Characteristics and controlling factors of effective reservoirs of Mesozoic low-permeability clastic rocks in Gaoqing Region, Dongying Depression, Bohai Bay Basin, China

The Mesozoic effective reservoirs in Gaoqing Region, Dongying Depression show strong heterogeneity and complex diagenetic evolution. In order to gain insight into the petrological characteristics and physical properties of the effective Mesozoic reservoirs and clearly demonstrate the controlling factors for the growth of effective reservoirs, tests including the thin section, scanning electron microscope(SEM), cathodoluminescence, high pressure mercury intrusion and fluid inclusion were performed, and a comparative study on pore structure and diagenesis characteristics between effective and non-effective reservoirs was carried out. Results indicate that, the Mesozoic effective reservoirs in Gaoqing Region are characterized by well-developed secondary pores, relatively good physical properties and strong heterogeneity of pore structure with a bimodal distribution of mesopore throats and micropore throats. Compared with non-effective reservoirs, the porosity reduction rate in effective reservoirs significantly decreases due to compaction and cementation, while the porosity enlargement rate with dissolution increases largely. The development of Mesozoic effective reservoirs in Gaoqing Region is controlled by multiple factors. Well-sorted sandstones with high content of rigid particles weaken the porosity reduction owing to compaction, and help compose the material basis of effective reservoirs. Early carbonate cements in effective reservoirs have the ability of "anti-compaction and pore-preserving". The relatively open water-rock system is favorable for dissolution, and inhibits the mid-stage calcite cementation. The leaching dissolution and organic acid dissolution, the dominant mechanisms for diagenetic modification, function to effectively improve the macroscopic reservoir quality and microscopic pore structures. The micro-fractures generated by faulting can improve the seepage capacity of effective reservoirs, and profoundly control the distribution of oil and gas in the Mesozoic. © 2019, OIL & GAS GEOLOGY Editorial Board. All right reserved.