Reservoir Characterization of the Strawn (Desmoinsian)
from St. Lawrence Field, Glasscock County, Texas
David J. Sivils (Sivildj@texaco.com) and Emily L Stoudt/Texaco Exploration and
Production Inc., P. O. Box 3109 Midland, Texas, 79702
Abstract
The Strawn Formation is one of the more enigmatic hydrocarbon producing
formations on the Eastern Shelf of the Midland Basin. Of the Strawn fields, St.
Lawrence trend in Glasscock County, Texas is one of most prolific. In order to gain a
better understanding of the nature of the reservoir properties of the Strawn a continuous
core was taken by Texaco. The core is 120 ft think and includes both the upper and
lower contacts with the overlying Canyon sand/shale sequence and the underlying
“Atoka” Formation.
The core is composed of 22 high-frequency shallowing-upward cycles. Each
cycle begins with a black, fissile shale or black argillaceous lime mudstone containing
and open marine fauna. These shaly and argillaceous carbonates represent the initial
relative rise in sea level. Overlying the initial muddy part of the cycles is a succession of
carbonate facies indicating a continuous shallowing, reflecting a reduction in
accommodation space due to sedimentation, as well as an increase in water energy.
Each cycle is capped by grain-rich packstones or skeletal grainstones. These grain-rich
facies represent the primary reservoir facies in the Strawn Formation. The highfrequency cycles can be grouped into three main composite sequences, which likely
represent three fourth-order sequences, and are correlatable with cycle data from
additional cores on the Eastern Shelf. Each of these cycles and the composite
sequences are interpreted as representing fluctuations in a sea level driven by
continental glaciation.
The tops of cycles exhibit signs of exposure, including karstification and soil
development. Diagenetic process associated with sub-aerial exposure in some cases
acted to enhance porosity and in other cases to reduce porosity through pervasive freshwater cementation. Porosity enhancement was through the dissolution of unstable
caronate grains resulting in the development of moldic and vuggy porosity. Maximum
prosity measured in core samples in on the order of 10%, and correlates well with log
porosity. Zones of high porosity represent the best reservoir development, and can be
correlated over a wide distance on logs.