Preservation of Intragranular Porosity Within the Harrodsburg Limestone (Middle Mississippian), Newtonville Consolidated Field, Spencer County, Indiana

John A. Rupp

Porosity preservation in the reservoir rocks within the Harrodsburg Limestone pool of the Newtonville Consolidated field, Spencer County, Indiana, is a function of both primary facies distribution and inhibited pore-filling sparry calcite cementation.

Reservoir facies of the Harrodsburg mark the initial shallow-water phase in an overall shoaling-upward, prograding ramp succession beginning with deep basinal clastic sequences of shales and turbidites (early middle Mississippian) and culminating with sabkha and shallow marine carbonate deposits (middle middle Mississippian). Grainstones composed of bryozoans and pelmatozoan bioclasts were deposited on the lower shoreface of a southwestward-deepening ramp in southern Indiana. Lateral distribution of coarse-grained, well-winnowed, southwestward and downdip-trending carbonate sequences was controlled by the undulatory and digitate nature of the ramp. Grainstones were deposited on topographic highs as prograding shoals and winnowed storm surge deposits, probably propagating into the deep r, interlobate regions below storm wave base. Grainstone-packstone sequences are enclosed in fine-grained, dark-gray dolomitic wackestones and mudstones representing deeper water, interlobate sedimentation.

Primary intragranular and minor intergranual porosity was preserved as early marine phreatic cementation created a rigid framework of grains resistant to further solution compaction. Fine-grained euhedral dolomite within proximal wackestones and mudstones formed as the product of a paleohydrologic system composed of plumes of fresh water that extended down through grainstone bodies and formed a peripheral zone of mixed meteoric and marine phreatic waters. Later coarse sparry calcite cement within peripheral grainstones resulted from burial cementation. Lack of significant water-filled porosity off the depositional structure indicates that the early presence of hydrocarbons within the primary pore system inhibited further cementation.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.