Paleokarstic Features and Thermal Overprints Observed in Some of the Arbuckle Cores in Oklahoma
Zuhair Al-Shaieb, Mark Lynch, 1993. "Paleokarstic Features and Thermal Overprints Observed in Some of the Arbuckle Cores in Oklahoma", Paleokarst Related Hydrocarbon Reservoirs, Richard D. Fritz, James L. Wilson, Donald A. Yurewicz
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Cores of Ordovician-age Arbuckle Group carbonates from Oklahoma were examined for evidence of paleokarst. The depositional and diagenetic fabric of the rock was analyzed and compared with outcrop analogs to illustrate the nature of sedimentary, karstic, and diagenetic facies. Burial diagenesis and hydrothermal alteration have in many cases obscured the original fabric of these rocks.
Arbuckle rocks in different tectonic settings and stratigraphic intervals in the subsurface of south-central and north-central Oklahoma display surprisingly similar suites of karstic and diagenetic phenomena. Dissolution cavities, solution-enlarged fractures, collapse breccias, and vugular porosity are present in many cores and attest to the predominance of fabric-destructive processes in the development of Arbuckle paleokarst Collapse breccias and sediment-filled solution features bear striking resemblance to outcropping analogs. Primary speleothemic precipitates were not readily observed; either they were not precipitated or were obscured by later dolomitization. Phreatic cements were more commonly encountered than vadose cements.
A complex history of exposure, subsidence, and diagenesis is recorded in these rocks. Although the actual physical manifestations of paleokarst are not difficult to identify, interpretation of the genesis and age of these features is decidedly problematic. Arbuckle carbonates have been exposed to surficial weathering for periods of variable intensity and duration numerous times in geologic history. Paleokarst horizons may have developed subjacent to disconformities within and between formations of the Arbuckle Group and where these rocks subcrop beneath regional unconformities. This complex hierarchy of unconformities can produce numerous porous horizons whose preservation potential may depend on subsidence rates rapid enough to prevent extensive low-temperature phreatic cementation, thereby preserving the open pore network of the karst profile.
Burial diagenesis is evidenced by the multi-event dolomitization of these rocks. Ferroan and nonferroan "growth-zoned" baroque and limpid phreatic dolomite cements commonly occlude vugular and fracture porosity. Host-rock carbonates have been extensively replaced or neomorphosed. Cathodolumincscent microscopy and chemical staining indicate that "growth-zoned" baroque dolomite is commonly uniform in composition and was precipitated under mildly reducing conditions. Dolomite cementation was arrested by the migration of oil into the remaining pore space.
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This volume is a compilation of papers relative to paleokarst and associated reservoirs. The examples illustrate many of the rock types, and stratigraphic, structural, and paleotopographic features of carbonate strata which result chiefly from solution and collapse due to ingress of meteoric waters at and below unconformities. Examples presented here range from settings with considerable dissolution and collapse to those with significant unconformities but little evidence of meteoric alteration. It is estimated that 20–30% of recoverable hydrocarbons are in some way related to unconformities. Paleokarst reservoirs may also be important future reservoirs for application of horizontal drilling technology.