The Big Clifty is a Chesterian-age Mississippian sandstone member of the Golconda Formation that extends from south-central Kentucky into adjacent Illinois and Indiana. Asphaltic deposits and conventional petroleum plays are distributed along the Pennyrile Fault System and Plateau in the southeastern portion of the Illinois basin. In south-central Kentucky anomalous oil-saturation geometries are observed in cored sections of the Big Clifty from wells in Warren and Butler counties along the margin of the Pennyrile Plateau. Petrographic study of the cores has revealed several diagenetic processes that have contributed to, or are directly responsible for, the anomalous oil saturation geometries and resultant reservoir partitioning. This study utilizes standard transmitted and reflected light microscopy, UVepifluorescence, optical cold-cathode cathodoluminescence microscopy, scanning electron microscopy, and alizarin red S and potassium ferricyanide staining in petrographic examination. Raman microscopy, energy-dispersive X-ray spectroscopy, transmitted electron microscopy, and X-ray diffraction assisted in identification and compositional analysis of minerals. Quantification of framework grains and cement compositions, porosity, and grain-size distributions was also conducted. This study’s data classified the Big Clifty as a fine to very-fine grained, quartz arenite with predominantly silicious and phyllosilicate cements and a mean of porosity of 7%. Oil-saturation geometries concordant to rock fabric are due to compaction, weathering of labile grains, and precipitation of authigenic cements (mainly quartz, kaolinite, and pyrite). A paragenetic sequence, established from textural evidence, places quartz, kaolinite, and pyrite phases early in paragenesis with labile grain dissolution, compaction, and precipitation of ferroan carbonate cement, with development of vugular porosity occuring later during burial. These phases and processes enhanced preexisting porosity and permeability heterogeneities within the rock. Nodular pyrite, oil emplacement, and precipitation of poikilotopic calcite are the latest diagenetic events. Nodular pyrite and poikilotopic calcite are responsible for oil-saturation geometries distinctly discordant with rock fabric. Poikilotopic calcite forms interstratal seals, occluding porosity, replacing framework grains and cements, and overprinting primary rock fabric. This study’s preliminary research into the poikiloptopic calcite suggests its occurrence is possibly associated with the cored wells’ shallow depths less than 500 feet (150 meters), and proximity to near-vertical faults and waters from drainage systems of karstified carbonate aquifers updip.