ABSTRACT

Lower Cretaceous sediments were deposited throughout the Gulf Coast during the Neocomian, Aptian, Albian, and Lower Cenomanian Ages. Basal sandstone extend across the entire region, onlapping Upper Jurassic terrigenous sediments. Following the initial stage of detrital rock deposition, a shallow epicontinental sea covered the western coastal plain and regions to the south and west. In this area, and also in the seaward parts of the eastern coastal plain, shallow-shelf carbonate rocks were deposited contemporaneously with subsidence. In east Texas and adjacent areas, periods of peneplanation of the bordering land and deposition of carbonates alternated with regressive periods when the hinterland was uplifted and deposition of the land-derived material exceeded subsidence. The southern Appalachians were rising, at irregular rates, throughout the Lower Cretaceous time, and furnishing sediments (mainly sand and shale) to the Mississippi Embayment and the eastern Gulf Coast.

Lower Cretaceous rocks are the most widespread and have the greatest volume of any major Gulf Coast stratigraphic division. They are believed to underlie an area of about 340,000 square miles and have a volume of more than 200,000 cubic miles. The area presently productive of oil and gas, extending from Mexico to southwestern Alabama, has an area of 83,000 square miles, and a volume of 60,000 cubic miles. One and one-half billion barrels of oil and 10 1/2 trillion cubic feet of gas have been produced from Lower Cretaceous sandstones and carbonates in this proven belt. Landward from the productive belt in Texas and Arkansas is a narrow belt which is considered non-prospective. This belt widens eastward across central Mississippi, southern Alabama, and Georgia and northern Florida where continental "red beds" with thickness up to 4,000 feet are present. A prospective belt and a speculative belt are gulfward from the proven area.

Depositional conditions of the extensive and thick Lower Cretaceous sediments were favorable for the development and preservation of vast amounts of hydrocarbon source materials and for the formation of many reservoir rocks and stratigraphic-structural traps. An environmental analysis of each stratigraphic unit indicates a very large petroleum potential for this group of sediments. Many accumulations will be found in the prospective belt, on land as well as under the continental shelf of Mississippi, Alabama and Florida. The undiscovered accumulations occur in deltaic sandstones, carbonate reefs, and shell mounds. In the speculative belt of coastal and offshore Louisiana and Texas, the objectives are limestone reefs which developed on the landward side of positive blocks. Many new fields will be found in the productive belt, and there will be lateral and deeper extensions of producing fields.