nature.Overall, the Turner is one of the most prolific plays in the basin. However, little work has been published on the Turner to understand the reservoir potential and its extent to predict future development and exploration. This thesis project researched the controls on reservoir quality and production in the Finn-Shurley Field in Weston County, Wyoming. The highly heterolithic nature of the Turner in the eastern PRB promotes studies to be conducted to best understand the intervals that contain the best rock quality for hydrocarbon accumulations to occur throughout the region. This was accomplished by integrating four cores (271 ft) with supplemental core data, petrographic thin section analysis, X-ray Diffraction (XRD), X-ray Fluorescence (XRF), Field Emission Scanning Electron Microscopy (FE-SEM) imaging, historical production data, and petrophysical well logs. Due to the coarse nature of conventional reservoir characterization, this integrated dataset was critical in facies classification across the multiple scales required to accurately understand factors contributing to reservoir quality and their relation to production. The rock and source quality elements contributing to reservoir quality from core-based measurements were used to identify four reservoir facies out of eight total identified facies.Within the study area, the Turner has been divided into eight facies based upon lithology, grain size, bioturbation intensity levels, diversity of ichnofauna, and sedimentary structures. Within these facies, porosity ranges between 10-16% and permeabilities generally range between 0.001-5 mD. There have been four potential reservoir facies identified which have porosity values greater than 12% and permeabilities greater than 0.1 mD, and suggest meso- to micro-pore throat sizes in these facies. These classifications aided in targeting the best reservoir intervals in the lower Turner interval for each core, which was scaled to petrophysical well logs for future targeting and completion of the Turner. The lower Turner one zone was determined to be the primary reservoir target in Finn-Shurley field based on the presence of reservoir facies, as well as high porosity, permeability, brittleness, and resistivity values.The integrated use of this multi-scale approach gave a more comprehensive understanding of the diverse and interstratified facies observed within the Turner at Finn-Shurley field. The variations in reservoir quality are investigated at multiple scales, which lead to improved knowledge of factors controlling reservoir quality and hydrocarbon production in the Turner. This includes understanding the role of diagenesis in this unconventional reservoir, implications of depositional processes, and visualizing the reservoirs mineralogical and elemental makeup at multiple scales.
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