Abstract

Staining of calcite cements (Dickson 1965) provides a way of mapping diagenetic events based on cathodoluminescent cements from updip areas downdip into the basin in the petroleum-producing Mississippian Newman Limestone, Kentucky. Updip, the limestones contain pre-compaction, pink-staining, calcite cements with nonluminescent to dull to nonluminescent zonation that formed during two phases of aquifer recharge from the northwest during which unstable carbonate grains were leached, and intergranular space was partly filled by meteoric cements. These cements pass downdip into dully luminescent 1) pink-, 2) purple-and 3) pink-staining cements and finally into purple- and blue-staining cements in the basin. These meteoric cements have depleted carbon and oxygen isotope compositions relative to marine calcites (Niemann and Read 1988). Burial calcite cementation was preceded or synchronous with burial dolomitization from waters with isotopic compositions between those of meteoric and saline waters. This was accompanied by a phase of burial leaching and chalky microporosity formation in ooid grainstones. Purple (zone 4) and blue (zone 5) calcite cements are the dominant cement type in the shaly limestones of the basin, but they also post-date pink cements (zones 1 to 3) updip. These cements are syn- and post-compaction burial cements that locally are associated with oil migration into the reservoirs and are Pennsylvanian to Triassic in age. A second phase of burial leaching may have preceded zone 5 blue calcite cement, which formed at maximum burial depths. Zone 6 (purple) and zone 7 (pink) cements are related to post-Paleozoic uplift. Pink-staining cements have less than 1,000 ppm Fe. Purple cements have 1,000 to 3,000 ppm Fe, whereas blue-stained cements have greater than 3,000 ppm Fe. Mn compositions are variable. Fe compositions of the cements suggest that the fluids became more reducing with depth. Fluid inclusions in burial cements suggest Na-Ca-Cl waters with salinities of 5 to 17 wt. % salt. Likely temperatures were 40 to 60 degrees C in Kentucky, increasing to 125 degrees C or more in the deeply buried fold-thrust belt. The delta 13 C values of purple/blue cements in the west show similar trends to meteoric calcites, in that they are lightest in the recharge area, becoming heavier downdip. Deltaic waters charged with isotopically light C penetrated deeply into the western part of the aquifer, becoming heavier downdip by rock-water reaction. The delta 18 O values of the purple/blue calcites become heavier in extreme basinal sections due to intense rock-water interaction. These basinal waters were generated by dewatering of the basinal shale section during Late Paleozoic tectonism. They were isotopically heavy and geopressured, and they precipitated burial calcites derived by pressure-solution.

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