东海西湖凹陷中央反转构造带古近系花港组致密砂岩储集层控制因素

Factors controlling the development of tight sandstone reservoirs in the Huagang Formation of the central inverted structural belt in Xihu sag, East China Sea Basin

采用岩石薄片鉴定、锆石U-Pb测年、扫描电镜、电子探针、激光微区碳氧同位素组成分析等技术手段,对东海盆地西湖凹陷古近系花港组致密砂岩储集层开展了岩石学特征、成岩环境演变以及储集层发育控制因素的综合研究。研究发现,中央反转构造带花港组砂岩物性整体较差,岩石类型以长石岩屑质石英砂岩为主,石英含量高,碎屑颗粒粗,杂基、高岭石、胶结物含量低;有机酸和大气淡水所形成的酸性成岩环境对次生孔隙的形成意义重大;花港组致密砂岩中相对优质储集层的发育与分布受控于沉积物源、沉积相带、成岩环境演变及异常高压。沉积物源及优势沉积相使得花港组砂岩具有良好的原始物性,是后期相对优质储集层发育的先决条件,异常高压对花港组储集层物性的改善主要体现在对原生孔隙的保护。纵向上,成岩环境演变差异性导致了花港组相对优质储集层集中分布于酸性成岩环境中;横向上,中北部相对优质储集层集中分布于异常高压发育的花港组下段,中南部集中分布于埋藏相对较浅的花港组上段。

By means of thin section analysis, zircon U-Pb dating, scanning electron microscopy, electron probe, laser micro carbon and oxygen isotope analysis, the lithologic features, diagenetic environment evolution and controlling factors of the tight sandstone reservoirs in the Huagang Formation of Xihu sag, East China Sea Basin were comprehensively studied. The results show that: the sandstones of the Huagang Formation in the central inverted structural belt are poor in physical properties, dominated by feldspathic lithic quartz sandstone, high in quartz content, low in matrix, kaolinite and cement contents, and coarse in clastic grains;the acidic diagenetic environment formed by organic acids and meteoric water is vital for the formation of secondary pores in the reservoirs;and the development and distribution of the higher quality reservoirs in the tight sandstones of the Huagang Formation are controlled by sediment source, sedimentary facies belt, abnormal overpressure and diagenetic environment evolution. Sediment provenance and dominant sedimentary facies led to favorable initial physical properties of the sandstones in the Huagang Formation, which is the prerequisite for development of reservoirs with better quality later. Abnormal overpressure protected the primary pores, thus improving physical properties of the reservoirs in the Huagang Formation. Longitudinally, due to the difference in diagenetic environment evolution, the high-quality reservoirs in the Huagang Formation are concentrated in the sections formed in acidic diagenetic environment. Laterally, the high-quality reservoirs are concentrated in the lower section of the Huagang Formation with abnormal high pressure in the middle-northern part;but concentrated in the upper section of Huagang Formation shallower in burial depth in the middle-southern part.