Reservoir Geology of Upper Mississippian St. Louis Limestone, South Mouser Field, Texas County, Oklahoma

Craig D. Caldwell, Harvey O. Vick III

Five porosity zones in the upper 80 ft (24.4 m) of the St. Louis Limestone have a cumulative production of over 1 million bbl of oil since St. Louis production was discovered at South Mouser field, Texas County, Oklahoma, in April 1983. A core from the upper 106 ft (32.3 m) of the St. Louis Limestone at South Mouser field is composed predominantly of bioclast, ooid, and mixed bioclast-ooid packstones, "washed" packstones, and grainstones. These lithologies reflect shifting of near-shoal and shoal environments on the shallow St. Louis shelf in the overall regressive sequence that includes both the upper part of the St. Louis Limestone and lower part of the overlying Ste. Genevieve Limestone. The lower 9 ft (2.7 m) of Ste. Genevieve Limestone in the core is a cross-stratifi d, fine-grained, sandy limestone.

Porosity in the cored St. Louis Limestone includes primary intergranular, leached grain, and minor vuggy, intragranular, and fracture. Within the five porosity zones, core porosity generally ranges from 6 to 14% and permeability from 1 to 100 md. Porosity development and cementation reflect, for the most part, meteoric diagenesis. Deposition and diagenesis of the upper 100+ ft (30.5+ m) of the St. Louis Limestone were markedly influenced by a local St. Louis paleohigh in approximately the same location as the present structural high at South Mouser field.

Cementation, compaction, and a significant amount of ineffective moldic porosity in the cored interval of the St. Louis Limestone result in increased pore system tortuosity. This indicates that an m value greater than 2.0 may be appropriate in S_w calculations. Additionally, in one porosity zone, ooids contain microporosity visible in SEM study. This porosity may hold irreducible water affecting S_w calculations made from wireline logs and resulting in an inaccurate assessment of the zone's ability to produce hydrocarbons.

Two distinct oil-water contacts occur in the upper St. Louis Limestone at South Mouser. The corresponding reservoirs are separated by a low porosity and permeability layer seen in core and correlated across the field using neutron-density logs.

AAPG Search and Discovery Article #91039©1987 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma, September 27-29, 1987.