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

摘要： 东海盆地西湖凹陷是中国近海面积最大的含油气凹陷,也是中国海上天然气勘探开发的主战场之一。近5年,基于"正韵律河道砂甜点"地震预测随钻监测技术、"渗透率分级-甜点厚度占比"高效井型设计与产能评价技术和油基泥浆长裸眼产能释放技术开展地质工程一体化技术体系创新与有效实践,西湖凹陷在多个低渗—特低渗领域获得商业性气流,展现出良好的低渗—特低渗天然气增储上产潜力。综合岩心、录井、测井、地震等地质资料和典型气藏勘探开发实践剖析,系统分析了西湖凹陷低渗—特低渗天然气藏的成藏特征及资源潜力。西湖凹陷始新统平湖组发育斜坡带三角洲平原—潮坪亚相煤系烃源岩和凹陷区潮坪—潟湖亚相含腐泥腐殖型烃源岩两类烃源岩,平湖组潮坪沉积体系发育"自生自储自盖"的天然气近源富集条件,天然气成藏条件优越。受"东西分带、南北分块"构造格局的控制,西湖凹陷中央背斜带发育3类天然气差异富集新模式。西部斜坡带平湖组岩性油气藏具有"纵向叠置、横向连片"的油气分布规律。优选中央背斜带北部的大型构造圈闭、中南部背斜翼部的岩性圈闭、南部深层平湖组和西部斜坡带近洼区为西湖凹陷低渗—特低渗气藏的重点勘探方向,天然气资源量超5千亿立方米。

关键词: 西湖凹陷, 低渗—特低渗, 天然气, 新领域, 资源潜力