Fault characterization of the Cretaceous Niobrara Formation using 3-D seismic of the Bonny Field, Yuma County, Colorado

Arnett, Bradley K.
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The Cretaceous Niobrara Formation (Fm) is an important hydrocarbon target within the Denver Basin of eastern Colorado, Wyoming, Kansas, and Nebraska. It is comprised of two members, the basal Fort Hays Limestone Member and the overlying Smoky Hill Member. Alternating units of chalk and marl represent the overall lithology of the Niobrara Fm. The latter is found at varying depths as it dips east to west across the basin. Bonny Field is the study area for this study. It is located in Yuma County, eastern Colorado, and produces biogenic gas from the Niobrara Fm. Bonny Field is located on the eastern flank of the Denver Basin. Within Bonny Field, the formation is heavily faulted and located at approximately 1,600 feet below surface. Niobrara faulting within the Denver Basin has also been evaluated in previous works, and will serve as comparisons to Bonny Field. The Bunting 3-D Survey area is located in the central portion of the Denver Basin, where the Niobrara Fm is located between 4,500 to 5,000 feet below surface. The faulting of the Niobrara Fm within this survey is interpreted as being caused by fluid expulsion and sediment compaction resulting in polygonal fault systems. Silo Field is located on the northern edge of the Denver Basin. There, the Niobrara Fm is located at approximately 7,100 to 8,800 feet below surface. The Niobrara Fm faulting within the Silo Field is interpreted as being driven by the nature of the basement. Faulting types present in Silo Field include wrench and listric faults, with areas of polygonal faulting. There are two fault systems present within Bonny Field: a polygonal fault system and a system dominated by listric faults. Similar to the Bunting Survey area, the polygonal fault system formed as a result of sediment compaction and fluid expulsion soon after Niobrara deposition. The listric faults formed later as a result of sediment slumping into the basin after more deposition occurred above the Niobrara. The faulting within Bonny Field is an important component of the petroleum system. The faults serve as structural traps, dropping the overlying Pierre Shale down against the top of the Niobrara to form a seal where gas can accumulate. Due to the numerous faults within the Niobrara horizon, there are many fault blocks to target. Producing wells are drilled into structural highs or on the high-side of individual fault blocks. There have been three stages of field development in Bonny Field. By better understanding the distribution of faults, the shape and orientation of fault blocks, there is potential for a fourth stage of development.

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