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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 57 (1973)

Issue: 4. (April)

First Page: 793

Last Page: 794

Title: Big Piney-La Barge Producing Complex, Wyoming: ABSTRACT

Author(s): Robert E. McDonald

Article Type: Meeting abstract


The Big Piney-La Barge complex has been the major gas producing area in Wyoming since 1957, and during the past 10 years it also has become a leading oil producing area. Recoverable natural gas is estimated at 3 Tcf and recoverable oil exceeds 100 million bbl.

The area is along the western margin of Wyoming's Green River basin, occupying a zone of transition between the overthrust belt and the basin. This position of transition typifies much of the depositional history for this part of Wyoming, whose stratigraphic record reflects both the influence of the craton on the east and the Cordilleran geosyncline on the west.

Productive reservoirs include the Triassic-Jurassic Nugget Sandstone, the Cretaceous Dakota, Frontier, and Mesaverde Formations, and sandstones of Paleocene age. Trapping mechanisms are diverse, and structural, structural-stratigraphic, and purely stratigraphic accumulations are represented.

Commencing in Late Triassic and extending into Early Jurassic time, the Nugget was deposited as a massive blanket of largely eolian sand. In Early Jurassic time, seas spread southward across the Cordilleran front and the Wyoming craton during a major marine pulse that deposited the Twin Creek Limestone on the west, and Gypsum Springs and Sundance Formations on the east. The Gypsum Springs appears to be recognizable, but the Twin Creek terminology is better applied at La Barge.

Jurassic time closed with continental deposition of the Morrison sequence. On the Wyoming shelf the transition from Jurassic to Cretaceous is commonly a straightforward transition from continental Morrison through transitional Lakota-Dakota and into marine Thermopolis Shale. La Barge appears to have been a hingeline in Early Cretaceous time and relations are not clear cut. All shelf units are recognizable and shelf terminology may be applied on the east side of the complex, but in a distance of 12 mi these units become nearly unrecognizable.

In early Paleocene time the platform underwent folding, thrusting, and erosion. The Hogsback thrust was moving eastward and the newly formed La Barge anticline was deformed further. North of the platform a major drainage system developed that flowed southward and southeastward through the Green River basin. As Paleocene progressed, the Hogsback thrust reached its present position and aggradation commenced across the Big Piney-La Barge area. Deposition was dominated by a coarse, commonly conglomeratic facies adjacent to the Hogsback thrust, which graded abruptly through an alluvial, piedmont, and small stream fluvial unit. This low-energy facies graded basinward into a high-energy fluvial and paludal facies

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of the large river environment. As the basin subsided further, the major drainage facies encroached onto the flanks of the La BArge platform, resulting in buildups of thick channel, bar, and point-bar sands into the lower energy muds and silts. It is at this transition between the high- and low-energy environments the Paleocene is most commonly productive.

In early Eocene time the disturbed belt gave a last shudder, reflected by the La Barge thrust and other faults that break Paleocene strata. Of particular interest are two east-dipping thrusts, and the tendency for the young thrusts to terminate as tear faults which assist in controlling accumulation. Tectonic activity since early Eocene time has been confined primarily to regional elevation and slight basinward tilting.

The Frontier sandstones are the major gas reservoirs on the platform, and demonstrate a marked thinning from west to east. The second Frontier appears to reflect parallic depositional environments and strikingly variable reservoir conditions. Following deposition of the second Frontier, transgression resulted in the deposition of a few thousand feet of marine shale, except over the western part of the La Barge area, where a major drainage from the west built a delta into the shallow Coloradan sea. This unit is locally called the first Frontier.

Regression from west to east in Montanan time deposited the fluival and paludal sediments of the Mesaverde Formation. Littoral transitional sands at the base of the Mesaverde commonly are productive both on subcrop and across structure.

Isopach maps of Cretaceous units suggest the existence of the Moxa arch, a structural feature that now plunges southward from the La Barge platform. It was a northward-plunging positive feature during Cretaceous deposition. In latest Cretaceous and early Paleocene times, the Big Piney-La Barge area was uplifted and folded into a large anticlinal feature, with erosion stripping away newly deposited Mesaverde.

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