Abstract: Porosity Distribution Beneath a Regional Sequence Boundary: Alabama Ferry Field, Texas

Julie A. Kupecz

Alabama Ferry field, Leon County, Texas, produces oil and gas from stacked carbonate grainstones of the Cretaceous Glen Rose "D" interval. A seismically defined sequence boundary has been correlated via synthetic seismograms and core to the top of the productive grainstone interval. In addition, depositional interpretation of cores indicate the presence of a lower sequence boundary; thus, the Glen Rose "D" represents an entire depositional sequence. The sequence consists of a basal transgressive erosional (ravinement) deposit that grades into open marine shales and carbonate mudstones; together these sediments comprise a transgressive systems tract. The highstand systems tract is represented by a series of shallowing-upward parasequences bounded by open marine shales, resulting in a s ries of stacked grainstones. The top of the grainstones coincides with the upper sequence boundary.

Effects of exposure at the sequence boundary (i.e., meteoric diagenesis) on the distribution of porosity and permeability are complex, and are controlled by a combination of primary mineralogy of allochems, presence or absence of algal coatings, grain packing, and distribution of early cement.

Ooid grainstones were cemented early under marine phreatic conditions, and were relatively impermeable to the effects of meteoric diagenetic fluids. Where subject to dissolution, they developed microporosity, and thus have low permeability (1-10% porosity; ^Lt1 md permeability). Algal-coated grainstones comprise 10-90% coated grains (mostly aragonitic allochems), with the remainder being aragonitic allochems (pelecypods, gastropods, ostracods), and Mg-calcite oysters. Coated grains are rarely leached due to the more mineralogically stable algal coatings, and where they are, the result is micromoldic porosity. Non-coated aragonitic allochems are commonly leached. Therefore, both porosity and permeability are variable (8-20% porosity, ^Lt1 to 300 md permeability). Bioclastic grainstones constitute the best production in the field. This facies is composed of abundant aragonite allochems, and where subject to meteoric dissolution, displays interconnected moldic porosity (up to 20%) and good permeability (up to 450 md).

As a result of burial, porosity was decreased by compaction, stylolitization and corresponding local calcite cementation, and local dolomite cement. However, reservoir quality in the field is ultimately controlled by depositional facies, with the effects being expressed differently in different facies.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994