RUDIST REEFS AS NATURAL GAS RESERVOIRS: THE LOWER CRETACEOUS STUART CITY TREND, SOUTH TEXAS

Rudist-bearing shelf margin reefs of the Middle to Late Albian Edwards Group constitute economically significant natural gas reservoirs within the Stuart City Trend of Texas. Extending 300 mi (480 km) from the U.S. – Mexico border across south Texas, the trend has produced over 1.3 trillion cubic feet of gas from approximately 700 wells. Recently acquired (post-2007) geologic and geophysical data provides new insights into the depositional facies and temporal-spatial variability of the Lower Cretaceous reef complex, as well as to the nature and character of reservoir pore systems. The Stuart City shelf-edge complex consists of massive reef wall, reef core, and reef flat facies containing microbial-coral-rudist framestones and bindstones, a back-reef facies assemblage containing abundant algal-coated grains and skeletal wacke-, pack-, and grainstones, and forereef slope facies. Calibration of core and wire-line log data to regional 3D seismic allows depositional facies to be mapped in detail, revealing spatial size and distribution patterns similar to modern barrier reefs. Facies distributions and shelf margin architecture appear to be controlled by a combination of eustacy, environment, and regional structural/tectonic elements. Physiography of the Middle to Late Albian Edwards margin varied greatly along strike, dependent primarily on the geometry and timing of associated faults and structure, including the position of the underlying Hauterivian-Barremian (Sligo) reef margin. A total of six producing sub-regions are currently defined for the Stuart City Trend based of structural setting, fault style, reservoir depth and thickness, and trap configuration. Reservoir quality within Stuart City facies varies considerably. Current burial depths are at maximum, ranging from 8,000 - 16,000 ft (2,400 – 4,900 m). Back reef grainstones and reef core boundstones generally constitute the most productive reservoir facies. Detailed analyses of thin sections reveal the major pore network in all Stuart City facies consists primarily of micropores (2 to 6 microns in size) formed by stabilization of abundant Mg-calcite in allochems (Lithocodium, foraminifera, peloids) and micrite rims.