Formation Mechanisms Of Jurassic Effective Reservoirs In The Zhengshacun-Yongjin Area Of Central Junggar Basin

This paper takes deep clastic reservoirs of the Middle and Lower Jurassic formations of central Junggar Basin as an object to recognize the main controlling factors of reservoir quality and the distribution of effective reservoirs.The characteristics of sedimentary microfacies,the diagenetic heterogeneity of different sandstones,as well as the relationship between porosity and overpressure were systematically studied through cores observation,thin section and scanning electron microscopy identification,etc.Subsequently,the formation mechanisms of effective reservoirs were summarized and the development pattern of effective reservoirs was established.Through the research of the thesis,the following achievements and understandings are achieved:The sandstones show strong heterogeneity in the lithology,physical property and oilbearing shows.Fifteen sedimentary lithofacies are identified based on the characteristic of grain size and sedimentary structure.Four lithofacies assemblages are summarized to represent distributary channel,mouth bar,interdistributary bay and sheet sand microfacies.The Jurassic sandstones are composed of feldspathic litharenite and litharenite,which shows relatively low compositional maturity.Authigenic minerals mainly include carbonate cementation,authigenic clay minerals,anhydrite and siliceous cementation.According to differences of petrology microstructure and diagenesis heterogeneity,three petrofacies have been defined: ductile-rich sandstone,calcite-cemented sandstone and ductile-lean sandstone.The diagenetic evolution and hydrocarbon charge are quite different for three petrofacies.Ductile-rich sandstones are characterized by strong compaction with weak cementation and dissolution.Calcite-cemented sandstones are characterized of strong cementation,weak compaction and weak dissolution.Both the two petrofacies became tightened during eodiagenesis,and hydrocarbon charging rarely occurred.Ductile-lean sandstones are medium compacted,weak-medium cemented and strong dissolved.The lower degree of compaction and cementation during eodiagenesis lead the sandstones retain more primary pores,and it can be dissolved by acid fluid and occurred oil charging during late deep burial,which make it become the effective reservoirs.The influences of overpressure on three petrofacies depended on the relationship between the development of overpressure and the process of the sandstone tightened.Ductile-rich sandstones and calcite-cemented sandstones became tightened before the formation of the overpressure,indicated that the quality of the two sandstones were not affected by overpressure.Ductile-lean sandstones maintained good properties,a certain of compaction was effectively counteracted when overpressure developed.The distribution of different petrofacies is generally controlled by sedimentary microfacies.Ductile-rich sandstones and most of calcite-cemented sandstones distribute in the upper part of distributary channel and lower part of mouth bar,and the calcite-cemented sandstones are also seen occasionally in the sandy conglomerate at the bottom of distributary channel.Ductile-lean sandstones with the best properties mainly distribute in the middle-lower part of distributary channel or the middle-upper part of mouth bar.The plane distribution of the effective reservoirs were predicted by integrating logging identification of petrofacies and facies-controlled modeling.