Use of Cores and Image Logs to Interpret Depositional Environments of Sandstones in the Dakota Group at South Baxter Field, Sweetwater County, Wyoming

Longman, Mark W.¹, Randy Koepsell², and Stephen Sturm^3
1Questar Exploration and Production Co, Denver, CO
²Schlumberger, Greenwood Village, CO
³Schlumberger Oilfield Services, Greenwood Village, CO

     The Lower Cretaceous Dakota Group is 230 ft thick in the area of South Baxter Field, southwestern Wyoming and was continuously cored in the South Baxter #27. The unconformable contacts with the underlying Jurassic Morrison Formation and the overlying mid- Cretaceous Mowry Shale were recovered in the core along with three sandstone packages that form important reservoirs in the field. Additionally, high-resolution image logs reveal a variety of features in the sandstones including crossbed dip angle and direction, root structures, scour surfaces, pyrite nodules, shale rip-up clasts, and natural and induced fractures.
     The sandstone package at the base of the Dakota is 21 ft thick and is composed of medium- to fine-grained chertarenites in a finingupward fluvial channel deposit. Porosity ranges from 16 to 28%. Measured oil saturations were only 7 to 25% and low resistivities of 3 to 10 ohm-m indicate the interval is water bearing.
     A sandstone package about 40 ft thick in the middle Dakota consists of stacked fining-upward fluvial channel sands, each about 5 to 10 ft thick. These sandstones are medium- to very fine-grained with more quartz grains (~80 to 90%) and less chert (5-7%) than the basal sandstone. Porosity ranged from 11 to 26% but low resistivities suggest this interval is water bearing.
     The sandstone package capping the Dakota is 43 ft thick with a 10-ft-thick fining-upward fluvial package at the base overlain by 33 ft of sandstone composed of 6- to 8-ft-thick amalgamated parasequences. These sandstones are very fine- to medium-grained cherty sublitharenites. As with the other sandstones, vector plots generated for this interval from the image log suggest a generally northerly sand transport direction off the ancestral Uinta Mountains. There is a sharp contact between this blocky sandstone and the overlying organic-rich deep marine shale of the Mowry.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah