B Oilfield, a 100-million-ton-class oil reservoir hosted in Archaean metamorphic rock buried hills, has been recently identified in the Bohai Bay Basin. Understanding the vertical reservoir properties is crucial the vertical reservoir properties is of crucial importance for the development of this oil accumulation. To this end, the present study carries out an investigation into the vertical reservoir characteristics by integrating multiple datasets, including XRD whole-rock clay analysis, cast thin section observations, and imaging logging data. Based on the degree of weathering, the Archaean metamorphic rock buried hill reservoir can be vertically stratified from top to bottom into three distinct zones: the intense weathering zone, the moderate weathering zone, and the internal basement zone. The dominant lithologies within these zones consist of migmatitic granite, cataclastic migmatitic granite, migmatitic granitic cataclasite, and migmatitic gneiss. From the intense weathering zone down to the internal basement zone, the reservoir displays consistent evolutionary trends: in terms of mineral composition, the relative abundances of quartz and clay minerals show a gradual decline, whereas those of feldspar and mica minerals increase steadily; with respect to clay mineral assemblages, the proportions of illite and kaolinite tend to decrease, while the contents of chlorite and illitesmectite mixed layers rise correspondingly. Overall, the reservoir is characterized by ultra-low porosity and ultra-low permeability, with its physical properties deteriorating progressively from top to bottom. Specifically, this deterioration is reflected in a reduced pore-throat radius, elevated displacement pressure, and weakened dissolution intensity. Concomitantly, the fracture aperture narrows, the fracture filling degree diminishes, and the fillings are predominantly composed of siliceous minerals, calcareous minerals, and mica.
Zhang et al. (Thu,) studied this question.