A Study On Diagenetic Evolution Of Low Permeability Sand Reservoirs Of The Fourth Number Of Shahejie Formation In Bonan-Luojia Area

This paper was guided by the theory and method of microscopic reservoir geology. The research was chiefly conducted by recovering the evolution history of porosity and diagenesis of reservoirs according to the observations of thin sections and scanning electron microscope as well as the parameter of reservoir quality and mercury injection test, combined with image analysis of microscopic photos, to investigate porosity and diagenesis of low-permeability sandstone reservoirs of Es4 in Bonan-Luojia area. Then, the types and characteristics of typical diagenetic facies as well as their influence on physical properties of reservoirs were discussed according to the recovery of evolution history of reservoir porosity in the study area.Es4 in Bonan-Luojia area could be divided into Es4 x and Es4 s, which could be subdivided into four sandstone units,among which fine sandstone, siltstone and the fine siltstone are main rock types. The diagenesis of low-permeability sand reservoir of Es4 in Bonan-Luojia area, indicated by the study, mainly include compression,overgrowth quartz,dissolution of feldspar and detritus, cementation of carbonate and iron carbonate and so on. The diagenetic stages of reservoirs mainly belonged to stage A1 and A2 of middle diagenesis phase. Diagenetic environment turned from acidic to alkalescence and then to weak acidic. Inter-granular dissolution is main type of reservoir space of low permeability sand reservoirs and there are dissolution pores in grains as well as molded pores in reservoirs. The characteristics of obvious low permeability of Es4 reservoirs in the study area were indicated by petrophysical analysis data, of which the average porosity was 11.0% and the average permeability was 14.59×10-3μm2.There are five diagenetic facies of low-permeability sandstone reservoirs in the study area, which are diagenetic facies of dissolution, quartz cementation, early carbonate cementation, late carbonate cementation, mudstone and gypsum rock compaction. According to the study, diagenetic model was estabilished in north-south profile, in which early carbonate cementation diagenetic facies was distributed on southern gentle slope, while gypsum mudstone compaction, late carbonate cementation,quartz overgrowth as well as limited dissolution diagenetic facies developed in central depression zone, northern steep slope and fault terrace zone of study area.The evolution history of porosity and diagenesis was rebuilt with the volume of calculated primary porosity and compacting porosity reduction combined with sediment burial history and hydrocarbon generation and expulsion history of hydrocarbon source rock for three typical diagenetic facies, including dissolution, quartz cementation and late carbonate cementation. According to further analysis, reservoir quality of dissolution daigenetic facies was the best, while that of late carbonate diagenetic facies was the worst. The formation of low permeability reservoirs was influenced by sedimentary environment, buried depth and diagenesis. On the basis of characteristics of low permeability reservoirs of three genetic types, including sedimentary, diagenetic and tectonic-fracture types, low permeability reservoirs of Es4 in the study area was known as reservoirs of diagenetic type in this paper.