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AAPG Bulletin, V.
Partial dolomitization of a Pennsylvanian limestone buildup by hydrothermal fluids and its effect on reservoir quality and performance
1Chevron Energy Technology Company, 1400 Smith, Houston, Texas 77002; [email protected]
2Department of Geological Sciences, University of Cambridge, Downing St., Cambridge, CB2 3EQ, United Kingdom; [email protected]
Dolomite was studied in the Reinecke field in west Texas to determine its origin, reservoir characteristics, and effect on oil production. The Reinecke reservoir includes approximately 25% dolomite in an upper Pennsylvanian to lowest Permian limestone buildup. Dolomites are fine to coarsely crystalline with undulose extinction. Petrography indicates that dolomite formed diagenetically late. Dolomites overgrow stylolites, indicating precipitation during deep burial. Low 18O values are consistent with primary fluid inclusions that indicate dolomite precipitation or recrystallization at 92 to 118C. These temperatures are much higher than those in the existing reservoir or what should have been encountered during regional burial.
Two types of dolomite are observed. One type is stratiform, replaced micrite-rich facies, and is interpreted as early dolomite that recrystallized in hydrothermal brines. The second type crosscuts stratigraphy and depositional facies and has vertically continuous morphologies that trend northwest-southeast. The latter dolomite is interpreted as forming in hydrothermal brines derived from latest Permian-aged highly evaporated seawater. These brines descended deep into the basin because of their greater density and then ascended because of heating and thermal convection. Reinecke dolomites have lower porosity but higher permeability than the surrounding limestones. As a result, dolomites provide raceways for fluids moving through the reservoir. These dolomites helped an artificially enhanced bottom-water drive to recover approximately 50% of the original oil in place. The dolomite has complicated a crestal CO2 flood. These hydrothermal Reinecke dolomites have lower porosity but much higher permeability than most nearby Permian dolomites formed in evaporated seawater during shallow burial.
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