The Research For Development, Evolution And Evaluation Of Reservoir Sandbodies In The Chang 6 Member, Upper Triassic, Fuxian Exploration Area

According to the comprehensive detailed analysis of data such as core, drilling, logging and outcrop section, three kinds of sedimentary facies, namely, delta, lake and sublacustrine fan, are recognized in the Chang 6 Member, Upper Triassic, Fuxian area. The genetic types of their sandbodies mainly include subaqueous distributary channel, river mouth bar, distal bar, subaqueous crevasse splay and levee, deltaic front sand sheet, sublacustrine gravity-flow channel, distal turbidite, and sand sheet and sandy shoal and bar in shallow lake. Developing in the rapid growing period of delta in Fuxian area, those facies are the inborn geological factors controlled the development and distribution of the reservoir sandbodies in the Chang 6 Member of this area. Through detailed study of high-resolution sequence stratigraphy, the Chang 6 Member of this area can be divided into three medium-term base-level cycle, while each medium-term base-level cycle is made up of four to six short-term base-level cycle. From the fact that development and space-time distribution of the reservoir sandbodies are mainly controlled by fluctuation of base-level cycle of different grades, the reservoir sandbodies division solution having muddy barriers deposited by flooding surface of different grades as the boundary is brought forward. On the basis of this solution, the Chang 6 Member of this area can be divided into three sand-suites. Using the sand-suites as the unit for mapping, it shows areal distribution of the stratum thickness, sandstone thickness and sedimentary facies of the three sand suites. According to the information of core description, slice identification, chemical examination and testing, reservoir bodies of the Chang 6 Member are comprised of medium-and fine-grained sandstone and siltstone, which belongs to arkose and lithic arkose, and are in the term A-B of late diagenetic stage after experiencing those destructive diagenesis such as compaction, cementation and replacement, and corrosion which causes increasing of porosity. The reservoir space of the sandbodies is dominated by secondary pore. Its. porosity ranges in 1.70%~16.30%, mainly in7%~12%, and permeability differs within 0.01—19.6 X10'3 u m2, and belongs to the kind of reservoir bed with lower porosity and lower permeability. The inhomogeneous characteristic of reservoir is illustrated macroscopieally and microscopically. It is indicated that plane inhomogeniety of reservoir beds is mainly controlled by the depositional environment and sedimentary fades, and four types of intrabed inhomogeniety are related to microfacies, depositional sequence and diagenetic reworking. Combining pore evolution with diagenetic evolution organically, the primary porosity of the sandbodies is recovered to 37%, and decreased to around 14% when oil and gas infilled in middle and late Cretaceous after experiencing diagenetic change such as compaction, cementation and corrosion. Later, due to degradation of oil into bitumen causing partial porosity filled up, the porosity currently redused to about 9%. With application of the model-building strategy of three stages and the metfaed of geologic model and random simulation, 3D model of stratigraphy-structure framwork, sedimentary fades, and reservoir parameter are established respectively for sandbodies of the Chang 6 Member. With the application of fuzzy judge methodology, a classification evaluation to the reservoir sandbodies is made, pointing out that Type I sandbody does not develop in this area, Type II sandbody is in scattered distribution, and Type HI sandbody is of abroad distribution.