Analysis and modeling of dolomite sealing potential

Abstract

Sihai Zhang, Yin Xu, Peng Lu

One of the challenges toward understanding and predicting carbonate reservoir quality is the complex syn- to post-depositional diagenetic processes. Dolomitization is a critical modifier for carbonate reservoir quality in transforming the original grain-pore framework. Dolomitization especially leads to the redistribution of pore structure and connectivity, permeability, pathways, and reservoir producibility. Dolomite may behave as either a conduit or a barrier to flow depending on the original depositional texture, the chemistry of dolomitizing fluid, previous diagenetic stages, the timing and type of dolomitization, and the presence of anhydrite. A quantitative conceptual model of dolomite sealing potential was established based on theoretical calculations of dolomitization for reservoir quality, supported by core and thin section data. The dolomite volume fraction-porosity relationships suggest that (1) generally, dolomitization can be divided into replacement and pore-filling (over-dolomitization) phases; (2) porosity preservation characterizes the former phase, and reduction of porosity for the latter phase; (3) grain-dominated limestone facies usually have less dolomitization potential (e.g., the dolomite fraction is usually < 0.8), whereas muddy facies have a higher dolomitization potential. Away from the dolomitizing fluid source, low flux and normal concentrations cause porosity preservation. Near the fluid source, high flux and high concentrations cause over-dolomitization. Tight dolomite can be an effective seal featuring high dolomite content, small pore throats, low porosity and low permeability. 3D geological models based on well logs and seismic data were built to help understanding the dolomitization mechanism, predict replacive dolomite and dolomite seal distribution. Modeling results very well proved the two phase’s dolomitization and the sealing potential of tight dolomite.

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