Study On Diagenesis Of Unconformity Semi-weathered Rock

The structure of unconformity can be divided into three layers: rocks above the unconformity interface, weathering clay and paleokarst zone, according to heterogeneity of erosion and rocks formed by transgressin. Semi-weathered rocks, which is mainly composed of carbonate, gravel and volcanic rocks, is an important part of the sturcture and the favorable hydrocarbon accumulation reservoir layer. The semi-weathered rock formation and evolution is closely related to the diagenesis, which controlled the formation and perservation of reservoir. Since the Paleozoic, it has undergone multi-periods tectonic movement such as Caledonian, Hercynian, Indo-China and the Himalayan, and formed several unconformities in Jizhong Depression and the Junggar Basin. These two area are rich in semi-weathered rock lithology and become the focus region of this study. Multiphase tectonic activities led to the different nature of the study area complex diagenetic environment, diagenesis and diverse, so that the original rock structure, composition, structure and other changes to different extent, but also changed the types and distribution of reservoir space. Tectonic activities provide the foundation of Diagenesis about semi-weathered. According to the nature of diagenesis of the reservoir can be divided into constructive and destructive diagenesis diagenesis of two types, the former including dolomitization, to plaster of dissolution (especially non-integrated atmospheric water during the formation of dissolution) and the role of rupture; latter recrystallization, cementation, compaction, pressure solution, and filling the role of silica-based. Dissolution and tectonic disruption are the main construal diagenesis, helping to increase the porosity and improve the reservoir quality. In particular, the dissolution is to produce semi-weathered rock with secondary porosity of the most important way. Mainly as acidic dissolution fluid dissolved carbonate, clay mineral cements and feldspar, volcanic debris and other soluble components of particles, development intragranular dissolved pores, intergranular dissolution pores and intragranular dissolution porosity and other secondary porosity of the storage physical property has played a positive and effective to improve the role of weathering and leaching into burial dissolution. Compaction and cementation are the main destructive effects on the reservoir, leading to reduction of primary and secondary porosity, and deterioration of properties. Compaction damages primary porosity strongly and is the main reason for porosity reduction about clastic reservoir. Cementation developmented extensivly in the diagenetic evolution, filling almost all the primary and most secondary pore with authigenic mineral cements, such as carbonate, silica and clay minerals and so on, which is the main reason for the deeturction of pore in varbonate rocks and volcanic lava reservoir.