Petrophysical Interpretation of the Oxfordian Smackover Formation Grainstone Unit in Little Cedar Creek Field, Conecuh County, Southwestern Alabama

Abstract

A petrophysical study of the upper grainstone/packstone reservoir of the Oxfordian Smackover Formation in Little Cedar Creek Field was conducted, integrating core description, thin section analysis, log interpretation and cathodoluminescense to characterize controls on oil production in the upper reservoir. Little Cedar Creek Field produces approximately 2.4 million barrels (bbls) of oil annually and is currently in secondary recovery. By analyzing petrophysical characteristics such as porosity and pore type and correlating them to facies changes, better predictions can be made to optimize secondary recovery.

The diagenetic history of the ooid-peloid grainstone records six separate events. Early marine phreatic dogtooth sparry rim cement helped create the framework that allowed it to maintain a good portion of its depositional porosity as it underwent subsequent compaction, dissolution and cementation events. The most common porosity types are vuggy, oomoldic and intercrystalline.

The Smackover Formation ooid-peloid grainstone/packstone unit consists of multiple alternating ooid-peloid grainstone and peloid packstone/wackestone facies with varying porosity types. The most common types are oomoldic and vuggy with a range of preserved intergranular porosity. Porosity in the grainstone facies averages 17% and 5.6% in the packstone/wackestone facies. The number of facies changes within the upper reservoir does not play a significant role in controlling well production. Facies changes are too thin to be identifiable utilizing well logs alone, although neutron and density well logs do trace a close relationship between log values and core plug analysis values of porosity. Core reports indicate that porosity and permeability correlate strongly with pore size and facies. Areas with thicker accumulations of grainstone facies have higher porosity and permeability values and have higher oil production. Isopach maps of the cumulative grainstone facies indicate thick build-ups parallel to strike for the formation, consistent with a shoal environment. The strongest predictor of well production is the cumulative thickness of grainstone facies within the grainstone/packstone unit of the Smackover Formation. The grainstone is thickest in the southwest part of the field and pinches out updip in the northwest. Secondary recovery gas injection would be most effective if applied in the southwestern portion of the field because it could effectively sweep the oil updip towards the stratigraphic trap.