Abstract The recognition of a forebulge in the subsurface is difficult because of its low amplitude and wide extent. It is further complicated by the subsequent tectonic modification (by the Laramide orogeny in this case) that may have overprinted the forebulge with complex younger structural patterns. Three-dimensional (3-D) flexural numerical modeling provides a strong supportive tool to help predict forebulge locations and focus subsurface search on their subtle isopach expression. Based on detailed well-log correlations and good outcrop control, three regional cross sections were established to identify Late Cretaceous forebulges in southwestern Wyoming. Along these sections in the Greater Green River Basin (two east-west and one northwest-southeast), the existence of forebulges was only recognized in the southern section. In response to the progressive eastward movement of the Crawford, early Absaroka, and late Absaroka thrusts, the forebulge migrated eastward to the Moxa Arch, the Rock Springs Uplift, and the Washakie Basin, respectively. The 3-D flexural modeling indicates that the forebulge was limited in its extent only to the southern part of the basin because of the distribution of thrust loads. The forebulge shifted southeastward over time because of the migration of these loads. The 3-D flexural modeling is critical to understanding Late Cretaceous forebulge migration across southwestern Wyoming.

A comprehensive surface and subsurface study of the Upper Cretaceous Lazeart Sandstone Member of the Adaville Formation and the lower coal-bearing part of the Adaville Formation in the southwestern Wyoming thrust belt reveals a complex inter-tonguing of marine and nonmarine strata. During late Santonian and early Campanian time, the Lazeart wave-dominated deltaic system prograded southeastward onto a storm-influenced microtidal shelf. Sediments of the Lazeart Sandstone Member accumulated within storm-dominated lower shoreface, barred fair-weather upper shoreface, foreshore, washover, mouth bar, flood tidal delta, and tidal channel subenvironments. Landward of the strand line, sediments of the Adaville Formation were deposited within active distributary, channel margin, bay-head delta, slough, interdistributary swamp, lake, interdistributary bay, salt marsh, lagoon, interdeltaic bay, and peat-forming swamp subenvironments. Nine transgressive-regressive pulses of this wave-dominated deltaic system were superimposed on the regressive portion of the Niobrara Cyclothem. Thicknesses of noncoal strata, grain size, and sand percent all decrease away from channels and thickness and lateral continuity of coal increase. Because of differential compaction, areas with thick peat deposits became sites of deposition of active channel, channel margin, and slough facies. Subsequently, many of these sites once again evolved into areas of thick peat accumulation. Individual seam thickness is limited by the magnitude of the associated transgressive-regressive pulse. During regressions of limited areal extent, coastal swamps remained in one place for a long period of time, and thick peats were deposited. High subsidence rates favored such limited regressions and the development of thick, localized coal seams. Local subsidence rates were highest when shoreface sands prograded into deeper water directly over marine muds. Thickest coal seams overlie and intertongue with thick shoreface, foreshore, and washover fan sandbodies. Subsidence of the sandbodies maintained a high water table and contributed to the development of stable peat swamps. In interdeltaic regions, lower sediment loads and rates of subsidence led to the accumulation of thinner peats. The distribution and total reserves of coal seams within the Adaville coal field is controlled by the number of transgressive-regressive pulses and their geographic extent. Vertical stacking of the regressive sand bodies maximizes total coal reserves for a given locality. Stacked shallow marine sandbodies and high total coal reserves for the Adaville Formation are a consequence of relative sea-level rise during a period of isostatic compensation to thrust and sediment loads of the Sevier Orogenic Belt.

Abstract Cores from the giant Whitney Canyon-Carter Creek gas field in western Wyoming show the nature of a siliciclastic filled karsted carbonate surface in an area with limited outcrop exposure. Regional outcrop studies by others document that a large carbonate platform was subaerially exposed during early Meramecian through Chesterian time across Wyoming, creating a karst surface on the Madison Limestone. From Chesterian to Morrowan time, a west-to-east marine transgression filled solution features with siliciclastic material of the Morgan (Amsden) Formation and equivalents. At Whitney Canyon-Carter Creek Field, the upper Mission Canyon Formation (Madison equivalent) contains this mixed carbonate and siliciclastic facies. The uppermost 70 to 100 ft (21 to 30 m) of the Mission Canyon at Whitney Canyon-Carter Creek is a correlatable transition zone from preserved evaporite and dolostone beds beneath to the overlying siliciclastics of the Morgan Formation. Cores contain fractured and stylolitized carbonates that underwent solution collapse and brecciation due to dissolution and the subsequent filling of cracks and vugs by siliciclastic sands. Solution-collapse breccias in the cores have a sandstone matrix and dolostone clasts that are rounded, stained to a pinkish hue, locally capped by red clay drapes, and sometimes aligned as if deposited by a current. Breccias interpreted as tectonic in origin have only cement with no matrix and contain angular clasts with matching clast boundaries, i.e., fitted, indicating that fracturing occurred without any extensive dislocation of the clasts. The mixing of carbonates and siliciclastics within brecciated intervals is significant because these observations suggest that the Mississippian karst plain and associated shoreline extended considerably further west than previously thought, especially when thrust belt structural positions are restored. Evidence presented here suggests that the maximum western shoreline position was at least 20 miles (32.5 km) west of the Utah-Wyoming border.

Abstract Geologic data from conventional cores and wireline logs have been used to better understand the controls on reservoir-quality porosity within the Mission Canyon formation of Whitney Canyon-Carter Creek Field. The Mission Canyon Formation (Madison Limestone Group) is a thick, regressive sequence that has been subdivided by previous workers into three major stratigraphic units: (1) a lower section of very fine-grained, black to dark gray limestone and dolomite deposited in a starved, anaerobic basin; (2) a middle "main" porosity zone of dolomite and limestone that accumulated on an oxygenated shelf; and (3) an upper fractured and brecciated section of sabkha dolomite and anhydrite. The upper unit was extensively altered during subsequent solution brecciation. All three stratigraphic units were observed in cores from the field, but the middle section was studied in most detail. Most Mission Canyon production in Whitney Canyon-Carter Creek Field occurs from porous dolomites of the middle "main" producing zone. This section is 300 to 350 feet thick and consists of porous dolomites interbedded with tight, finely crystalline calcitic dolomites or tight lime grainstones. The reservoir dolomites contain intercrystalline, moldic, fracture, and vug porosities. The best reservoir intervals have skeletal-moldie and intercrystalline porosities of greater than 9% and permeabilities exceeding 0.7 md. Although the earliest stages of diagenesis seem to have created the pprosity patterns observed in the reservoir, the carbonates were intensely stylolitized and fractured during burial and subsequent thrusting. Porosity was partially filled with later-stage calcite and anhydrite cements that are possibly related to fracturing. A similar vertical succession of facies was recognized in all of the cores. The wells are oriented roughly parallel to depositional strike of the facies and little variation occurred in that direction; therefore, the facies and porous dolomite zones are correlated easily where faults or other structural problems do not limit communication. Good stratigraphie horizontal communication apparently exists throughout the reservoir as only minor rock variability and differential cementation are present along layers. Vertical communication may be restricted by thin, tight limestone layers.

Interaction of the Rocky Mountain foreland and the Cordilleran thrust belt has taken place in the Snider Basin area, west of Big Piney, Wyoming. Here, the central segment (La Barge Platform) of the Moxa Arch, a Late Cretaceous, basement-cored uplift, plunges northwestward beneath, and disrupts the continuity of, the Darby, Hogs-back, and Prospect thrusts. Although to the north and south of the La Barge Platform, basement shortening appears to have been along low-angle thrust faults, this was probably not the case for the platform. The 2,600 m of west-verging structural relief along the southwest limb of the La Barge Platform occurs over about 5 km of horizontal distance and might be the result of shortening of the basement across a zone of discrete, closely spaced reverse faults, generally restricted to basement. This shear zone (here termed the La Barge shear zone) dips about 60°NE. Shortening in the basement appears to have been matched by shortening of the cover rocks along a pair of west-dipping reverse faults, the upper of which is the Tip-Top thrust. According to our interpretation, Tip-Top thrusting was synchronous with the emplacement of the La Barge Platform and thus predated the Hogsback thrust. Our work confirms earlier interpretations that the Darby and Prospect thrusts link in the subsurface below Snider Basin and continue to the south as the Hogsback thrust. According to our interpretation, Hogsback thrusting is a composite of Paleocene Prospect thrust movement and later Darby thrusting. In the area of the La Barge Platform, the Hogsback thrust was localized by following west-dipping Cretaceous beds on the monocline above the La Barge shear zone.

Abstract The lower Frontier Formation in the Moxa arch area of southwestern Wyoming is one of the most prolific gas–producing formations in the Rocky Mountain region. In this study, sedimentologic and stratigraphic analyses were conducted on cores and logs of Frontier wells from the Whiskey Butte and Moxa fields. Lower Frontier sediments were deposited as strandplains and coalescing wave–dominated deltas that prograded into the western margin of the Cretaceous interior seaway during Cenomanian time. Twelve sedimentary facies have been identified. The most common sequence consists of burrowed to cross–bedded nearshore marine (delta front, shore–face and inner–shelf) sandstones disconformably overlain by cross–bedded (active) to soft–sediment deformed (abandoned) distributary–channel sandstones and conglomerates. The sequence is generally capped by delta plain mudstones and silty sandstones. Low permeability sandstone reservoir facies are nonhomoge– neous and include crevasse splay, abandoned and active distributary channel, shoreface, foreshore, and inner shelf sandstones. Distributary–channel facies form 80% of perforated intervals in wells in the southern part of the Moxa area but only 50% to the north. Channel sandstone bodies occur on the same stratigraphic horizon, are laterally discontinuous with numerous permeability barriers, and are occasionally stacked. Upper shoreface and foreshore sandstones thicken to the north and east and are more laterally continuous than channel facies. The percentage of perforated intervals in upper shoreface and foreshore facies increases from 20% in the south to 50% in the north. The lower Frontier sandstones contain strike–oriented shoreface (delta front) and dip–oriented distributary channel sand bodies in approximately equivalent amounts. Delta–plain mudstones thin to the north and east and are an important stratigraphic seal. Highest gas production rates are from distributary channel sandstones closer to the axis of the Moxa arch. However, there appears to be little correlation between the thickness of any reservior facies and net production.