The Cementation And Impact On The Porosity Evolution Of Cretaceous Reservoir In Kuqa Foreland Thrust Belt

Kuqa Foreland Thrust Belt is one of the main exploration zone of Tarim oil field,Multiple stages of diagenesis lead Cretaceous reservoir evolution complicated,and cementation is significant on the development and distribution of high quality reservoir.Combined with previous studies,The thesis using a variety of analysis and testing techniques as casting thin sections,cathodoluminescence,SEM and X-ray analysis,researches on the petrological characteristics,cementation characteristics and porosity evolution of the reservoir sandstone from the Bashijiqike Formation.The thesis clarifies the cementation types and diagenetic evolution sequence,analyses the influence of cementation to porosity evolution and establishes the evolutionary model of porosity which controlled by cementation of Cretaceous reservoir.Carbonate cementation,the uppermost type of cementation,is widely distributed and well developed,and often occurs in poililotopic and filling-pore forms.Carbonate cementation has two effects on the porosity evolution of reservoir.On the one hand,early-time carbonate cementation can protect porosity against compaction,the study on Kela area presents that early-time carbonate cementation can recover porosity for 2%~5% after dissolution.On the other one hand,late-time carbonate cements occupy pores and decrease porpsity.Carbonate cementation totally decreases the porosity of reservoir about 8%.Then siliceous cementation and albite cementation are relatively developed,but are short in amount.siliceous cements almost occur as overgrowth quartz,albite cements occur as overgrowth of feldspar and tiny euhedral crystal.Both of siliceous and albite cementation decrease the porosity of reservoir,the porosity reduction by siliceous cementation would be 0.6%~1% and albite cementation reduces porosity about 0.5%~0.8%.The least is anhydrite cementation,anhydrite cements only appear in the areas nearby the fault,and the amount of anhydrite cements become fewer with depth increasing.Therefore the effect on decreasing porosity of anhydrite cementation is limited.Besides the cementation above,the clay cements of reservoir is mainly constituted of illite,mixed-layer illite/smectite and a spot of chlorite.Illite cements occur as flakes and bridge-shaped fibres,mixed-layer illite/smectite cements occur in reticular texture,and chlorite cements occur as curving flakes.Clay cements mianly plug up pores and throats,thus leading to the decline of permeability.