Reservoir heterogeneity in a carbon sequestration target: The Donovan Sand member of the Rodessa Formation, Citronelle field, Alabama

The Citronelle field, located in Mobile County, Alabama, has been chosen as a U.S. Department of Energy (DOE) Carbon Capture and Storage (CCS) pilot site, and supercritical CO2 has been injected for the combined purposes of long-term storage testing and enhanced oil recovery (EOR). The target formation for injection is the Early Cretaceous Donovan Sand of the Rodessa Formation, and is characterized by discontinuous fine- to medium-grained, heterogeneous arkosic fluvial sandstones interbedded with fissile mudstones and micaceous silts. Based upon core analysis of the Donovan Sand, depofacies indicate a fluvio-deltaic/estuarine depositional environment for the Donovan Sand. Probable sources of the Donovan sediments are the Motts Gneiss and the Hospilika Granite, both of which are exposed within the Pine Mountain Window at the Georgia/Alabama state border and into central Georgia. Through synthesis of core, thin section, and well log analysis, reservoir heterogeneity and porosity development within the Upper Donovan Sand is highly variable, with porosity averages of ∼2-5%, but locally can be up to ∼13%. In more porous zones there is more evidence of grain alteration and creation of secondary porosity, while in the lower porosity sections cementation or high matrix content fill interstitial space and the main pore type is primary pores. Porosity and cement percentages, rather than mineralogic composition, were used to categorize sandstones into petrofacies due to the consistent mineral composition of the Donovan Sand. According to petrofacies-to-well log correlations, the most porous petrofacies are present within the same stratigraphic zone (defined herein as a high porosity zone ) and this zone is interpreted to represent a porous (\u3e 5% &phis;) fluid migration pathway in the subsurface. An Upper Donovan Sand structure map, thickness map, and average porosity map generated using transform equations from SP and bulk density curves show that the thickest portions of the Upper Donovan do not directly correlate with the most porous areas within the formation. This research may serve as an analog for other mature oil and gas fields and CCS sites targeting fluvial-deltaic reservoirs