Diagenetic Control on Permeability Baffles and Barriers, Carter Sandstone, North Blowhorn Creek Oil Unit, Black Warrior Basin, Alabama

KUGLER, RALPH L., Geological Survey of Alabama, Tuscaloosa, AL

Mississippian (Chesterian) Carter sandstone is the most productive oil reservoir in the Black Warrior basin of Alabama. In North Blowhorn

Creek oil unit, very fine- to medium-grained quartzarenite and sublitharenite occur in an elongate, isolated northwest-southeast trending body, surrounded by shale. The sandstone was deposited in a beach-barrier environment. Most production is from ripple-laminated and horizontal- to low-angle-planar laminated shoreface and foreshore deposits in the central part of the reservoir body. Uneven distribution of diagenetic features creates permeability baffles and barriers at several scales within the reservoir, ranging from microscopic to macroscopic, and increases tortuosity of fluid flow. Early authigenic calcite and ferroan calcite occur only in shell lags deposited in channels within the reservoir body. These originally porous and permeable layers are completely cemented by calcite, f rroan calcite, and ferroan dolomite. Carbonate-cemented shell lags form discontinuous permeability barriers that may be laterally continuous between adjacent wells. Ferroan dolomite is the most abundant cement in Carter sandstone and occludes all pores near the margins of the reservoir body. The pore system within the high-quality portion of the reservoir consists of modified primary and secondary intergranular porosity and microporosity within patches of kaolinite. Porosity and permeability relationships are controlled by the distribution of quartz overgrowths, kaolinite, deformed mudstone fragments, intergranular pressure solution, and stylolite seams. The lateral extent of baffles and barriers created by these diagenetic features is related to depositional texture and ranges from micr meters to meters.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)