Chemical Water-rock Interaction During Diagenesis Of Carbonate Reservoirs In The Northern Slope Of The Central Tarim Basin

Carbonate reservoirs have a large oil and gas reserves,but their complex diagenesis makes it difficult to recover the diagenetic evolution process.The change of carbonate reservoirs pores is mainly the combined effects of chemical water-rock interaction such as cementation,dissolution,and dolomitization.This paper takes the Ordovician carbonate reservoirs in the Northern slope of the central Tarim Basin as the research object.Some experiments are designed to study the influence mechanism of different factors of chemical water-rock interaction in carbonate reservoirs.Using TOUGHREACT numerical simulation software to establish laboratory-scale numerical models corresponding to experiments.According to laboratory-scale numerical models,time scale and diagenetic conditions are changed to study the influence on porosity of chemical water-rock interaction in different diagenesis stages.Numerical models of different sedimentary facies are established to recover the porosity evolution curve quantitatively,and provide a basis for the evaluation and exploration of oil and gas in the carbonate reservoir.The results show that:Dolomitization follows the Ostwald ripening rule,it is a process whereby Mg²⁺in a solution enters from the defect lattice of calcite through surface adsorption,gradually replaces Ca²⁺in the calcite,and finally generates dolomite.Dolomitization can be initiated by high temperature,high c(Mg²⁺)/c(Ca²⁺)acidic fluid,suitable proportion of calcite and dolomite.Constructive diagenesis such as dissolution and dolomitization occurred in the sedimentary-parasyngenetic stage,supergene stage,shallow burial stage,medium-deep burial stage,developing a lot of secondary pores which is beneficial to form high-quality reservoir.Calcite cementation mainly occurs on the edge slope facies and continental shelf-basin facies,resulting in poor development of pore,it is difficult to storage oil and gas;The restricted platform facies,wide mesa facies and platform edge facies is highly reformed by atmospheric freshwater,and pores have been better preserved,which has the potential to generate high-quality reservoirs.