An Integrated Depositional Model for the Late Pennsylvanian to Early Permian Mudstone Deposits in the Midland Basin, West Texas

Abstract

Late Pennsylvanian to early Permian mudstone formations in the Midland Basin are host to some of the most prolific unconventional hydrocarbon accumulations in the continental United States. Yet, studies to characterize these deposits have only recently gained attention. The aim of this study is to provide a better understanding of mudstone heterogeneity in marine offshore systems, using as a case study the deposits of the Midland Basin. To both document and explain the ranges, types, and causes of mudstone heterogeneity, a holistic approach is taken by integrating high-resolution sedimentological and geochemical observations and techniques on ~1300 ft. of mudstone from three cores. The cores span the Wolfcamp, Dean, and Lower Spraberry Shale Formations. In addition to detailed sedimentologic and stratigraphic descriptions, data collected on these cores include: elemental compositions from handheld x-ray fluorescence, wireline log data, total organic carbon, x-ray diffraction, programmed pyrolysis, porosity and permeability, thin sections, scanning electron microscopy images, and high-resolution photography. Preliminary results show nine facies: black claystone, dark gray fine siltstone, calcareous fine siltstone, planar laminated medium siltstone, calcareous medium siltstone, tan bioturbated medium siltstone, graded-to-non-graded coarse siltstone, skeletal wackestone, and skeletal packstone. Four facies associations representing distinct depositional envrionments are interpreted. These include: distal basin plain (FA1), distal to medial basin plain (FA2), medial to proximal basin plain (FA3), and proximal basin plain(FA4). The dominant mode of deposition in FA 1 was suspension settling under anoxic to euxinic benthic redox conditions. Classic models of suspension settling in anoxic conditions can only be related to this one facies association, which is not particularly abundant in the succession. FA2 reflects both suspension settling and traction currents sedimentation in a suboxic to anoxic environment. FA 3 is dominated by traction current sedimentation in an oxic to suboxic environment. FA 4 is dominated by sediment gravity flows. Finally, preliminary interpretations indicate 1) mudstone heterogeneity in the Midland Basin is directly related to facies and depositional environments, and 2) depositional mechanisms and environments are more varied and complex than those portrayed by historical models.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018