Hydrocarbon exploration in Lower Cretaceous rocks of western Gulf Coastal Plain dates from 1920's when such shallow fields as Luling, Darst Creek, and Salt Flat were discovered. Deeper exploration for objectives in Lower Cretaceous increased sharply following 1954 discovery of Stuart City field, LaSalle County, Texas. The rock column under consideration, dates from Neocomian to upper Albian including, in ascending order, following stratal units: Trinity, Fredericksburg, and Washita. While deep Lower Cretaceous activity is often referred to as the Edward play, objective horizons actually fall within 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 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 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 Edwards and Glen Rose where dolomitization and dissolution have greatly improved reservoir End_Page 353------------------------------ and from structural closures coexistent with rudistid facies. Initial production and productive history of reservoirs in rudistid-bearing rocks have been disappointing. Exploration in trend has been based on close correlation of seismic field efforts and regional stratigraphic studies. Detailed studies of 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. End_of_Article - Last_Page 354------------