The Sequence Stratigraphic and Paleogeograhic Distribution of Reservoir-Quality Dolomite, Madison Formation, Wyoming and Montana TAURY SMITH AND GREGOR EBERLI The primary aim of this study is to show the sequence stratigraphic and paleogeographic distribution of dolomite and porous dolomite in the Madison Formation of Wyoming and Montana. This study integrates the regional sequence stratigraphic approach of Sonnenfeld (1996,) with the detailed reservoir characterization of Westphal et al (1999) to show the exploration-scale distribution of porous dolomite within a regional high-resolution sequence stratigraphic framework. In particular, a hypothesis is tested that the dolomite occurs preferentially in the transgressive portions of sequences and cycles. The findings of this study include: o Most of the porous dolomite occurs within the transgressive portion of the second-order supersequence. o Dolomitization varies from updip to downdip on the ramp. The pervasively dolomitized mid-ramp sections are the optimum location for new exploration plays. o Downdip on the ramp, porous dolomite occurs in laterally extensive muddominated strata in the transgressive portions of the two composite sequences. o In the transition between the downdip sections and the mid-ramp, fabricselective dolomitization is still common but porosity occurs within the transgressive portions of progressively higher frequency sequences and cycles. o Dolomite reservoir quality and distribution can be predicted at various points along the Madison ramp using the data presented in this study. Setting The Lower and Middle Mississippian Madison Limestone was deposited on a gently dipping carbonate ramp (sensu Read, 1995) that extended from New Mexico to western Canada and was bounded to the east by the Transcontinental Arch and to the 1
west by the Antler Highlands. Most of the ramp was shallow marine, but deeper marine conditions prevailed in the Antler Foredeep, Williston Basin and Central Montana Trough (Sando et al., 1981; Rose, 1976). We studied outcrops along a transect that trends perpendicular to the Transcontinental Arch from updip in southeastern Wyoming to downdip in the Central Montana Trough (Fig. 1). Results These exploration-scale cross-sections show that porous early dolomite is most commonly found in the transgressive portions of a hierarchy of sequences and cycles and that pervasive dolomitization of all rock types only occurs in the middle part of the ramp. The Madison Formation is composed of a second order supersequence that consists internally of two composite sequences and numerous third-order sequences and higher frequency cycles. Most of the porous dolomite occurs within the transgressive portion of the second-order supersequence (Fig. 2). Downdip on the ramp, fabric selective dolomitization was dominant as more than 9% of the mud-dominated strata and less than 5% of the grain-dominated strata were dolomitized. The only porous dolomite, however, occurs in laterally extensive mud-dominated strata in the transgressive portions of the two composite sequences. Moving updip, fabric-selective dolomitization is still common, but porosity occurs within the transgressive portions of progressively higher frequency sequences and cycles (Fig. 2). In the middle part of the ramp all rock types in the TST of the second-order supersequence were pervasively dolomitized and porosity occurs throughout the interval. Further updip, the amount of dolomite and porous dolomite decreases upward within the TST of the supersequence, and rock fabric played little role as 7% of the grainstones and only 5% of the mudstones were dolomitized. In the dolomitized portions of the Madison limestone, facie s can be correlated to the permeability. Grainstones have higher permeability than packstones with the same total porosity, packstones greater than wackestones and wackestones greater than mudstones. This suggests that mold size and abundance and crystal size have a significant impact on permeability. Grainstones and packstones are typically composed of coarser crystals than mudstones and wackestones. Implications for future exploration and production The variations in dolomitization and porosity distribution produce very different reservoirs at different positions along the ramp. The optimum location for Madison 2
production is within the mid-ramp grainstone fairway where Sequences I through III are pervasively dolomitized (Fig. 4). Opportunities exist outside of this fairway, but reservoir quality progressively decreases in both updip and downdip of the mid-ramp. In the downdip sections, explorationists should expect two laterally extensive muddominated targets (base of sequences I and III) with relatively low permeability to porosity ratios. The reservoirs are partitioned at the composite sequence scale. At the transition from the downdip to the mid-ramp sections, mud-dominated carbonates in higher frequency cycles in sequences I and II are porous as well as the laterally extensive mud-dominated zones at the bases of the composite sequences. In the mid-ramp all grainand mud-dominated carbonates are dolomitized and can be porous. The occurrence of abundant dolomitized grainstones leads to a higher permeability to porosity ratios for fields within the grainstone fairway. This is the ideal location for high-risk deep prospects because reservoir quality will be the highest. References Rose, P.R., 1976, Mississippian carbonate shelf margins, western United States, in J.G. Hill, ed., Geology of the Cordilleran Hingeline, RMAG, Denver, CO, p. 135-151. Sandberg Sando, W.J., C.A. Sandberg, and R.C. Gutschick, 1981, Stratigraphic and economic significance of Mississippian sequence at North Georgetown Canyon, Idaho: AAPG Bulletin, v. 65, p. 1433-1443. Sonnenfeld, M. D., 1996, Sequence Evolution and Hierarchy within the Lower Mississippian Madison Limestone of Wyoming, in M. W. Longman, and M. D. Sonnenfeld, eds., Paleozoic Systems of the Rocky Mountain Region, Rocky Mountain Section SEPM, p. 165-192. Westphal, H., G.P. Eberli and G.M. Grammer, 1999, Outcrop cyclostratigraphy as analogue for the Mississippian Madden Deep Gas Reservoir (abstract): AAPG Annual Meeting Program with Abstracts, p. A148. 3
NW Benbow Mine Road 2 1 Shoshone Canyon Sheep Mountain (modified from Sonnenfeld, 1996) 4 2 1 Mississippian Madison Formation: Facies, and Sequence Stratigraphy 4 2 1 Wind River Canyon 4 2 1 Tensleep Canyon 4 2 1 Casper Mountain 2 1 Fremont Canyon 2 1 Hartville Quarry 2 1 SE DATUM - Base of Breccia in Seq. IV M W P G BR M W P G BR vuggy Feet Meters vuggy vuggy porous dolomite 8 BM 6 Montana Wyoming 5 miles HC M moldic Feet 5 SM 4 SH TC WRC 2 FC CM MWPG BR M schematic VI V IV III fault breccia in middle II Breccia Types Facies 3rd-order sequence boundary 3rd-order mfs Intermediate Cycle Boundary Lithology Dolomite Limestone Hydrothermal Evaporite Solution Collapse Karst Stromatolites (laminated mudstone) Proximal lagoon (mudstone and wackestone) Distal lagoon (hard-peloid grainstone) Ooid Shoal (ooid grainstone) Skeletal Shoal (skeletal grainstone and packstone) Near shoal heterolithic (tidally influenced mudstone to grainstone) Foreshoal (storm-bedded mudstone to grainstone) I Breccia Laminites (<1 cm) Thin beds (1-4 cm) Cross-beds Ooids Corals Pellets/peloids Quartz sand Burrows Brachiopods Chert Echinoderms/generic skeletal Figure 1. Cross section showing detailed measured sections and porosity logs for measured locations. was determined through measurement of plugs drilled directly from outcrops. mud
2nd- Order CS NW 3rd- Order V IV Benbow Mine Road (modified from Elrick, 1991) Shoshone Canyon Sheep Mountain (modified from Sonnenfeld, 1996) Wind River Canyon Dolomite vs. Limestone Cross-Section Tensleep Canyon Casper Mountain Fremont Canyon Hartville Quarry DATUM - Base of Breccia in Seq. IV III chalky II Feet Meters Dolomite Grst/Pkst I Mdst/Wkst 8 BM 6 SM SH Montana Wyoming 5 4 TC Limestone Grst/Pkst Mdst/Wkst 5 miles WRC 2 FC CM Figure 2. Dolomite vs. Limestone distribution in the Madison Formation. Dolomitzation is enricjed in the transgressive portion of the second order supersequence. Pervasive dolomitization of 3rd-order sequences I-III in TST is confined to the middle part of the ramp. Laterally extensive dolomitized intervals occur at bases of both composite sequences. HC
Sequence I Sequence II Sequence III Sheep Mountain 315 31 5 95 295 29 9 285 28 275 85 27 265 26 8 255 25 245 75 24 235 2 7 225 22 215 21 65 25 2 195 6 19 185 18 55 175 17 165 5 16 155 15 45 145 14 135 1 4 125 12 115 35 11 15 95 9 85 8 25 75 7 65 6 55 5 15 45 4 35 1 25 2 15 1 2 5 5 MWPGBr Figure 3. Composite Sequence with basal two sequences and 4th and 5th order cysles. In this downramp section dolomitization is not complete but is mostly occurring in the transgressive portions of the higher frequency cycles.
8 BM 6 5 miles Montana Wyoming SH SM WRC 4 TC 5 CM FC 2 Dolomitized Grainstone Fairway HC Figure 4. The middle part of the Madison ramp is pervasively dolomitized and consists of highly permeable grainstones. This grainstone fairway is the optimum location for production in the Madison limestone. Opportunities exist outside this fairway but reservoir quality progressively decreases in both the updip and downdip direction.