Abstract
The lower Glen Rose Formation in the subsurface of south Texas exhibits three cycles of shoal-water complexes, consisting of high-energy bank, bar, and biogenic reef deposits that developed on the Pearsall arch. Facies distribution shows that these elongate complexes trend northeast-southwest for at least 78 mi (125 km) and are located 44 mi (70 km) seaward of the Lower Cretaceous shoreline. Although barren of oil and gas, these vertically stacked cycles contain facies development and porosity preservation essential for attractive exploration targets. Each cycle represents three major depositional facies: open-marine shelf, shoal-water complex, and protected lagoon. The open-shelf facies is characterized by terrigenous mudstone to wackestone. The shoal-water complex consists of skeletal and oolitic grainstone surrounded by lower energy packstone deposits. High-energy patch reefs of coral, stromatoporoid, and caprinid boundstone cap the grainstones. Lagoonal deposits of low-energy wackestone and laminated mudstone overlie each of the shoal-water sequences and indicate seaward progradations, which were interrupted by transgressions of open-marine shelf deposits of the succeeding cycle. The high-energy patch reefs may have prograded seaward across the shelf as the initial build-up of the Stuart City shelf margin. Diagenesis during burial has resulted in loss of porosity in the rocks. The greatest remaining porosity occurs in the grainstone facies. There are two major porosity types: primary interparticle and secondary moldic. The primary porosity resulted from early meteoric cementation and preservation of high initial interparticle pore space prior to extensive grain compaction. The secondary porosity is the result of aragonite allochem dissolution. End_of_Article - Last_Page 1424------------
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