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Regional Stratigraphy and a High-Resolution Geochemical Model of the Upper Pennsylvanian Cline Shale, Midland Basin, Texas

Abstract

The Cline Shale is an organic-rich mudrock deposited in the Midland Basin during the Late Pennsylvanian. Exploration and production activity in this unconventional resource play has increased in recent years. A depositional model of the Cline Shale is interpreted using regional wireline log stratigraphy coupled with a core-based, high-resolution geochemical model. The study area was a restricted deep-water, epicratonic basin near the southern extent of Laurussia during the late Pennsylvanian. It is widely thought that slow sedimentation rates and oceanic oxygen depletion controlled the accumulation and preservation of organic matter. Large amplitude relative sea-level changes contributed to significant depositional heterogeneity both laterally and vertically throughout the Cline. The eastern boundary of the Cline Shale is the Eastern Shelf, where it grades into Canyon and Cisco age carbonates, shales, and sandstones. The Central Basin Platform and the Horseshoe Atoll are the western and northern boundaries of the Cline system respectively. Distinct changes in wireline log character from the Eastern Shelf into the Midland basin are apparent, and these changes can be attributed to basin-scale heterogeneities within the Cline Shale. High-resolution (2-inch sampling interval) x-ray fluorescence (XRF) geochemical data were collected from four Cline Shale cores. Three of the cores are discontinuous and represent deposition at or near the basin center. One core preserves a continuous Cline section and represents strata deposited on the slope of the Eastern Shelf. The XRF data delineate both mineralogical composition and depositional conditions, i.e., periods of oceanic oxygen depletion, throughout the interval at sub-facies resolution. Chemical facies can be interpreted on a 2-inch scale using agglomerative hierarchical cluster analysis; a statistical method that identifies similar groups of data in large datasets. This high-resolution facies scheme is coupled with strategic TOC and XRD measurements in order to create a robust geochemical model for the Cline Shale. The basin-centered cores and the core on the slope of the Eastern Shelf exhibit two distinct depositional and redox environments. These differences can be related to paleogeography and the transgressive nature of the formation.