西湖凹陷平北斜坡平湖组潮坪环境砂体沉积模式及控制因素

Sedimentary models and controlling factors of sand bodies in tidal flat environment of Pinghu Formation on Pingbei Slope of Xihu Sag

潮坪沉积是西湖凹陷西部斜坡带平湖组的主要沉积相类型,潮坪沉积体系整体为富泥背景,砂体规模和微相类型变化较快,弄清规模性砂体的微相类型及其展布规律是平湖组潮坪背景下岩性勘探的关键点之一。通过对平北斜坡带钻井岩性剖面、岩心及分析化验资料的分析,结合现代沉积调研,系统梳理了平北斜坡带平湖组潮坪体系下规模岩性砂体的成因机制及分布规律。研究表明,平北斜坡带受不同潮差控制发育3类潮坪模式,形成了多类型规模性砂体:小潮差背景下发育障壁潮坪模式,规模性砂体为障壁砂坝、冲越扇砂体;中潮差背景下发育低凸起遮挡潮坪,规模性砂体为潮汐砂坝、潮道;大潮差背景下发育开阔海岸潮坪,规模性砂体为潮汐砂脊。这3类潮坪受构造演化、古地貌和海平面共同控制:①SQ1层序构造活动强,海平面上升,低凸起形成障壁,形成了障壁潮坪模式,障壁砂坝及冲越扇砂体主要分布在障壁之上及侧翼;②SQ2层序构造活动减弱,海平面最大,且水进-水退频繁变化,形成水下低凸起遮挡型潮坪模式,水进期在水下低凸起内外侧发育规模性潮汐砂坝,水下低凸起之间发育强潮汐通道,通道内外侧发育规模性潮道、潮汐砂坝砂体;③SQ3层序构造活动最弱,海平面下降,形成开阔海岸型潮坪模式,岬湾地貌受强潮流及沿岸流的改造,发育规模性潮汐砂脊砂体。研究成果可有效指导潮坪背景下岩性圈闭的勘探和评价。

Tidal flat deposition is the primary sedimentary facies type of the Pinghu Formation on the western slope belt of the Xihu Sag.The tidal flat depositional system is generally characterized by a mud-rich background,with rapid changes in sand body scale and microfacies types.One of the key points for lithologic exploration in the tidal flat environment of the Pinghu Formation is to understand the microfacies types and distribution patterns of large-scale sand bodies.By analyzing the lithological profiles,cores,and analytical laboratory data from drilling wells in the Pingbei Slope Belt,combined with modern sedimentary investigations,the genesis mechanisms and distribution patterns of large-scale lithologic sand bodies within the tidal flat system of the Pinghu Formation on the Pingbei Slope Belt were systematically examined.The research reveals that three types of tidal flat patterns develop on the Pingbei Slope Belt,controlled by different tidal ranges,leading to the formation of multiple types of large-scale sand bodies:barrier tidal flats develop under low tidal range conditions,characterized by barrier sandbars and override fan sand bodies;low-relief sheltered tidal flats develop under moderate tidal range conditions,featuring tidal sandbars and tidal channels;open-coast tidal flats develop under high tidal range conditions,where large-scale tidal sand ridges form.These three types of tidal flats are jointly controlled by tectonic evolution,paleogeomorphology,and sea-level changes:①During the SQ1 sequence,intense tectonic activity and rising sea levels lead to the formation of low-relief barriers,resulting in the barrier tidal flat pattern.Barrier sandbars and override fan sand bodies are mainly distributed above and along the flanks of the barriers.②During the SQ2 sequence,tectonic activity weakens;sea levels peak,and there are frequent fluctuations between transgression and regression,giving rise to a submerged low-relief sheltered tidal flat pattern.During transgression,large-scale tidal sandbars develop inside and outside the submerged low reliefs,while strong tidal channels form between them,with large-scale tidal channels and tidal sandbars developing inside and outside the channels.③During the SQ3 sequence,tectonic activity is minimal,and sea levels decline;an open-coast tidal flat pattern emerges.The headland-bay landforms are reshaped by strong tidal currents and longshore currents,leading to the development of large-scale tidal sand ridges and sand bodies.The research findings can effectively guide the exploration and evaluation of lithologic traps in a tidal flat environment.