ABSTRACT

The Stuart City Trend, South Texas, represents a climax biogenic development along the lower Cretaceous (upper Aptian, Albian, and lower Cenomanian) shelf margin. Landward of this trend, a wide variety of shallow-water shelf carbonates accumulated on a broad, relatively flat platform. Seaward, the entire section consists of dark-colored planktonic foraminifer-bearing argillaceous carbonates. The sediments of the Stuart City Trend make up the Stuart City Limestone, which attains a total thickness of 2,000 to 2,500 feet. Time-equivalent outcropping rocks in Central Texas are the Glen Rose and Edwards Formations. Between 1954 and 1961 a number of wells, ranging in depth from 11,000 to 20,000 feet, were drilled with the Stuart City Formation as their final objective. Of the 19 wells from which cores were obtained for this study, 12 were considered gas wells with initial production ranging from 1.5 to 36.5 mmcfgd. Six of these wells are reported as still producing gas today. The objectives of this paper are to describe the deposition facies and environments which occur along this trend, to show their relationship to the diagenesis and porosity development and to provide a model for further hydrocarbon exploration, both along the Stuart City Trend and also the deeper Sligo Trend.

The Stuart City carbonates have been assigned to five major environments of deposition: shelf lagoon, shelf-margin, upper shelf slope, lower shelf slope, and open marine. The shelf-lagoon facies include miliolid wackestone, mollusk wackestone, toucasid wackestone, and mollusk-miliolid grainstone. These facies accumulated under generally lowenergy condition in water depths from 0 to 20 feet. In contrast, the narrow band of shelf-margin carbonates is made up of coral-caprinid boundstone, requienid boundstone, and rudist grainstone, all of which accumulated in moderate to high energy conditions and in less than 15 feet of water as a complex of reefs, banks, bars and islands. Seaward of the shelf margin, the upper shelf-slope environment comprises the caprinid-coral wackestone and coral-stromatoporoid boundstone facies, the lower shelf slope, the intraclast-grainstone, echinoid-packstone, and echinoid-mollusk-wackestone facies. Further seaward in water depths greater than 60 feet, the open marine environment is represented by the planktonic-foraminifer wackestone.

Porosity in the carbonates of the Stuart City Trend is divisible into two main types: those which are fabric related and those which are non-fabric related. Primary fabric-related types consist of intraparticle and interparticle. Primary intraparticle porosity, openings within the body chambers of the rudists, occurs in the caprinid-coral wackestone, coral-caprinid boundstone and requienid boundstone. Primary interparticle porosity was originally very high (greater than 30 percent) in the rudist grainstone facies, but cementation soon after deposition due to submarine, phreatic and meteoric conditions reduced the porosity to less than 10 percent and late subsurface cementation filled the remaining porosity. Primary interparticle porosity now occurs only in a few very thin intervals.

Secondary fabric related porosity consists of solution-enlarged interparticle and moldic. Both occur in the boundstone and grainstone facies but in very thin restricted units. The poor development of solution-enlarged interparticle and moldic porosity reflects the minor role that subaerial exposure played during the development of Stuart City Trend.

Non-fabric related porosity consists of vertical fractures. Open, non-lined, vertical fractures are extremely abundant in a few wells.

The low porosities encountered in the past along the Stuart City Trend are the result of lack of significant periods of subaerial exposure for development of secondary types and of massive cementation which destroyed primary types. Further exploration along this trend should be aimed at identifying areas which may have been exposed soon after deposition, for development of secondary types; or areas which subsided more rapidly, for preservation of primary types.