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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Dedolomitization and Its Significance to Carbonate Diagenesis
(1) Hangzhou Institute of Petroleum Geology, Hangzhou, China.
The calcitization (dedolomitizaition) of the sedimentary carbonate sequences are generally considered the consequence of subaerial condition. But recent researches reveal that the dedolomitization can take place in different diagenetic conditions. We divided it into 4 types. The dedolomitization relevant to fracture have lower O isotope value (-6.3 to -9.6‰, PDB), higher C isotope (1.0 to 2.6‰), medium Fe and very low Sr content. The dedolomitization relevant to stylolite have close relationship with hydrocarbon, and have very low O isotope value (-8.0‰) , low C isotope value (-8.6‰). The factors which influence the dedolomitization mainly are: the dolomites’ property, degree of order, the source of Calcium and the fluid properties. The fluid with High Mg/Ca ratio easily leads to dedolomitization. The higher the Ca/Mg ratio of the solution, the higher releasing rate, and the rapider dedolomitization. The Ca/Mg ratio are controlled by the calcite-dolomite equilibrium of the fluids in the pore, and the degree of order of dolomite have a great influence on it. Two general mechanisms have been proposed: a reaction of dolomite with calcium sulfate solutions or the alteration of ferroan dolomite by oxygenated meteoric water. The sulfate ions needed for the reaction are believed to be derived from the oxidation of pyrite or from gypsiferous solutions. Near surface dedolomitization is often related to dissolution of gypsum and dolomite in vadose or phreatic meteoric waters. Meteoric waters falling on outcropping gypsum generate a solution with a very high Ca/Mg ratio. In some cases, dedolomite may be indicative of subaerial exposure. Gypsum dissolution drives the precipitation of calcite, thus consuming carbonate ions released from dolomite. Prerequisites for the formation of near-surface dedolomite is a solution high in dissolved calcium and low in magnesium, low C02 partial pressure, and temperatures < 50°C. The dedolomitization can be thought as a kind of process that the Mg2+ is gradually released. The ways of dedolomitization are: from centre to rim, from rim to centre and the spotted calcite spread in rhombus dolomites. The dedolomitization is significant in recognizing exposure, unconformity and reconstructing the pore. The laterally continuous distribution of dedolomitization can indicate unconformity. But some research show that maybe the dedolomitization relevant to precipitation will destroy the primary pore.