关键词: 西湖凹陷, 低渗—特低渗, 天然气, 新领域, 资源潜力

Abstract: Xihu sag in the East China Sea Basin is the largest hydrocarbon-bearing sag in China's offshore area, and is also one of the main battlefields for offshore natural gas exploration and development in China. In the past five years, based on the "positive rhythm channel sand dessert" seismic prediction while drilling monitoring technology, "permeability classification-dessert thickness ratio" high-efficiency well type design and productivity evaluation technology, and oil-based mud long open hole productivity release technology, the geological-engineering integration technology system innovation and effective practice have been carried out. Xihu sag has obtained commercial gas flow in many low-ultra-low permeability fields, showing good potential for low-ultra-low permeability natural gas storage and production. Based on the analysis of core, mud logging, logging, seismic and other geological data and the exploration and development practice of typical gas reservoirs, the accumulation characteristics and resource potential of low permeability and ultra-low permeability natural gas reservoirs in Xihu sag are systematically analyzed. The Eocene Pinghu Formation in the Xihu sag develops two types of source rocks:the delta-tidal flat facies coal measure source rock in the slope zone and the tidal flat-lagoon facies sapropelic humic source rock in the sag area, showing a full-sag distribution pattern. The tidal flat sedimentary system of Pinghu Formation has the near-source enrichment conditions of natural gas with "self-generation, self-storage and self-cover", and the natural gas accumulation conditions are superior. Controlled by the structural pattern of "east-west zoning and north-south block", three new models of differential enrichment of natural gas are developed in the central anticline belt of Xihu sag. The lithologic oil and gas reservoirs of Pinghu Formation in the western slope zone have the oil and gas distribution law of "vertical superposition and horizontal connection". The large-scale structural traps in the north of the central anticline belt, the lithologic traps in the wing of the central and southern anticlines, the deep Pinghu Formation in the south and the near-sag areas in the western slope belt are the key exploration directions for the low-permeability and ultra-low-permeability gas reservoirs in Xihu sag. The natural gas resources exceed 500 billion cubic meters.

Key words: Xihu sag, low permeability and ultra-low permeability, natural gas, new fields, resource potential