中国东海陆架盆地西湖凹陷西部斜坡带古近系宝石组沉积充填特征

Sedimentary filling characteristics of the Paleogene Baoshi Formation in the western slope of Xihu Sag， East China Sea Shelf Basin

王辉 秦兰芝 徐靖琦 陈永军 张威 代勇 陈贺贺

Hui WANG Lanzhi QIN Jingqi XU Yongjun CHEN Wei ZHANG Yong DAI Hehe CHEN

东海陆架盆地西湖凹陷的油气勘探主要集中在始新统平湖组和渐新统花港组，而宝石组基础研究相对薄弱，沉积环境和沉积特征尚未完全明确。根据西湖凹陷的地震资料、钻井及测井数据、古生物资料和元素地球化学分析数据，研究了宝石组沉积充填特征。研究表明：① 地震反射界面T与T之间为宝石组碎屑岩地层，顶、底界面具有明显的上超和削截地震反射特征。宝石组在凹陷中心厚度可达2 000 m，具备形成烃源岩的物质基础。② 宝石组沉积受断层分段性活动控制，形成了多个相对独立的沉积中心。在西部斜坡带自北向南构造样式有变化。北段杭州斜坡带为反向协调构造缓坡型，中段平湖斜坡带为多期断裂叠加陡坡型，南段天台斜坡带为同向协调构造缓坡型。③ 宝石组为微咸水弱氧化-弱还原沉积环境，古盐度自北向南呈升高趋势，水体较浅，为湖/海平面升降频繁的海-陆过渡相沉积。④ 宝石组南、北沉积体系具有差异。北段杭州斜坡带主要发育辫状河三角洲-海湾沉积体系，南段天台斜坡带和平湖斜坡带（除平北区）主要发育局限海沉积环境下的潮坪-潮汐三角洲-滨浅海沉积体系，中段平湖斜坡带水下低凸起分隔南北，为低凸起遮挡下中段-北段的潟湖和海湾沉积体系，厚层泥岩发育，是西湖凹陷宝石组的主力烃源岩。

Hydrocarbon exploration in the Xihu Sag of the East China Sea Shelf Basin is concentrated primarily in the Eocene Pinghu Formation and the Oligocene Huagang Formation. In recent years， it has shifted to the Baoshi Formation in the western slope of the sag. However， fundamental research on the Baoshi Formation remains limited， leading to an unclear understanding of its sedimentary environment and characteristics. In this study， we investigate the sedimentary filling characteristics of the Baoshi Formation in the Xihu Sag using seismic， borehole， log， and paleontological data， as well as data from elemental geochemical analysis. The results indicate that the Baoshi Formation occurs between the T and T seismic reflection interfaces. This formation consists of a series of clastic layers， with its top and bottom boundaries characterized by pronounced onlapping seismic reflection and truncation. It has a thickness of up to 2 000 m at the sag center， providing a material basis for the formation of source rocks. The Baoshi Formation is deposited under the control of segmented fault activities， resulting in the formation of multiple independent depocenters. The western slope exhibits varying structural styles from north to south. Specifically， the northern Hangzhou slope zone occurs as a reversely harmonic tectonic ramp， the central Pinghu slope zone consists of a steep slope structure shaped by multi-stage fault superimposition， while the southern Tiantai slope zone represents a normally harmonic tectonic ramp. The Baoshi Formation is formed in a brackish， weakly oxidizing to weakly reducing sedimentary environment， with paleosalinity increasing from north to south. This formation consists of the deposit of the marine-continental transitional facies in shallow water with frequent lake/sea level fluctuations. It exhibits distinct sedimentary systems in its northern and southern portions. Specifically， the northern Hangzhou slope zone is characterized by a braided river delta-bay sedimentary system， whereas the southern Tiantai and Pinghu slope zones （excluding the Pingbei area） are dominated by a tidal flat-tidal delta-littoral neritic sedimentary system in a restricted marine sedimentary environment. The northern and southern sedimentary systems along the western slope are dissected by a low salient in the central Pinghu slope zone， which serves to shelter lagoon and bay sedimentary systems in a restricted setting. The thickly laminated mudstones in these sedimentary systems act as the major source rocks of the Baoshi Formation in the Xihu Sag.

10.11743/ogg20250314