Core-calibrated regional facies distributions of the Cline Shale, Midland Basin, TX

Abstract

The Cline Shale is an organic-rich mudrock deposited in the Midland Basin during the uppermost Pennsylvanian (Canyon and Cisco time). The Cline Shale facies are constrained by the Central Basin Platform to the west, the Horseshoe Atoll to the north, the Eastern Shelf to the East, and the Ozona Arch to the South. It sits stratigraphically above the Pennsylvanian Strawn Formation and below the Permian Wolfcamp Formation. The Cline Shale is approximately 250 feet to 400 feet thick in the Midland Basin. It is both regionally and locally heterogeneous; investigating these heterogeneities is the first step to understanding the potential of this unconventional resource play.

Three cores from the basin center and one core from the toe of the Eastern Shelf slope were used in this study. X-ray fluorescence (XRF) data points were analyzed on a two to three inch resolution for the composition of 29 different elements. Using the XRF as a reference, 86 samples were drilled from the four cores and were analyzed for x-ray diffraction (XRD) and total organic carbon (TOC); these samples were used as calibrations for mineralogy and organic matter content. Agglomerative hierarchical cluster analysis, a multivariable statistical method, was used to group the dataset into statistical clusters. Each cluster was interpreted as a geologic facies based on empirically derived cutoffs of mineralogy and organic matter content. This core analysis reveals previously unrecognized facies stacking patterns and vertical organic matter distributions.

From this core study, we distinguished core-calibrated gamma ray (GR) facies cutoffs. Gamma ray (GR) logs were first normalized to the same maximum and minimum values. The facies are: GR<75 API = carbonate, 75120 API = siliciclastic organic matter-rich. Using these cutoffs, we synthesized facies distribution maps across the basin for the gross Cline section and for Canyon deposition, Lower Cisco deposition, and Upper Cisco deposition. Throughout the gross Cline interval, Carbonate distribution is greatest near the shelf edges, and has a greater relative importance within the basin in the northwestern portion of the basin. Siliciclastic organic matter-poor facies are considerably thick (commonly more than 200 ft. of the total Cline Shale deposition) throughout the basin; however, the net-to-gross diminishes considerably to the northwest as the facies is diluted by carbonates. A fairway of the largest net-to-gross siliciclastic organic matter-rich facies (greater than 0.2 net-to gross) trends northwest-southeast, and extends through Mitchell, Sterling, and Coke Counties. The fairway of organic matter-enriched rock is controlled in part by a lack of carbonate deposition, again serving as a diluent, in the area. Regional facies distribution maps through time also provide an insight to the sequence-stratigraphy of the Cline Shale. Understanding regional facies distributions from core-calibrated logs can lead to more strategic exploration and development of the Cline interval.

AAPG Datapages/Search and Discovery Article #90214 © 2015 Southwest Section AAPG Annual Convention, Wichita Falls, Texas, April 11-14, 2015