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Hydrocarbon exploration in Lower Cretaceous rocks of the western Gulf Coastal Plain dates from the 1920's when such shallow fields as Luling, Darst Creek, and Salt Flat were discovered. Deeper exploration for objectives in the Lower Cretaceous increased sharply following the 1954 discovery of Stuart City field, LaSalle County, Texas.
The rock column under consideration, dates from Neocomian to upper Albian including, in ascending order, the following stratal units: Trinity, Fredericksburg, and Washita. While the deep Lower Cretaceous activity is often referred to as "the Edward reef play," objective horizons actually fall within the Edwards, Glen Rose, and Sligo limestones.
The middle Trinity Pearsall shale and middle to late Washita Del Rio shale are widely distributed, easily mapped units. Limestones between these key beds are composed of three generalized lithofacies grading from north-northwest to south-southeast as follows: (1) Carbonate rocks of shallow-water origin characterized by mudstones, wackestones, and packstones in which miliolid and larger foraminifers, oolites, and algal structures are common. Evaporites are locally abundant and dolomite is widely developed; (2) Wackestones, boundstones, and grainstones of shallow-water origin in which the dominant faunal elements are rudistids, corals, algae, and stromatoporids; (3) Carbonate mudstone of somewhat deeper-water and more open-sea origin bearing pelagic foraminifers and calcispheres.
Lithofacies 1 and 3 have widespread distribution whereas the rudistid-bearing rocks are limited to a rather narrow band along platform margins, and have thus become known as "the Edwards reef."
These Lower Cretaceous rocks produce from fault closures in the Edwards and Glen Rose where dolomitization and dissolution have greatly improved the reservoir
and from structural closures coexistent with the rudistid facies. Initial production and productive history of reservoirs in the rudistid-bearing rocks have been disappointing.
Exploration in the trend has been based on close correlation of seismic field efforts and regional stratigraphic studies. Detailed studies of the "reef complex" in an attempt to determine areas of best porosity and engineering studies related to reservoir stimulation, are necessary before this trend becomes economically more attractive.
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