Browsing by Subject "Dolomitization"
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Item Outcrop-constrained flow and transport models of reflux dolomitization(2009-12) Garcia-Fresca, Beatriz, 1973-; Lucia, F. Jerry; Sharp, John Malcolm, Jr., 1944-Two hydrogeologic models explore reflux dolomitization using two outcrop datasets at different scales to constrain transient boundary conditions and heterogeneous petrophysical properties. A platform-scale petrophysical model of the Permian San Andres Formation was built from outcrop and subsurface data following a reservoir modeling approach that preserves outcrop heterogeneity and incorporates a sequence stratigraphic framework. This model was used as input for hydrogeological simulations of hypersaline fluid flow and solute transport during the accumulation and compaction of the platform. Boundary conditions change over time, as relative sealevel fluctuations drive sedimentation, depositional environment migration, topographic gradients, and location, size and salinity of the brine source. The potential volume and distribution of dolomite formed is inferred by a magnesium mass-balance. The composite result of reflux events at various orders of stratigraphic hierarchy is a complex dolomite pattern that resembles that observed on San Andres outcrops. Dolostone bodies across the platform may be generated by different combinations of favorable conditions, including proximity to the brine source, zones of higher permeability, permeability contrasts, and latent reflux. A meter-scale reactive transport model of the Albian Upper Glen Rose Formation simulates deposition of three high-frequency cycles punctuated by three brine reflux events. The simulator determines flow, solute and reactive transport along the flow paths, revealing the spatial and temporal distribution of calcite dissolution, and precipitation of dolomite and sulfate. The model recreates fully and partially dolomitized cycles within the time and lithological constrains on Glen Rose outcrops. Our results show that the distribution of dolomite within a high-frequency cycle may be the net result of intercycle processes, whereby dolomitizing fluids sourced from younger cycles flow across stratigraphically significant boundaries. We also show that variations in dolomite abundance and the unfulfilled dolomitization potential control the contemporaneous propagation of multiple dolomite fronts and the coalescence of discrete dolomite bodies. Results show that reflux is an effective and efficient mechanism to dolomitize carbonate formations that progresses simultaneously with sediment accumulation. Dolomitization is the cumulative result of many short-lived reflux events, sourced in different locations and times, and amalgamation of successive dolostone bodies. This model contrasts with previous studies that approached dolomitization of a carbonate platform as a discrete reflux event and current interpretations that relate dolomite bodies to their most immediate stratigraphic surfaces.Item Pore Characterization and Classification in Carbonate Reservoirs and the Influence of Diagenesis on the Pore System. Case Study: Thrombolite and Grainstone Units of the Upper Jurassic Smackover Formation, Gulf of Mexico(2014-07-10) Tonietto, SandraThe grainstone and the thrombolite units of the Smackover Formation at Little Cedar Creek Field, in Alabama, USA, were analyzed to determine their reservoir characteristics. The Smackover Formation reservoirs in this field have only minor dolomitization, and most of the depositional texture of the reservoirs is preserved, making Little Cedar Creek Field a unique location to study facies distribution and diagenetic alteration of these reservoirs. Depositional facies define good quality reservoirs of Smackover Formation, but diagenesis plays an important role on enhancing or reducing their porosity and permeability. Thrombolite and ooid-oncoid-peloid grainstone are the most prolific reservoir facies of the Smackover Formation, whereas dolomitization and dissolution are the main diagenetic processes improving porosity and permeability. A paragenetic sequence based on petrography, cathololuminescence, and minor and trace elements analysis was determined on both reservoirs types. Image analysis of scanned thin sections calculated the percentage of grains, pores and cements in the samples. Both reservoirs record distinct early diagenetic events, but similar late diagenetic evolution. The microbial thrombolite was exposed only to marine diagenesis, but the ooid-oncoid-peloid grainstone also was exposed to meteoric phreatic waters. Samples of the dolomitized Smackover Formation thrombolite unit from Appleton and Vocation fields were analyzed and compared to Little Cedar Creek Field thrombolite samples. Porosity, permeability and capillary pressure analysis was completed on thrombolite samples with no dolomitization and samples with distinct degrees of dolomitization. The dolomitization, associated with dissolution of calcite, created an intercrystalline pore network in the thrombolite, increasing porosity and pore connectivity (permeability), and usually reducing pore size. These processes also caused the high petrophysical heterogeneity of the thrombolite to decrease laterally and vertically, resulting in a more homogeneous pore system. In this study a new pore characterization applied to carbonate rocks was developed. It encompasses pore geometry, pore connectivity and the influence of diagenesis in the pore system by generating a quantitative result in order to identify and map reservoir flow units and diagenetic trends. This new pore characterization is based on features observed in thin sections, being a fast and less expensive method to evaluate porosity characteristics.