Paleoenvironment and Reservoir Distribution of Upper Glen Rose Formation at Alabama Ferry and Fort Trinidad Fields, Houston and Leon Counties, Texas

Allen K. Cregg

Alabama Ferry and Fort Trinidad fields (Houston and Leon Counties, Texas) are located updip of a break or reentrant in the Lower Cretaceous shelf-margin reefs. The reentrant probably allowed an unusual amount of tidal energy to pass from the Gulf of Mexico into the relatively shallow East Texas basin and affected formation of shoal complexes throughout much of the Early Cretaceous.

Alabama Ferry and Fort Trinidad fields produce oil and gas from stratigraphic traps in ooid-skeletal bars contained within shoal complexes of the upper Glen Rose zones A through G. These zones represent cyclic transgressive-regressive limestone-shale sequences deposited across much of the East Texas basin. At Alabama Ferry field, each cycle is generally 50-200 ft thick. The cycles are composed of various high-energy shoal-complex grainstones and packstones bounded above and below by lower energy shelf interior to lagoonal mudstone and/or wackestone or shales. Reservoirs are generally restricted to the 10 to 50-ft skeletal-ooid grainstone bars of the shoal complex. There are also 1 to 8-ft occurrences of more porous coarse-grained skeletal clastic grainstones, interpreted as tidal-chan el lag deposits associated with grainstone bars. The cyclic sedimentation present in the East Texas upper Glen Rose may have been achieved by interaction of an oscillatory variation in sea level with a linear rate of subsidence.

A thickening in the Cretaceous units above the Glen Rose occurs between the two fields. This thickening may present a barrier between the predominant oil pay at Alabama Ferry and the gas pay with an oil rim at Fort Trinidad. The diagenetic overprint of the rock is commonly favorable with respect to reservoir quality. Within ooid-skeletal grainstones, the amount of secondary moldic porosity created by freshwater leaching of ooid nuclei and skeletal clasts usually outweighs the emplacement of various calcite cements. Both of these events occurred in the meteoric phreatic environment and probably fairly early in the diagenetic history of the rock.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.