Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Partial Dolomitization of a Pennsylvanian Limestone Buildup by Hydrothermal Fluids and Its Effect on Reservoir Quality and Performance, West Texas
(1) Chevron Energy Technology Company, Houston, TX.
(2) Department of Earth Science, Cambridge University, Cambridge, United Kingdom.
Dolomite was studied in Reinecke Field in west Texas to determine its origin, reservoir characteristics and effect on oil production. The Reinecke reservoir includes ~25% dolomite in an upper Pennsylvanian to lowest Permian limestone buildup. Petrographic and geochemical characteristics indicate that dolomite precipitation and recrystallization were diagenetically late, and occurred in a deep burial environment. Dolomites are medium to coarsely crystalline with undulose extinction. The dolomite’s δ18O values are consistent with fluid inclusions that indicate precipitation or recrystallization at 92-118 degrees C. Those temperatures are much higher than the existing reservoir or what should have been encountered during regional burial (~60 degrees C).
Two types of dolomite are observed in Reinecke Field. One type is stratiform, replaced micrite-rich facies, and is interpreted as early dolomite that recrystallized in hydrothermal brines. The second type cross-cuts stratigraphy and depositional facies and has vertically continuous morphologies that strike northwest-southeast. The latter dolomite is interpreted as forming in hydrothermal brines derived from latest Permian highly-evaporated seawater. These brines apparently descended deep into the basin because of their greater density, and then ascended due to heating and compaction of their host rock. 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 ~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 nearby Permian dolomites formed in evaporated seawater during shallow burial.