Clay-Mineral Distribution in Permo-Pennsylvanian Shales of Val Verde Basin and Yates-Todd Arch, Texas

Abstract

The purpose in studying the clay-mineral distribution in the Val Verde basin, Yates-Todd arch, and Midland basin of West Texas is to obtain information which will aid in working out the geologic history in this area. A knowledge of the geologic history will aid in locating oil or gas accumulations beneath the Permo-Pennsylvanian shale interval. A clay-mineral stratigraphic framework has been constructed from identifications of the clay minerals contained in these Permo-Pennsylvanian shales. Clay-mineral ratio profiles have been plotted from analyses of samples from 20 wells to make up this framework. Three clay-mineral zones can be traced consistently through the cross section represented by these wells. A lower zone (Zone I) contains illite, chlorite, kaolinite, mixed-layer clay, and small amounts of montmorillonite. A middle zone (Zone II) contains illite and minor amounts of chlorite, kaolinite and mixed-layer clay. Mixed-layer clay in this zone, however, is more abundant than in Zone I. An upper zone (Zone III) contains a richer assemblage of clays and is more variable than the two lower zones. The characteristic clays of Zone III are illite, chlorite, kaolinite, montmorillonite, vermiculite (?), and mixed-layer clay. Throughout most of the framework, the clay-mineral zone boundaries are favorably comparable with tentative electric log correlations. A major departure, however, occurs in the Magnolia's L. M. Morrison No. 1 of northwestern Val Verde County. This anomaly may reflect either large-scale thickening or the presence of additional section in this well. Zone I clay content is relatively homogeneous and correlatable variations are difficult to establish. Several wells in the Val Verde basin contain more than 5,000 feet of Zone I shale in which there is little variation in the clay content. A lower zone of relatively abundant chlorite, however, can be recognized in four wells. Chlorite is also more abundant in Zone I of the Val Verde basin than in Zone I of the Midland basin. Zone II is relatively uniform throughout the framework except in the Yates-Todd arch area. It is unidentifiable in two wells here. This absence reflects influence of the arch during Zone II deposition or is related to the geographic-oceanographic location of this area during Zone II time. Zone III is more variable both laterally and vertically. These variations could probably be correlated with more complete sampling. Abundant mixed-layer clay in this zone suggests slower deposition than that which occurred during Zone II time and (or) a different physical-chemical environment.