Diagenetic Controls on Porosity and Permeability in Upper Miocene Carbonates, La Molata, SE Spain
Relating diagenetic processes to porosity and permeability is essential for carbonate reservoir quality prediction. We utilize Miocene heterozoan, photozoan, and oolitic - microbial sequences in SE Spain where paleotopography is preserved to evaluate the effect of a mesohaline mixing-zone and multiple events of meteoric diagenesis on porosity within a well-constrained stratigraphic framework and known sea-level history. Seven subaerial exposure surfaces allow evaluation of exposure duration, and climate on meteoric diagenesis. Only minor diagenesis occurred during early arid and short-lived subaerial exposure events. Later end-Miocene dolomitization and dissolution had the most profound effect on porosity. Dolomite δ13C and δ18O range from +0.9 to +6.0‰ PDB, -4.5 to +3.0‰ PDB, respectively. Covariation suggests fluid mixing. Downdip areas have more enriched δ18O and δ13C. Isotopic evaporation modeling of the most positive δ18O suggests a salinity of 42 ppt. Fluid inclusion measurements of freezing point depression yield Tm-ice ranging from -0.2 to -2.3 °C, indicating salinities ranging from 4 ppt to 43 ppt, with highest values in downdip areas. These data confirm fluid mixing and rule out physical mixing or recrystallization of multiple dolomite phases. Dolomitization was from a mixture between meteoric water and slightly evaporated seawater, here termed mesohaline mixing, a possible predictable type of dolomitization given specific climate and hydrogeologic conditions. Petrographic relationships indicate mesohaline mixing created major moldic and vuggy dissolution. Dolomitization was followed by 5 million years of subaerial exposure, during which times of more humid climates and erosion during uplift resulted in two zones of significant calcite cementation, separated by a zone with little calcite cementation. Calcites have negative δ13C and δ18O, and fluid inclusion Tm-ice of 0.0 °C, indicating precipitation from meteoric water. The two cemented zones likely represent two different paleo-water tables formed during uplift and erosional downcutting. The results of mesohaline mixing enhancing porosity and permeability in relation to paleotopography, and later stages of meteoric calcite cementation decreasing porosity and permeability are predictable and can be incorporated into geomodels for better prediction of porosity and permeability distribution in carbonate reservoirs.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California