Abstract: Diagenetic Constraints on Use of Predictive Sedimentary Models in Exploration-Exploitation of Carbonate Reservoirs

Clyde H. Moore, Jr., Jack W. Becker

The Jurassic Smackover Limestone long has been a prolific Gulf Coast oil producer. The regional distribution of favorable reservoir facies, primarily lime grainstones, can be explained in terms of the epeiric sea-shelf sedimentary model. The Smackover and the overlying Buckner record the progressive offlap of the environmental elements of this model during a major Jurassic regressive episode. The oolitic lime grainstones of the upper Smackover reservoir facies represent a shallow high-energy zone at the intersection of wave base with gently sloping shelf. The pelleted, oncolitic lime mudstones, wackestones, and dolomites of the middle and lower Smackover members represent the slightly deeper, low-energy offshore environment below wave base. The overlying Buckner shales, d lomites, and evaporites represent a landward, low-energy restricted lagoonal to sabkha complex. Jurassic salt tectonism regionally modifies the shelf model by localizing high-energy conditions over isolated salt-related topography. The southwestern Persian Gulf is a modern analog of Smackover-Buckner carbonate sequences.

The Walker Creek field, southern Arkansas, seems to confirm the Smackover-Buckner shelf model. The reservoir facies has been described as a series of stacked oolite sand bodies isolated by lagoonal muds resulting in multiple reservoirs. Petrographic studies reveal that the reservoir actually consists of a continuous sequence of lime grainstones with little intercalated lime mud. Reservoir isolation is the result of early water-table cementation on periodic exposure during deposition. Early carbonate cements were precipitated in the meteoric-phreatic zone while primary porosity was being preserved in the overlying meteoric-vadose zone. Primary porosity has been preserved preferentially over active diapiric salt structures because vadose conditions persisted longer across these topograp ic highs.

The initial development of porosity at Walker Creek is the result of sedimentologic processes considered in the predictive model of the Smackover-Buckner. However, the preservation of this porosity is the result of the diagenetic environment during and shortly after deposition.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC