The Thermodynamic Simulation Of Diagenetic Processesin Cretaceous High Quality Reservoir Of The Kela 2 Gas Fieldin Kuqa Depression Of Tarim Basin

The reservoir quality of sedimentary clastic rocks depends on the porosity and permeability that are two critical economic elements to be used to prospect oil and gas resources. Diagenetic processes ultimately control the pore geometry and types. Especially, the rock-water interaction in the deeply buried sediments yields dissolution and precipitation of some minerals that have important impact on the secondary pores. This study takes the reservoir of the Kela 2 gas field of the Tariam basin, the western China, as an example for understanding the relationship between the porosity and permeability and autogenetic minerals and textures of the clastic rocks on the vertical depth scale. The secondary pores are essential reservoir space in this gas field, moreover, their origin is closely related to dissolution of the frame grain of plagioclase and the cements of calcite. On the basis of Gibbs free energy changes , ΔG, the Gibbs free energy changes of the dissolution reactions of plagioclase and calcite in the ground waters, the mineral alteration or precipitation trend can be determined. The anorthite component of plagioclase from Kela 2 reservoir was dissolved so completely by the pore fluid that the dissolution made great contribution to the pore volume, and the albite component suffered partially dissolution; by contrast K-feldspar did not dissolved in the ground water at all. Calcite dissolution and precipitation depend on pore chemistry. At the temperature T<90℃calcite precipitates to form cement of the sediments; when T≈90~105℃, there is such much calcite to be dissolved that a great deal of secondary intergrain dissolved pores form. This is an important reason that the secondary pores origin in the reservoir of Kela 2 gas field. Based on the secondary pore geometry, connectivity and distributions derived from dissolution of calcite cements and plagioclase frame grains, three secondary pore association patterns can be subdivided, i.e. CF, F, and C types, which are illustrated with the secondary pore association characteristics. It is clearly showed that the secondary pore association patterns have impact on rock porosity and permeability.