Delta-Destructive Spit Complex in Black Warrior Basin: Facies Heterogeneity in Carter Sandstone (Chesterian), North Blowhorn Creek Oil Unit, Lamar County, Alabama

Abstract

ABSTRACT The Carter sandstone in North Blowhorn Creek oil unit is the most productive oil reservoir in the Black Warrior basin and is a localized, elongate, quartzarenite body enclosed by shale. The reservoir represents a spit-style beach system in a shoal-water delta complex; this type of reservoir is not widely recognized in ancient rocks, but similar, localized beach deposits are abundant in the Black Warrior basin and perhaps in many other sedimentary basins. To understand facies heterogeneity and the resulting controls on flow within this poorly understood reservoir type, this study employed an interdisciplinary approach that synthesized stratigraphic, sedimentologic, petrologic, and petrophysical data. The North Blowhorn Creek reservoir is composed of many imbricate, clinoformal sandstone lenses that decrease in size along the length of the reservoir. Rock types, sedimentary structures, and log signatures vary systematically within the lenses. This variation reflects a transition from shoreface and foreshore environments in the depositionally downdip parts of the lenses to backshore environments in the updip parts. Shoreface and foreshore sandstone contains the best interconnected pore system and is thus the principal reservoir rock, whereas backshore deposits contain abundant depositional and diagenetic baffles and barriers to fluid flow that adversely affect reservoir quality. Flow is favored in amalgamated sandstone lenses along the axis of the reservoir, but segmentation of the reservoir as the size of the imbricate lenses decreases favors complex flow patterns.