--> --> Implications of Tectonic Signatures within and above the Niobrara Formation in Cherry Creek Field, Eastern Denver Basin.

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Implications of Tectonic Signatures within and above the Niobrara Formation in Cherry Creek Field, Eastern Denver Basin.

Abstract

On the eastern flank of the Denver Basin, biogenic gas is produced from the uppermost chalk member of the Niobrara Formation. Methane gas accumulates in small localized structures created by extensional faulting. Timing of the faults has been previously hypothesized to be post Laramide. However, tectonic signatures within a 15 square mile 3D seismic volume, located in Cheyenne County, KS, indicate there is potential for localized extensional faulting that began much earlier. This study further constrains the initiation of faults within the Niobrara, and describes the mechanisms responsible for faulting.

Across the Denver and Powder River basins, mechanisms proposed to create faulting of the Niobrara Formation include Permian salt dissolution, differential compaction over paleotopographic highs, folding of strata over basement features, compaction and dewatering of chalks, hydrocarbon pore pressure, and reactivation of wrench faults.

Evidence from Cherry Creek 3D study area, Cheyenne County, KS, suggests multiple controls are responsible for faulting within the Niobrara Formation. Reactivation of basement shear, dissolution of Permian strata below the Stone Corral Formation, and differential compaction has all influenced Niobrara faulting. Timing and recognition of the deformation were determined by tectonic signatures directly offsetting structural features. A tectonic signature is defined as a characteristic within sedimentary rocks that suggest deposition coeval with deformation.

Initial faulting of the Niobrara is interpreted as shear through part of the study area, with associated minor vertical movement and even several small ‘pop-up’ structures due to localized shortening. Timing of the shear faulting is determined to have been Cretaceous, during deposition of the Niobrara and Sharon Springs Member of the Pierre Shale.

Qualitative evidence indicates that extensional faulting of the Niobrara initiated almost immediately post deposition of the Sharon Springs Member, but low resolution seismic, from shallow imaging issues above the Sharon Springs Member, prevented quantitative interpretation. Still, the technique presented within this study could prove useful in deeper parts of the Denver and Powder River basins, where seismic data has been acquired, and shallow imaging resolution issues are not a concern.