Reservoir characterization of the Mississippian Madison Formation, Wind River basin, Wyoming

Significant heterogeneity in petrophysical properties, including variations in porosity and permeability, are well documented from carbonate systems. These variations in physical properties are typically influenced by original facies heterogeneity, the early diagenetic environment, and later stage diagenetic overprint. The heterogeneities in the Mississippian Madison Formation in the Wind River basin of Wyoming are a complex interplay between these three factors whereby differences from the facies arrangement are first reduced by pervasive dolomitization, but late-stage hydrothermal diagenesis introduces additional heterogeneity. The dolomitized portions of the Madison Formation form highly productive gas reservoirs at Madden Deep field with typical initial production rates in excess of 50 MMCFGD. In the study area, the Madison Formation is composed of four third-order depositional sequences that contain several small-scale, higher frequency cycles. The cycles and sequences display a facies partitioning with mudstone to wackestone units in the transgressive portion and skeletal and oolitic packstone and grainstone in the regressive portions. The grainstone packages are amalgamated tidally influenced skeletal and oolitic shoals that cover the entire study area. The basal three sequences are completely dolomitized, whereas the fourth sequence is limestone. The distribution of petrophysical properties in the system is influenced only in a limited manner by the smaller scale stratigraphic architecture. Porosity and permeability are controlled by the sequence-scale stratigraphic units, where uniform facies belts and pervasive dolomitization result in flow units that are basically tied to third-order depositional sequences with a thickness of 65–100 ft (20–30 m). The best reservoir rocks are found in regressive, coarse-grained dolomites of the lower two sequences. Although the amalgamated shoal facies is heterogeneous, dolomitization decompartmentalized these cycles. Fine-grained sediments in the basal transgressive parts of these sequences, along with caliche and chert layers on top of the underlying sequences, are responsible for a decrease of porosity toward the sequence boundaries and potential flow separation. Good reservoir quality is also found in the third sequence, which is composed of dolomitized carbonate mud. However, reservoir-quality predictions in these dolomudstones are complicated by several phases of brecciation. The most influential of these brecciations is hydrothermal in origin and partly shattered the entire unit. The breccia is healed by calcite that isolates individual dolomite clasts. As a result, the permeability decreases in zones of brecciation. The late-stage calcite cementation related to the hydrothermal activity is the most important factor to create reservoir heterogeneity in the uniform third sequence, but it is also influential in the grainstone units of the first two sequences. In these sequences, the calcifying fluids invade the dolomite and partly occlude the interparticle porosity and decrease permeability to create heterogeneity in a rock in which the pervasive dolomitization previously reduced much of the influence of facies heterogeneity. Hildegard Westphal studied geology in Tubingen, Brisbane, and Kiel, where she received her Ph.D. in 1997. After a postdoctoral position at Rosenstiel School for Marine and Atmospheric Sciences, University of Miami, she became an assistant professor at Hannover University. Currently, she is a member of the Paleontology Department at Erlangen University. Her work focuses on early diagenesis of carbonates; the genesis, diagenesis, and paleoenvironmental record of limestone-marl alternations; and paleoecological interpretation of carbonate platforms.Gregor Eberli received his Ph.D.s from the Swiss Institute of Technology (ETH), Zurich, Switzerland, in 1985 and the University of Miami in 1991. With his colleagues at the Comparative Sedimentology Laboratory, he conducts research in sedimentology, stratigraphy, geochemistry, and petrophysics of modern and ancient carbonates. In several projects, he investigated the influence of sea level changes on sedimentary architecture. He was an AAPG Distinguished Lecturer in 1996–1997 and a Joint Oceanographic Institutions/U.S. Science Advisory Committee Distinguished Lecturer in 1998–1999. Langhorne Smith currently heads the Reservoir Characterization Group at the New York State Museum. He holds a B.S. degree from Temple University, a Ph.D. from Virginia Tech, and did postdoctoral work at the University of Miami. He also worked for Chevron as a development geologist for two years. His current research interests are in carbonate reservoir characterization and hydrothermal alteration of carbonate reservoirs. G. Michael Grammer is an associate professor at Western Michigan University. His research includes high-resolution carbonate sequence stratigraphy and early diagenesis and their application to reservoir characterization. He was an AAPG Distinguished Lecturer for 2002–2003 and is a coleader of AAPG's modern carbonate field course. Previously, he was a senior research associate for Texaco and has consulted for domestic and international oil companies. He received his Ph.D. from the University of Miami in 1991. Jason Kislak received his bachelor's degree from Franklin & Marshall College in Lancaster, Pennsylvania. He is currently working toward his master's degree at the Rosenstiel School of Marine and Atmospheric Science at the University of Miami.