Abstract: Under the influence of an alkali(first)-acid(second) diagenetic environment, tight reservoirs developed in the early stage of diagenesis in the saline lacustrine basin. To reveal the formation mechanism and petroleum geological significance of the diagenesis tight zone, we took Es₄^s of Bonan Sag as an example and analyzed physical property, casting sheet, and mineral X-ray diffraction data. Research has shown that the cementation rate of carbonate cements in the diagenesis tight zone of E₄^s is greater than 0.5 and the compaction rate is less than 0.2. Primary cores were lost due to strong cementation in the early stage of the alkaline diagenetic environment; the contact area between organic acid and soluble minerals reduced later, and the occurrence of large-scale dissolution lagged in time. Therefore, the diagenesis tight zone was formed at 2 800-3 200 m. Reservoir porosity in the diagenesis tight zone was generally less than 12%, hence, the more favorable reservoir is located above and below the diagenesis tight zone, in delta front, mixed sedimentary rock beach-bar, and near-shore subaqueous fan front environments. The inferior physical properties of the diagenesis tight zone not only stops secondary migration of hydrocarbons, so that hydrocarbon accumulation modes differ above and below the zone, but also causes the degree of hydrocarbon enrichment and deliverability to differ in the inner and outer parts of the zone.

Key words: diagenesis tight zone, formation mechanism, geological significance, saline lacustrine basin, Bonan Sag