Abstract:

The origin of effective reservoir in mid-deep clastic reservoir has always been the popular research content of petroleum exploration. The reservoir petrological characteristic, physical properties and diagenetic characteristics of the upper part of the Fourth Member of the Shahejie Formation(Es^s₄)were systematically studied through cores observation，rock thin section identification，physical properties analysis and fluid-inclusion analysis, et al in Hexiwu structural zone of Langgu depression . Moreover, the authors analyzed controlling factors of effective reservoirs on the basis of the physical properties parameter cutoff calculation, summarized the genetic mechanism and established the genetic model of effective reservoirs. The results show that the rocks of Es^s₄ reservoirs are mainly composed of lithic arkose and a small amount of feldspthic lithic sandstone, and they have medium compositional maturity and high structural maturity. In diagenesis, compaction is strong, cementation is obviously difference in north and south, and dissolution is weak. The reservoirs have more primary and less dissolution pores, so porosity and permeability correlation is well. Reservoir cement content, sorting coefficient, sedimentary microfacies and formation pressure are the main controlling factors of effective reservoirs. The fine sandstone, moderate coarse sandstone and pebbled sandstone with little matrix contents, well sorted, and little carbonate debris in branch channel of braided delta front are favorable material foundation in effective reservoirs forming process. Early overpressure restrained compaction and protected primary pore, also early hydrocarbon charging restrained cementation and protected pore space. They both together contributed the main reason for the formation of effective reservoir. In addition, dissolution formed a little amount of secondary pore and early quartz overgrowths around particles edge restrained compaction, which played supporting roles in the formation of effective reservoir.

Key words: physical properties parameter cutoff, effective reservoir, controlling factor, genetic mechanism, Langgu depression