Diagenesis of organic-rich shale in Second Member of Kongdian Formation in Cangdong Sag and its influence on reservoir characteristics

The diagenesis and evolution of organic-rich shale have an important influence on the characteristics of shale reservoirs， but the relationship between diagenesis and reservoir characteristics of organic-rich shale in the Second Member of Kongdian Formation in Cangdong Sag of Huanghua Depression is not clear， which restricts the exploration and development of shale oil and gas. By means of X-ray diffraction （XRD）， petrographic thin section， scanning electron microscope， and other experimental measurements， the diagenesis types and controlling factors of organic-rich shale in the Second Member of Kongdian Formation in Cangdong Sag were studied systematically， and its diagenetic evolution stage was defined. The influence of diagenesis on the micro-pore structure of shale was also discussed. The results showed that the organic-rich shale in the Second Member of Kongdian Formation in Cangdong Sag has many diagenetic types， such as compaction， metasomatism， dissolution， cementation， and thermal evolution of organic matter， and the main body is in the middle diagenetic A2 stage. The sedimentary environment and diagenetic fluid jointly control the diagenesis， which respectively determines the material composition of the rock and the formation and evolution of the secondary pores in the later stage. Dissolution and thermal evolution of organic matter belong to constructive diagenesis. Organic acids induced by hydrocarbon generation are of great significance to the dissolution transformation and optimization of reservoirs， which are beneficial to the formation of high-quality shale reservoirs. Compaction and cementation belong to destructive diagenesis. The diagenetic difference of different lithofacies has a significant influence on the pore structure， in which feldspathic shale facies has good mechanical brittleness and reservoir capacity， which is conducive to the formation of effective reservoirs， and the dissolution of mixed shale facies is weak as a whole. Destructive diagenesis causes pore reduction， which is not conducive to the formation of effective reservoirs. Carbonate shale facies has widely developed dissolution pores， but their mineral cementation limits porosity and pore connectivity and is not conducive to the formation of effective reservoirs.