Study On Diagenetic Facies And Distribution Of Ahe Formation In The Eastern Kuqa Depression

Lower Jurassic Ahe Formation in Yiqikike area,Kuqa Depression,Tarim Basin is characterized by complex diagenesis,strong heterogeneity,low porosity and low permeability.This paper takes the tight sandstone reservoir of Ahe Formation in Yiqikik area as the research object.The main diagenesis experienced by Ahe Formation is compaction,cementation,dissolution and rupture based on the observation of core,thin section,scanning electron microscope and other geological data.The main cementation type is carbonate cementation.The method of "qualitative + semi-quantitative" was used to determine the basis of diagenetic facies.By comparing the advantages and disadvantages of different diagenetic facies logging identification methods,the appropriate one is selected to identify the diagenetic facies.Finally,the geological characteristics of different diagenetic facies are analyzed and the favorable areas are predicted combined with sedimentary facies.The main lithologies of Ahe Formation in Yiqikelike area are lithic sandstone and feldspar lithic sandstone.The diagenesis types mainly include compaction,pressure solution,cementation,dissolution and rupture.According to diagenesis and diagenetic minerals,the diagenic facies of target strata can be divided into five types: compaction facies,carbonate cementation facies,unstable component dissolution facies,dissolution micro-fracture facies and micro-fracture facies.The former two types of diagenetic facies have low porosity,low permeability and poor physical properties,which are non-reservoirs.The last three types have good physical properties,which are favorable reservoirs.Different diagenetic facies have different logging responses through cross-plot processing of conventional logging data.However,it is impossible to identify different diagenetic facies by cross-plot because of the information overlap in logging responses of different diagenetic facies.Therefore,the dimensionality of the logging data is reduced by factor analysis method,of which the extracted factors are given practical geological meaning.Five types of diagenetic facies can be effectively identified by three-dimensional factor cross plot.The logging identification of diagenetic facies is mapped from low dimensional linear indivisible problem to high dimensional nonlinear divisible problem by using BP neural network.The trained learning model has higher accuracy and it was verified by comparing the results with the thin section identification as well as the pore and permeability data.It provides a basis for reservoir diagenetic facies logging identification in the well section lacking coring.On the plane,the tight sandstone of Ahe Formation has high content of quartz and feldspar and low content of rock debris in Mingnan,Dibei and Yishen area.The main diagenetic facies is unstable component dissolution facies.In addition,the Kezi and Yixi areas have more microfractures,and the diagenetic facies type is mostly microfracture facies.In terms of sedimentary facies,the unstable component dissolution facies and dissolution microfracture facies are mostly developed in braided channel and underwater distributary channel,and their proportion in mouth bar is also relatively high.