KUGLER, RALPH L., JACK C. PASHIN, and RICHARD E. CARROLL, Geological Survey of Alabama, Tuscaloosa, AL

ABSTRACT: Controls on the Occurrence and Producibility of Carter Sandstone (Chesterian) Oil Reservoirs in the Black Warrior Basin of Alabama

More than 80 percent of oil produced from the Black Warrior basin in Alabama was extracted from Carter sandstone. More than 95 percent of that production is from a northwest-southeast trending belt of lensoid, very fine-grained quartz-arenitic beech sequences deposited in a shoal-water deltaic complex developed along the Bangor carbonate bank margin. The remaining Carter production is from northeast-trending lobate and elongate sandstone bodies representing deposition in constructive, river-dominated, deep-water deltaic lobes to the southwest. Reservoirs within the shoal-water deltaic complex consist of imbricate, amalgamated to discrete sandstone lenses, but the geometry of sandstone bodies is variable. For example, sandstone in North Blowhorn Creek oil unit, which accounts for two-t irds of total oil production, was deposited in a spit complex consisting of a series of clinoformal lenses that decrease in size toward the southeastern terminus of the reservoir. This reservoir is the largest, best preserved sandstone body in the shoal-water deltaic complex. Smaller, less productive reservoirs are isolated, reworked remnants of similar beach systems.

Shoreface and foreshore deposits contain the best interconnected pore system, whereas backshore deposits have abundant depositional and diagenetic baffles and barriers to fluid flow, lower effective porosity, and higher water saturation. Pore and pore-throat size distributions in reservoir sandstone are polymodal owing to a mixture of effective intergranular macroporosity and ineffective microporosity in authigenic and detrital clay. The combined effects of variation in grain-size and in the abundance of clays result in local order-of-magnitude scale permeability contrasts within shoreface and beach deposits that affect sweep efficiency during improved-recovery operations. Ferroan dolomite cemented zones formed around shell accumulations are discontinuous barriers to fluid flow. Altho gh the present diagenetic character of Carter reservoirs is the result of late-stage events during burial diagenesis, the distribution of diagenetic features is directly related to depositional components of the reservoirs. Patterns of oil production from individual reservoirs also closely correspond to depositional architecture. Characterization of the spatial distribution of fluid-flow properties of Carter reservoirs through integrated sedimentologic, petrologic, petrophysical, and geostatistical modeling provides a valuable supplement to engineering analysis of reservoir performance during planning and evaluation of improved-recovery operations.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.