04206nas a2200217 4500008004100000245009700041210006900138300001200207490000700219520346800226653002303694653001703717653002903734653001303763100001703776700001903793700001803812700002003830700001503850856012303865 2004 eng d00aReservoir Characterization of the Mississippian Madison Formation, Wind River Basin, Wyoming0 aReservoir Characterization of the Mississippian Madison Formatio a405-4320 v883 a

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 65ndash100 ft (20ndash30 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.

10acarbonate systems.10apermeability10apetrophysical properties10aporosity1 aWestphal, H.1 aEberli, G., P.1 aSmith, L., B.1 aGrammer, G., M.1 aKislak, J. uhttp://archives.datapages.com/data/bulletns/2004/04apr/0405/0405.HTM?doi=10.1306%2FF48FC4B5-2545-4973-AAEA24831A05723B