陆相盆地滩坝砂体沉积特征及其形成与保存条件 ——以青海湖现代沉积为例

滩坝砂体由于经历了湖浪作用的反复淘洗物性良好，成为近年来石油勘探界关注的热点。通过对我国最大的内陆湖泊（青海湖）现代沉积特征的实地考察，从沉积环境、水动力及沉积特征等诸多方面建立了滨岸滩坝沉积的综合鉴别特征，指出低角度冲洗交错层理、反粒序（滩）、正粒序（坝）以及不含泥质等特征是识别滩坝砂体的典型标志。分析结果表明:物源—水动力—湖盆底形—湖岸线—湖平面（基准面）是决定青海湖滩坝是否发育（形成+保存）的主控因素，其中物源为滩坝形成提供了物质基础，水动力为沉积物改造及滩坝形成提供了源动力，湖盆底形与湖岸线决定了滩坝形成的平面位置与规模范围，湖平面（基准面）的升降变化决定了已形成滩坝沉积保存与否的地质命运。陆源滩坝砂体的形成较为容易，但保存较难，只要具备物源、湖盆底形及水动力等基本条件，低位域、高位域及湖侵域都可能形成滩坝砂体，但只有位于长期基准面旋回早期或三级层序湖侵期的滩坝沉积才能得到较好的保存。这对我国陆相盆地岩性油气藏勘探开发具有一定借鉴意义。

Study of the Development and Preservation of Lacustrine Beach and Bar Based on the Modern Sedimentary Characteristics of Qinghai Lake

Beach and bar sandbodies are an important research area in the petroleum exploration industry,since they are the result of repeated elutriation by wave action and are therefore favorable to the development of hydrocarbon reservoirs. A field investigation of modern sediments at Qinghai Lake (the largest inland lake in China)has established detailed identifying markers for beach and bar sandbodies from many aspects,such as sedimentary environment,hydrodynamics and sedimentation characteristics. Low-angle forewash cross-bedding,inversely graded (beach sand), normally graded (bar sand)and the absence of mud are typical for identifying signs. Further analysis showed that the material source and provenance,hydrodynamic forces,bottom morphology of the lake basin,lake shoreline and lake base level cycles are the main controlling factors marking the development,both formation and preservation,of beach and bar sandbodies. The provenance supplies the material base for beach and bar formation,and hydrodynamic forces boost sediment reshaping and beach and bar formation. The bottom morphology and shoreline of a lake basin determines its position in plan,and the extent of beach and bar formation;fluctuations in the lake base level determine the extent to which the beach and bar formations have been preserved. This study also points out that the formation of beach and bar sandbodies in a lacustrine basin occurs relatively easily,but their preservation is problematic. When suitable conditions of provenance,lake basin bottom morphology and hydrodynamics are present,low-stand system tracts,high-stand system tracts and lacustrine transgressive system tracts may form beach and bar sand deposits,but only the sand deposits at the early stage of the second order sequence,or at the lacustrine transgression phase of the third-order sequence,are well-preserved. Since beach and bar sand deposits are universal in continental lacustrine basins,this study has a particular reference significance for oil and gas reservoir lithology for exploration and development in continental basins in China.