Abstract
With extensive ancient karsts and abundant oil and gas resources, the Ordovician carbonate reservoirs in the Huanghua depression have broad prospects of exploration and development. The types, characteristics, distribution and controlling factors of karsts were investigated by means of the analysis of cores, thin slices, logging, seismic, and geochemical data in this study. The study reveals that the Ordovician carbonate reservoirs in the Huanghua depression falls into three main categories, pore type, pore-vug-fracture type and fracture type, of which, pore-vug-fracture reservoirs account for 62%. The epigenetic uniform karst mainly developed in the Late Ordovician to Early Carboniferous, while the differential karst developed in the Mesozoic. The uniform karstification mainly include chemical dissolution of the original carbonate rock, while the differential karstification not only include chemical dissolution but also hydraulic erosion and biochemical dissolution by organic acid. The burial karstification mainly involved the dissolution of the Ordovician reservoir by large amount of acidic fluids generated by the overlying Upper Paleozoic coal measure source rock while generating hydrocarbon. The dissolution caused by upwelling magmatic-tectonic hydrothermal fluids along faults also contributed to the formation of burial karst reservoirs. The sedimentation-diagenesis and paleogeomorphology-tectonic movement have certain control effects on the karstification of the Ordovician reservoirs in the Huanghua depression, determining the reservoir quality.
Highlights
Studies on karst and reservoirs started in the 1960s abroad (Roehl 1967; Ford and Ewers 1978; Estebar and Klappa 1983; James and Choquette 1984)
The dolomite and calcite minerals fillings of the samples were tested for δ13C and δ18O, and the results show that dolomite and calcite minerals produced by karstification have small ranges of δ13C (PDB) and δ18O (PDB) (Fig. 6), with δ13C (PDB) values ranging from − 5.52 to − 1.33, − 1.07 ‰ on average; and δ18O (PDB) values ranging from − 15.9 to − 6.03, − 9.37 ‰ on average
The mineral species associated with burial environment or hydrothermal activities can be seen on cores and thin sections, which include deformed dolomite, sphalerite, The temperatures of the fluid inclusions in calcite filling in dissolved pores and vugs of the Ordovician reservoir were tested
Summary
Studies on karst and reservoirs started in the 1960s abroad (Roehl 1967; Ford and Ewers 1978; Estebar and Klappa 1983; James and Choquette 1984). With the rise of the oil and gas industry, study on karst reservoir has advanced and gradually become a systematic discipline, karstology. The oil and gas exploration targeting Ordovician karst reservoir of Huanghua depression in the central of the Bohai Bay Basin started early in the 1970s but failed to make major discoveries due to the lack of basic geological understanding. Industrial oil and gas flows were tapped in several Ordovician carbonate karst reservoirs in Gangbei buried hill, Wumaying–Wangguantun buried hill, Chenghai buried hill, Zhaodong buried hill and Nandagang buried hill (Fu et al 2002, 2010, 2013; Zhang et al 2019a). To study karstification of the Ordovician carbonate rocks in the Huanghua depression, we examined the petrologic features and classified the karst reservoirs
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