Characteristics of lacustrine carbonate rocks within the upper section of the fourth member of Shahejie Formation in Zhanhua-Chezhen area, Jiyang Depression, Bohai Bay Basin

Lacustrine carbonate rocks of the upper section of the fourth member of Shahejie Formation (E s₄) are extensive around the Yihezhuang high in the Zhanhua-Chezhen area in the Jiyang Depression, Bohai Bay Basin. With a great number of oil reservoir discoveries and plenty of drilling data, it has become an ideal location for lacustrine carbonate reservoir study. The carbonate rocks mainly include limestones (internal debris limestones, bioclastic limestones, composite granular limestones and micritelimestones), dolomites (mainly crystalline dolomites, including algaldolomites, algalframework dolomites and so on) and mixed ones (carbonate and clastic rocks). Primary pores, secondary pores and fractures are the main pore types in the reservoir. Both porosity and permeability are low. Reservoir physical properties areclosely related to rock type. Sedimentary environment determines reservoir quality. Compaction, cementation and dissolution deform the reservoir. The sedimentary paleogeographic environmentis a main controlling factor. Reservoirs are most favorable onthe steep slope in the south and east of Yihezhuang high, less favorable on the gentle slope in the north of Chenjiazhuang and Yihezhuang highs, and poor or unfavorable between the reef and beach or in lake and semi-deep water areas.