湖盆扇三角洲露头特征及与油气的关系

Lacustrine fan-delta survey characteristics and significance for petroleum geology

湖盆扇三角洲一般发育在断陷湖盆陡坡带紧靠山地前缘或构造形成的深大断裂、陡崖一侧,断裂活动强烈,物源供给充分,山间急流发育,为扇三角洲形成提供了先决条件。通过对柴达木盆地西部阿尔金南缘中侏罗统大煤沟组、潮水盆地红柳沟下白垩统庙沟群以及滦平盆地桑园营子侏罗系扇三角洲露头的实地考察,共观测到3个亚相、8个微相:扇三角洲平原亚相为扇三角洲水上部分,结构特征与沉积构造表现为冲积扇环境,以粗粒重力流沉积为特征;岩性主要为互层砾岩和砂岩,前部废弃河道和河道间见紫红色泥质层等暴露标志;分为辫状河道、辫状河道间和泥石流沉积微相。扇三角洲前缘亚相是扇三角洲沉积的主体,分布范围最大,具有成因类型多,沉积构造丰富,泥岩、砂岩、砾岩呈不规则透镜状分布等特征;沉积物整体偏细,结构和成分成熟度均偏高;分为水下分流河道、河口坝、席状砂及河道间微相。前扇三角洲亚相处于较深湖地带,物源区的水流很难到达该区域,主要为悬浮质进入较深水中的静水沉积物,局部地区夹杂有滑塌浊积岩沉积;分为深湖—半深湖和滑塌浊积岩微相。研究结果表明,扇三角洲平原亚相沉积为近源、快速、杂乱堆积,不稳定组分和填隙物含量高,储层物性相对较差;扇三角洲前缘亚相砾岩、砂岩受河流及沿岸流等多种水流的综合作用,泥岩细粒沉积物被淘洗干净,储集物性整体较好,该处形成的砾岩、砂体常与较深湖暗色生油岩呈指形交互状,且位于油气运移的上倾方向,非常有利于扇三角洲前缘砂体捕获到生油洼陷所生成的大量油气,便于成藏;前扇三角洲亚相深湖—半深湖中滑塌浊积岩通常被包裹在前缘生油泥岩中,亦便于成藏。

Fan delta in lake basincommonly develops along the front edge of the mountain within the fault-block region or on one side of the structural cliff where fault activities are intensive that may have supplied preferential conditions for the development of fan delta such as enough source and rapid streams.By means of survey on the Dameigou formation of Middle Jurassic,Miaogou group of Late Cretaceous in Chaoshui basin and the fan delta of Jurassic in Luanping basin,three intrafacies and eight microfacies are observed and classified.Fan delta plain is the terrestrial part of the fan delta,its architectural feature and sedimentary structure display the environment of alluvial fan,and characterized by coarse grain density current sedimentary feature,its lithologic character is mainly featured by alternative layer of the conglomerate and sandstone,and exposing marks such as crimson muddy members are observed in the abandoned front channel and interchannel.Fan delta plain can be subclassified into microfacices as mud-rock flow,braid channel and among braid channel.Fan delta front is the main part of the fan delta,and its distribution range is the largest,and characterized by multiple occurrence types,abundant sedimentary structure,and irregular lenticular distribution of the mud,sandstone and conglomerate.Deposition is generally fine and maturity generally higher.Fan delta front can be further divided into microfacices of distributary channel,estuary bar,sheet sand and interchannel. Fan front—delta subfacies can be divided into deep to semi-deep lacustrine facies and fluxoturbidite that existed in the central lake area where the streams from the source area can hardly reach this area,and its lithologic character is mainly quiescent water deposition with fluxoturbidite in the parts of depositional area.It is demonstrated that plain fan delta is near- source and fast accumulated in a jumble with high unstable component and matrix,and with relative poor reservoir physical property.Conglomerate and sandstone of fan delta front is controlled by river,coastal current and so on.Fine grained sediment is well washed,which makes the entire reservoir physical property good,moreover,the conglomerate and sandstone in this area constantly alternated like finger with the dark source rock in the deep lake,and this area lies just along the updip direction of the gas and oil migration line.This is very favorable for the accumulation of the gas and oil;fluxoturbidite in the deep—semideep lacustrine area of fan-prodelta is usually packaged in the oil source rock, which is also beneficial for the accumulation of the gas and oil.