Phayles Sandstone (Upper Cretaceous) Deltaic and Shelf-Bar Complex, Central Wyoming: ABSTRACT

Abstract

Outcrop and subsurface studies of Phayles Sandstone (basal Mesaverde Group), southeastern Wind River basin, indicate rapid deltaic progradation and subsequent formation of a shelf-bar complex. Net sand distribution determined from 175 well logs indicates a major deltaic lobe with maximum thickness of 45 m prograded at least 20 km basinward from exposures of deltaic and shoreface deposits. The West Poison Spider field is located approximately 8 km southeast and downdrift from this deltaic lobe. Study of 33 logs and 13 cores from this field indicates the reservoir is associated with elongate shelf sandbars. The bar complex is at least 10 km long, 8 km wide, and 15 m thick; bar-axes are oriented N40°W. Study of shoreface sandstones in outcrop suggests the paleoshoreline trended N50°W. Several distinct sandstone bodies are stacked within the complex. Three component facies are recognized: (1) cross-stratified medium-grained sandstone; (2) parallel-bedded (hummocky cross-bedded?) fine-grained sandstone; and (3) bioturbated fine-grained sandstone. These sandstones occur in repetitive successions with facies 1 capping the sequence and facies 3 forming the basal member. Log-response compares favorably with core descriptions permitting detailed facies correlations. Within the field, individual bars range from 3 to 8 m in thickness and pinch out both seaward and landward interfingering with bioturbated and rippled shelf siltstones. Sedimentary structures, stratigraphic relations, and petrography suggest the bar complex was derived from sands reworked from the deltaic lobe. The shelf bars migrated shoreward and stacked during a major transgression resulting in the deposition of the Wallace Creek Tongue of the Cody Shale. This shale overlies and forms an updip seal for the reservoir facies. End_of_Article - Last_Page 545------------