Alabama-Ferry Field, Leon County, Texas: Facies Types and Diagenetic Aspects of an Upper Glen Rose Stratigraphic Trap

Anthony J. Lomando, Dana L. Roy, Lawrence Bruno

Alabama-Ferry field is a mid-shelf cyclic shoal complex located approximately 15 mi (24 km) landward of the Glen Rose shelf margin. Since its discovery in 1983, over 110 wells have been drilled to develop the field and define the limits.

The interplay of dominant particle type, texture, and diagenesis controls reservoir quality. Grainstone and some packstone textures composed of skeletal, intraclast, and ooid particle types are the principal reservoir facies. Overall particle composition of grainstone units varies from near-pure end members to various mixed subfacies, with accessory peloids, forams, echinoderm fragments, and others. Grainstone facies belts vary from east to west across the field. Skeletal grainstones dominate the eastern portion of the field in a roughly north-south belt. Intraclast, ooid, skeletal, and mixed grainstone subfacies are spread across the broad western portion of the field.

The highest-quality reservoir textural type is skeletal grainstone, with interparticle and well-developed moldic porosity. Intraclast and ooid grainstone end members are good and moderate-to-poor reservoir rock types, respectively, with interparticle and microporosity as the principal pore types. Abundant microporosity, often found within ooids and intraclasts, is associated with relatively higher irreducible water saturations and less-effective pore systems (lower interconnectivity). The overall reservoir quality of different mixed-particle subfacies is generally dependent on the proportion of dissolution-prone skeletal fragments (moldic porosity).

The suite of pore types and diagenetic events in Alabama-Ferry field are similar to many Lower Cretaceous carbonate reservoirs in the Gulf Coast province. Interparticle and early formed moldic pore spaces are commonly lined with marine fringing cements, often isopachous. Compaction, in several forms, can have a major effect on reservoir quality. Micro- and macro-stylolitization, primarily in intraclast and ooid grainstones, reduces pore space through volume reduction and release of calcite in solution, which reprecipitates locally as spar cement. The amount of calcite spar cement can range from sparse up to near-complete pore volume occlusion in any facies. Even without significant compaction, porosity in some skeletal grainstones with formerly high interparticle and moldic pore volum can be completely reduced by a combination of calcite spar and baroque dolomite cements. Throughout the field, variable amounts of baroque dolomite, as well as accessory anhydrite and authigenic quartz, occur as late-stage cement types. Brittle interparticle and moldic pore collapse reduces some pore volume but can enhance permeability. Spalled ooid cortices and cement rinds are usually associated with plastic deformation of ooids and intraclasts. Fractures are sparse and contribute little to reservoir pore volume or permeability. Therefore, in the cyclic shoal complex at Alabama-Ferry field, as with many carbonate grainstone reservoirs, optimum facies combined with porosity preservation are the keys to ultimate reservoir quality.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.