Study On The Formation Mechanism Of The High-quality Dolomite Reservoir Of Dengying Formation In Nanjiang Area

The Dengying Formation in Nanjiang area located at the bottom of the superimposed basin, characterized by the old age, the multi-stages of tectonic movement and complex diagenesis. Thus, formation mechanism of high-quality dolomite reservoir from overseas simple-structure basin is no longer applicable for the superimposed basin. Study based on characters of lithology, fluid injection in cavities and the types of diagenesis etc. in Dengying Formation, some significant conclusions can be suggested as follow.The tidal flat-lagoon of restricted platform is the main sedimentary environment of the Denying Formation, the microfacies include algal-bearing muddy flat, algal flat, algal grain bank, lagoon, grain bank. Dolomite styles of algal dolomite, crystallization dolomite, dolarenite, arenaceous dolomite and siliceous dolomite have been recognized. The genesis of dolomite is original or dolomitization in eogenetic diagenesis, and the features of hydrothermal fluid activities are common.Research shows that the existing porosity types are basically secondary pores related to the dissolution and recrystallization, and most of these pores are transfered from palaeo-karstification cavities which formed earlier. Abundance porosity such as inter-crystal (dissolution) pores, inter-particle dissolution pores, great dissolution pores, stylolite dissolution pores, and lots of survivals cavities etc are identified. The development and distribution of existing pores and cavities are controlled by primary sedimentary microfacies and rock types.The features of isotopic geochemical and reservoir lithology show that there were two stages meteoric karstifications developed respectively at the middle and end of the Denying Formation. The first stage meteoric karstification is characterized as the massive botryoidal structure and significant negative deflection ofδ13C andδ18O under unconformity surface, while the second stage meteoric karstification is characterized as development of secondary dissolution pores and negative deflection ofδ13C andδ18O, the palaeo-karstification resulted in significant increase of the primary rock porosity.The generational relationship of minerals in cavities of Dengying Formation dolomite is complicated. By comparison study of the minerals infilled in different cavities, multi-stage injection sequence of fluid can be reconstructed as follow: 1st generation fibrous-columnar dolomite→2nd generation quartz→3rd generation fine-coarse crystal granular dolomite→1st generation bitumen→4th generation quartz/coarse crystal granular dolomite→2nd generation bitumen→5th generation megacryst dolomite. The fibrous-columnar dolomite developed in first stage is the product of palaeo-karstification formed in the middle of Denying Formation period, the bitumen of the first and second generation formed between Devonian- carboniferous and Triassic, respectively.The post-depositional process of Dengying Formation in Nanjiang is classified into five stages. The main types of diagenesis include compaction-pressure solution, dolomitization, dissolution (meteoric karstification and burial dissolution), recrystallization, sillicification, tectonic fracture etc. According to the effect of diagenesis to the reservoir property, diagenesis types of constructive, destructive and destroy-constructive are divided.Massive micropores were brought out by dolomitization, which is helpful to the formation of later dissolution pores and preservation. The unconsolidated sediment being consolidation and dehydration, while the primary porosity is destroyed fatally, is due to the compaction and pressure solution. Meteoric karstification formed abundance karst cavities. Burial dissolution-precipitation formed secondary dissolution pores, and partial porosity lose result from infilled minerals. Hydrothermal activity and recrystallization took place along karst cavities formed by palaeo-karstification earlier.The studies based on primary and secondary controling factors indicate that formation of high-quality dolomite reservoir in Dengying Formation is mainly controlled by the meteoric palaeo-karstification during hypergene and the bury dissolution during deep bury. The intensity of palaeo-karstification is controlled by the primary sedimentary microfacies and rock types, the burial dissolution result in enlargement or shrinkage of the karst cavities.