Integrated Outcrop and Subsurface Studies of the Interwell Environment of Carbonate Reservoirs: Clear Fork (Leonardian-Age) Reservoirs, West Texas and New Mexico Final Technical Report October 1, 1998, to January 31, 2002 F. Jerry Lucia, Editor January 2002 Prepared for U.S. Department of Energy under Contract No. DE-AC26-98BC15105 The Semiannual Technical Report for the reporting period October 2001 to January 2002 is incorporated herein. Bureau of Economic Geology John A. and Katherine G. Jackson School of Geosciences The University of Texas at Austin Austin, Texas 78713-8924 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The view and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or an agency thereof. ii TABLE OF CONTENTS EXECUTIVE SUMMARY..............................................................................................................v INTEGRATED GEOLOGICAL AND PETROPHYSICAL STUDIES OF CLEAR FORK RESERVOIR ANALOG OUTCROPS: SIERRA DIABLO MOUNTAINS, TEXAS....................1 Stephen C. Ruppel, W. Bruce Ward, Eduardo Ariza, and James W. Jennings, Jr. CYCLE AND SEQUENCE STRATIGRAPHY OF THE CLEAR FORK RESERVOIR AT SOUTH WASSON FIELD: GAINES COUNTY, TEXAS.....................................................59 Stephen C. Ruppel and Eduardo Ariza CALCULATION AND DISTRIBUTION OF PETROPHYSICAL PROPERTIES IN THE SOUTH WASSON CLEAR FORK FIELD..........................................................................95 F. Jerry Lucia and James W. Jennings, Jr. 3-D MODELING OF STRATIGRAPHICALLY CONTROLLED PETROPHYSICAL VARIABILITY IN THE SOUTH WASSON CLEAR FORK RESERVOIR .............................143 James W. Jennings, Jr. FRACTURE ANALYSIS OF CLEAR FORK OUTCROPS IN APACHE CANYON AND CORES FROM SOUTH WASSON CLEAR FORK FIELD.............................................191 J. F. W. Gale, S. E. Laubach, R. M. Reed, J. G. Moros Otero, and L. Gomez MODELING COUPLED FRACTURE-MATRIX FLUID FLOW IN GEOMECHANICALLY SIMULATED FRACTURE PATTERNS...........................................227 Zeno G. Philip, James W. Jennings, Jr., Jon E. Olson, and Jon Holder iii INTEGRATED OUTCROP AND SUBSURFACE STUDIES OF THE INTERWELL ENVIRONMENT OF CARBONATE RESERVOIRS: CLEAR FORK (LEONARDIAN-AGE) RESERVOIRS, WEST TEXAS AND NEW MEXICO Contract No. DE-AC26-98BC15105 Executive Summary This is the final report of the project Integrated Outcrop and Subsurface Studies of the Interwell Environment of Carbonate Reservoirs: Clear Fork (Leonardian-Age) Reservoirs, West Texas and New Mexico, Department of Energy contract no. DE-AC26-98BC15105 and is the third in a series of similar projects funded jointly by the U.S. Department of Energy and The University of Texas at Austin, Bureau of Economic Geology, Reservoir Characterization Research Laboratory for Carbonates. All three projects focus on the integration of outcrop and subsurface data for the purpose of developing improved methods for modeling petrophysical properties in the interwell environment. The first project, funded by contract no. DE-AC22- 89BC14470, was a study of San Andres outcrops in the Algerita Escarpment, Guadalupe Mountains, Texas and New Mexico, and the Seminole San Andres reservoir, Permian Basin. This study established the basic concepts for constructing a reservoir model using sequence- stratigraphic principles and rock-fabric, petrophysical relationships. The second project, funded by contract no. DE-AC22-93BC14895, was a study of Grayburg outcrops in the Brokeoff Mountains, New Mexico, and the South Cowden Grayburg reservoir, Permian Basin. This study developed a sequence-stratigraphic succession for the Grayburg and improved methods for locating remaining hydrocarbons in carbonate ramp reservoirs. The current study is of the Clear Fork Group in Apache Canyon, Sierra Diablo Mountains, West Texas, and the South Wasson Clear Fork reservoir, Permian Basin. The focus was on scales of heterogeneity, imaging high- and low-permeability layers, and the impact of fractures on reservoir performance. In this study (1) the Clear Fork cycle stratigraphy is defined, (2) important scales of petrophysical variability are confirmed, (3) a unique rock-fabric, petrophysical relationship is defined, (4) a porosity method for correlating high-frequency cycles and defining rock-fabric flow layers is described, (5) Clear Fork fractures are described and geomechanical modeling of fractures is investigated, and (6) most importantly, new statistical v methods are developed for scaleup of petrophysical properties from the core to the layer scale and for retaining stratigraphic layering in simulation models. The outcrop study defined two high-frequency sequences in the lower Clear Fork and three high-frequency sequences in the upper Clear Fork. Unfortunately, much of the upper Clear Fork is eroded at this location. Importantly, the Clear Fork was found to be highly cyclic with both subtidal mud-to-grain cycles and well-developed tidal-flat-capped cycles. The cycles formed layers that are continuous across as much as 2,000 ft of outcrop. This finding contrasts with the prevailing concept that the Clear Fork is highly discontinuous. Characterizing the Clear Fork as composed of continuous high-frequency cycles was important for the construction of the subsurface model. Petrophysical studies of the outcrop demonstrated a gradual lateral change in porosity and permeability from tidal-flat facies to subtidal facies. As has been observed in all three outcrop studies, the petrophysical properties are highly variable and poorly correlated at the inch to foot scale, but a longer range pattern does appear to have spatial correlation. The concepts of the spatial distribution of petrophysical properties developed using this outcrop data together with previous data have a major impact on methods for building carbonate reservoir models. A sequence-stratigraphic model was built for the South Wasson Clear Fork field using the outcrop sequence model, vertical succession of facies from core descriptions, and seismic data. The two high-frequency sequences defined in the lower Clear Fork in outcrop were also identified in the subsurface. High-frequency sequences were more difficult to define in the upper Clear Fork because of poor log resolution and incomplete coverage of the reservoir interval by outcrop section. Nevertheless, three longer range sequences were defined, one of which coincides with the reservoir top. Core descriptions show cyclicity at a scale similar to that of the outcrop. A surprising result of the rock-fabric, petrophysical study was that the South Wasson Clear Fork reservoir can be characterized by a single porosity-permeability transform and a single saturation, porosity, and reservoir-height equation despite the presence of a robust variety of rock fabrics. This allowed porosity models to be converted to permeability directly without having to integrate a rock-fabric number. However, this made it difficult to identify cycles from wireline logs. Carbonate cycles could not be identified using the gamma-ray log because of the large amount of diagenetic uranium in the Clear Fork, and cross plots of porosity and water saturation are of little use because the fabrics group into one petrophysical class. A statistical study of vi porosity and rock fabrics shows the mud-dominated fabrics have lower porosity than the grain- dominated fabrics do on the average. Therefore, vertical increases in porosity were interpreted to mean a change from mud-dominated to grain-dominated fabrics. There are two reservoirs in the South Wasson Clear Fork field: a lower and a middle Clear Fork reservoir. The seal for the lower Clear Fork is the Tubb sandstone, and the seal for the middle Clear Fork is at a sequence boundary within the upper Clear Fork. Using porosity as a surrogate for rock fabric, 21 cycles were mapped in the middle Clear Fork and 21 cycles in the lower Clear Fork. To maintain high and low permeability in the flow model, each high-frequency cycle was divided into a lower, low-permeability mud-dominated layer and an upper, high- permeability grain-dominated packstone layer. The layers, which are referred to as rock-fabric flow layers, are the basic elements in the construction of a flow model. Integrating outcrop and subsurface data shows that petrophysical heterogeneity in the South Wasson Clear Fork reservoir is composed of a large-scale stratigraphically controlled component and a small-scale poorly correlated component. Scaleup from core-plug scale to reservoir-layer scale results in effective permeability increasing by a factor of 2. The large-scale variability exists as a flow-unit-scale petrophysical layering that is laterally persistent at interwell scales and produces highly stratified reservoir behavior. Modeling these large-scale heterogeneities as trends using moving