Eastern Section Meeting

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A Diagenetic Partitioned Big Clifty Sandstone Reservoir in South Central Kentucky


The Chesterian Big Clifty (Jackson) Sandstone positioned along the southeastern portion of the Illinois Basin in Warren and Butler counties of Kentucky in the shallow subsurface less than150 meters depth is minimally a localized diagenetically partitioned heavy-oil reservoir or asphaltic deposit. This detailed petrographic, cathodoluminescence (CL) and scanning electron microscope (SEM) study of diagenetic minerals in the sandstone reveals several mineral emplacements in the reservoir. The use of SEM in conjunction with Energy-Dispersive X-ray Spectroscopy (EDS) was utilized to examine the composition of important textural relationships in cements. Textural evidence reveals a generalized paragenetic sequence consisting of: 1) rare intergranular calcite 2) quartz overgrowths on quartz framework grains 3) illite and kaolinite in intergranular porosity and 4) calcite that is both poikilotopic and replacive. Calcite is volumetrically the most important cement and forms a series of seals for multiple reservoir zones. The most important porosity is typified by intergranular pores in relatively open-framework quartz arenite. Minor feldspar dissolution is also present in the reservoir, contributing to some minor porosity. Similarly, microporosity in authigentic clays is also observed. Calcite occludes porosity both along rock fabric elements such as ripple beds, crossbeds, and planar beds but in other cases, calcite precipitation is discordant to bedding or bedforms. Such discordant calcite has many cuspate or pointed boundaries, thus forming a geometrically complex seal/reservoir transition. The spatial occurrence of such diagenetic partitioning of the Big Clifty is thought to be rather limited throughout the deeper portions of the Illinois Basin however, our study suggests calcite partitioning relatively late in the paragenetic sequence is fairly common near the outcrop location of this reservoir along the southeastern part of the basin.