干旱环境河流扇概念与鄂尔多斯盆地延长组“满盆砂”成因新解

河流扇是陆相盆地中近年来日益受到重视的一种新沉积模式，其主要形成于干旱气候环境中。该文阐述了河流扇的概念演变与发展由来、沉积特征、鉴别标志与控砂机理等，讨论了河流扇与分支河流体系、末端扇及浅水三角洲等相关概念之间的区别与联系，认为河流扇形成主要受“分支型”河道控制，而末端扇及浅水三角洲主要受“分流型”河道控制。在对鄂尔多斯盆地晚三叠世古气候环境深入分析的基础上，运用河流扇概念对延长组“满盆砂”形成机制进行了重新解释。主要结论是:1）延长组沉积时期，古气候具有三分性——早期为干旱环境，中期为湿润环境，晚期再次转变为半干旱—半湿润环境；2）在干旱气候环境下，缺少大面积汇水区，以洪水搬运—事件沉积作用为主，形成了别具特色的河流扇体系，这可能是造成延长组“满盆砂”的主要原因之一；3）干旱环境河流扇砂体与潮湿环境水进域富含有机质泥岩相匹配，有利于形成大型岩性油气藏，从而使延长组由“满盆含砂”变为“满盆含油”。该研究改变了以往大型坳陷湖盆以“三角洲模式”为主导的传统认识，对丰富发展我国陆相盆地沉积理论认识及指导油气勘探实践均有积极意义。

The Concept of Fluvial Fans in an Arid Environment: A new explanation of the origin of"sand-filled basins"in the Yanchang Formation,Ordos Basin

The relatively new sedimentary model of ‘fluvial fans’ has been increasingly investigated in recent years. Fluvial fans mainly form in arid climates. In the present study， the concept， sedimentary characteristics， identification signs and sand control mechanism of fluvial fans are described. It is pointed out that it is only possible to establish a sedimentary model that reflects geological reality by absorbing the recent achievements and new knowledge gained from international research on inland depression lake basins， if we are to improve the exploration success rate and promote the development of China’s petroleum industry. An in-depth analysis of the paleoclimatic environment of the Upper Triassic in the Ordos Basin， adopting the ‘fluvial fan’ concept， reinterprets the ‘sand-filled basin’ development mechanism in the Yanchang Formation. The main conclusions are: （1） During the overall sedimentation of the Yanchang Formation， three paleoclimatic periods existed: an arid environment in the early stage， a humid environment in the middle stage， and a semi-arid， semi-humid environment in the late stage. （2） In the arid-climate environment， the lack of a large-scale catchment area led to flood transport sedimentation， forming a unique fluvial fan system， which may be the basic reason for the ‘sand-filled basin’ evident in the Yanchang Formation. （3） In the arid environment， the fluvial fan sand bodies correspond to the richly organic mudstones in the wet environment transgressive system tract， which is highly conducive to the formation of large-scale lithological oil and gas reservoirs， thereby changing the Yanchang Formation from a sand-filled basin to an oil-filled basin. The study has completely changed the previous understanding of a ‘large depression’ model dominated by a delta. This theoretical re-interpretation of continental basin sedimentation has positive significance in guiding oil and gas exploration practice in China.