Core- to log-scale analysis of the Wolfcamp Formation in the Thunder C20-13 #2H core, Delaware Basin, Reeves County, Texas

Abstract

The Pennsylvanian-Permian Wolfcamp Formation of the Permian Basin in West Texas and New Mexico is currently one of the most pursued unconventional plays in the world. In most areas, the Wolfcamp is over 2,000 ft thick and contains multiple stacked pay intervals. A 2018 United States Geological Survey (USGS) assessment evaluated the potential of technically recoverable and undiscovered resources of the Wolfcamp Formation in the Delaware Basin to contain 29,476 MMbbl of oil, 220,824 Bscf of gas, and 14,907 MMbbl of natural gas liquids (NGLs). The vast opportunities available have sparked interest in understanding the complexity and heterogeneity of the Wolfcamp strata to their implications on reservoir performance. The Wolfcamp Formation records deepwater deposition of organic-rich mudstones interbedded with calcareous mudstones, calcareous siltstones, carbonates, and argillaceous mudstones that were deposited within mixed carbonate-siliciclastic fan systems. These fan systems resided in a semi-restricted basin with sediment originating from multiple sources. The Wolfcamp Formation is informally subdivided into four intervals, from youngest to oldest, as Wolfcamp A, B, C, and D. Understanding the geological processes and characteristics that comprise each member is critical for exploration and in selecting the best landing zones. The Wolfcamp Formation is comprised of mass movement and sediment gravity flow deposits separated by background hemipelagic sedimentation. While the Wolfcamp Formation has been widely correlated using wireline logs, Wolfcamp event beds and the facies that make up these event beds are often below log resolution. The objective of this study involves using core, core associated data, and well logs sourced from the Wolfcamp play to investigate the vertical variability seen in the Wolfcamp intervals. The dataset used in this study is from the Cimarex Energy Thunder C20-13 #2H well located in Reeves County, Texas. The total length of the core used is 738.4 ft and includes the following: Wolfcamp A (215.5 ft), Wolfcamp B (175.9 ft), Wolfcamp C (310.3 ft), and Wolfcamp D (36.7 ft). Integration of detailed core description, X-ray Diffraction (XRD), X-ray Fluorescence (XRF), Routine Core Analysis (RCA), Source Rock Analysis (SRA), geomechanical analysis, and well logs were used in this study to identify facies, facies characteristics, and associated reservoir properties. Four facies groups and nine lithofacies were identified based on mineral composition, grain size, grain shape, sorting, color, and sedimentary structures. From these facies, porosity ranges from 4.5 to 12.1 percent and permeability from 0.1 to 340.0 nD. Total organic carbon (TOC) was observed to be highest in siliceous mudstones, with TOC up to 6.3 wt.%. Unconfined compressive strength (UCS), a method to assess rock strength from core, was measured using an Equotip Bambino micro-rebound hammer. Carbonate-rich lithofacies were found to have higher rock strength compared to clay-rich facies. The skeletal packstone has the highest average rock strength of 65 MPa. The lowest rock strength was observed in the argillaceous mudstone with an average UCS of 45 MPa. The distribution of Wolfcamp lithofacies is highly stratified. However, cyclicity is seen in both carbonate and siliciclastic event beds. Each event bed ranges from less than an inch to tens of feet in total thickness. Source rock analysis was used to examine the quantity of organic matter, hydrocarbon generative potential, the extent of thermal maturity, and kerogen type. The reservoir properties of each lithofacies were evaluated to identify reservoir prone and nonreservoir prone facies. Detailed core analyses were compared to a normal log suite to determine the controls of rock and fluid properties on well log responses.