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.