Origin of four Upper Pennsylvanian (Missourian) cyclothems in the subsurface of western Kansas : application to search for accumulation of petroleum

Abstract

Thin, widespread, carbonate-dominated cyclothems of the Lansing, and Kansas City groups were deposited across Kansas during the Missourian on a gently tilted shelf bordering the rapidly subsiding Anadarko basin. The possible causes of this cyclothemic sedimentation are re-evaluated based upon a comparison of the stratigraphy, sedimentology, and diagenesis of four, similar, successive cyclothems over a wide area in the subsurface of western Kansas using cores, cuttings, and wireline logs. The observations and interpretations are used to evaluate the distribution of petroleum reservoirs in these cyclothems.

The four cyclothems are characterized by rapid transgression and relatively slow regression of the sea across a gently tilting shelf. A thin, basal transgressive carbonate is overlain by marine shale, representing maximum inundation by the sea. The marine shale completely covered the study area but for local areas over the more significant positive areas of the shelf and in one of the cyclothems, the I-Zone. The black organic-rich facies in the marine shale covers much of the southern two-thirds of the shelf excluding portions of the structurally positive areas and again in the I-Zone. The anoxic bottom conditions that brought on the deposition of the black shale resulted primarily from the combination of stagnant bottom waters due to a thermocline and scavenging of oxygen by large amounts of terrestrial organic matter.

Progressive, contemporaneous, epeirogenic deformation of the shelf significantly affected sedimentation and diagenesis in these cyclothems. These changes included varying but generally less subsidence over previously active structures such as the Central Kansas and Cambridge uplifts and increased subsidence along portions of the shelf closest to the Anadarko basin, most active during deposition of the earliest two cyclothems. These epeirogenic adjustments were however secondary in importance to a rapidly fluccuating sea level in controlling the formation of the cyclothems. Periodic changes in eustatic sea level due to continental Gondwana glaciation are deemed most plausible for producing the rhythmic flooding and subaerial exposure of the broad, gently-dipping shelf.