Carbonates: Investigating an Enigma
Date: 04/16/2010
Dolomite defies easy description. It forms in a variety of distinctly different settings and can change as conditions change. It is a complex, metastable carbonate whose mode of formation influences its morphology.
Dolomite can precipitate directly from solutions containing magnesium, calcium, and carbonate ions to form cement or unlithified sediment. However, most originates through the chemical alteration of precursor carbonate rock or sediment—primarily limestone or calcareous muds. These carbonates tend to be unstable, composed chiefly of calcite or its more thermodynamically unstable polymorph, aragonite. When these precursor materials are exposed to magnesium-rich fluids, some of the calcium ions can be replaced by magnesium ions to form a more stable magnesium calcium carbonate known as dolomite.
Dolomite is found in a wide range of settings including hydrothermal veins, lakes, shallow oceans, lagoons, and evaporative basins. Theories surrounding its origins continue to evolve. For a given span of geologic time, it may reside in one form, only to pass to another state when its equilibrium is disturbed—primarily through changes in pressure, temperature, or chemistry. The crystals may even grow in size. In this manner, early generations of crystals may subsequently be recast into more stable forms.
This recrystallization is beneficial to reservoir formation when it generates intercrystalline porosity, but these porosity gains may later be wiped out by growth of large interlocking dolomite crystals or by precipitation of pore-filling dolomite cements. Despite the inherent chemical complexities and hidden modes of origin, some dolomites host exceptional reservoirs of high porosity and permeability. For this reason, E&P companies continue their endeavors to predict where drilling targets will most likely encounter quality reservoirs.
An article in the Autumn 2009 issue of the Oilfield Review describes various modes and settings in which dolomite is formed, as well as processes that are responsible for enhancing or destroying its porosity. “Dolomite: Perspectives on a Perplexing Mineral” also reviews problems encountered when interpreting data from conventional logging suites and provides a glimpse into advanced tools and methodologies used for evaluating reservoirs in this enigmatic rock.
Read the full article at the Oilfield Review Web site. Clients are invited to register as premium content users to access this and other articles in the Oilfield Review.
Reference
Al-Awadi M, Clark WJ, Moore WR, Herron M, Zhang T, Zhao W, Hurley N, Kho D, Montaron B, and Sadooni F: “Dolomite: Perspectives on a Perplexing Mineral,” Oilfield Review 21, no. 3 (Autumn 2009): 32–45.
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