Characteristics, Diagenesis, Controlling Factors and Formation Mechanism of Deep-Burial Ordovician Carbonate Reservoirs in the Yangshuiwu Area, Jizhong Depression, Bohai Bay Basin.

The deep-burial Ordovician carbonate reservoirs in the Yangshuiwu area (Jizhong Depression) have shown significant hydrocarbon potential since 2016. In this paper, based on the systematic investigation of characteristics and diagenesis of the reservoirs using cores, thin sections, logging, drilling, and geochemical analysis, their controlling factors and formation mechanism were discussed. The three high-quality reservoir belts (RBs) developed in the Ordovician strata are named RB1 (karst reservoirs), RB2 and RB3 (dolostone reservoirs) from top to bottom, each with its own set of pores, vugs, caves, and fractures. The results show that dolostones have relatively higher porosity and permeability than limestones and breccias. Unfilled and partly filled fractures, in particular, multiply enhance reservoir permeability. Additionally, the diagenetic paragenesis developed in marine, meteoric, and burial environments leads to the porosity evolution of Ordovician carbonates, mainly including compaction, calcite cementation, dolomitization, dissolution, and fracturing. Notably, sedimentation and dolomitization controlled by sea-level fluctuations are critical for the reservoirs formation. The tidal-flat sequence of each cycle of sea-level fluctuations influences their distribution. Subaerial meteoric water dissolution triggered by the Late Ordovician uplift plays an important role in the RB1, but has a negligible effect on the RB2 and RB3. Most importantly, the conductive high-angle fractures induced by the activities of the Yangshuiwu and Hexiwu faults from the Eocene to Pliocene can enhance all RBs potential, contributing to the ultimate reservoir quality. Therefore, a four-stage theoretical development model of the Ordovician reservoirs was established in combination with the tectonic background. [ABSTRACT FROM AUTHOR]