Berea reservoirs in eastern Kentucky are developed in tight siltstone and sandstone. Precision horizontal drilling provides access to thin (<10 ft) stratigraphic traps, and a new shallow oil play has resulted. Bedding, porosity and permeability data, x-ray diffraction mineralogy data, and petrography are examined and compared in five cores to better understand stratigraphic traps and pore types.
Upper Berea reservoirs are developed in storm-dominated shelf and upper-slope siltstones and sandstones, whereas lower Berea reservoirs occur in downslope siltstones and sandstones. Variable bedding on broad clinoforms, soft-sediment deformation, and cementation contributes to reservoir heterogeneity. Permeability and porosity vary within individual types of bedding, although hummocky and swaley bedding generally have higher permeability than that of deformed bedding. For permeability >0.1 md, porosity is generally >10%. Cement in Berea samples includes quartz overgrowths, ferroan dolomite, siderite, pyrite, and kaolinite. In addition, these rocks were subject to varying degrees of intergranular compaction, quartz overgrowths, and secondary porosity formation (grain dissolution). In thin section, three pore types were encountered: (1) primary intergranular pores, (2) secondary moldic pores, and (3) microporosity. Intergranular porosity is most common.
Mercury capillary injection tests show that moderate-permeability (0.02–0.2 md) Berea has pore size distributions of 1 to less than 0.1 µm, but mostly from 0.5 to 0.6 µm, which are commonly found in gas reservoirs. Higher-permeability samples (1.2–2.0 md) have pore-throat diameter distributions of 2 to >0.1 µm. The highest pore volumes (near 3.25%) have pore diameters of 1–2 µm, which is more characteristic of oil-producing reservoirs.