咸化湖盆细粒重力流沉积特征及其页岩油勘探意义——以准噶尔盆地玛湖凹陷风城组为例

Fine-grained gravity flow sedimentary features and their petroleum significance within saline lacustrine basins:A case study of the Fengcheng Formation in Mahu depression,Junggar basin,China

细粒重力流沉积作用是咸水深湖环境重要的沉积作用过程之一,它能把浅水细粒碎屑和有机质搬运到深湖,形成页岩油的甜点储层和优质源岩。开展咸化湖盆细粒重力流沉积特征的研究对陆相盆地页岩油评价具有重要的意义。准噶尔盆地玛湖凹陷风城组(P1f)沉积时期发育了一套咸水湖泊环境下的湖底扇沉积体系,研究表明该体系扇缘可发育8类细粒重力流岩相组合:①细粒浓缩密度流-细粒过渡流-细粒碎屑流岩相组合;②细粒异重流岩相组合;③细粒浓缩密度流-细粒碎屑流岩相组合;④细粒浓缩密度流-泥流-安静水体空落岩相组合;⑤细粒过渡流-细粒碎屑流岩相组合;⑥细粒碎屑流-泥流岩相组合;⑦细粒碎屑流-湍流尾流岩相组合;⑧细粒下部过渡塞流-细粒上部过渡塞流-准层状泥(塞)流岩相组合。这8种岩相组合是细粒重力流沉积作用相互转化的结果,它们属于不完整的混合事件层,可构成一个完整的混合事件层。咸化湖盆、深层热卤水和凝灰质的加入使该湖底扇沉积体系的沉积物普遍含白云石、碳酸钠钙石、苏打石和硅硼钠石等矿物。其中,白云石为化学沉淀的方解石在准同生期和浅埋期形成的准同生白云石,碳酸钠钙石、苏打石和硅硼钠石等矿物可能来自深层热卤水。湖底扇近端粗粒重力流沉积物是致密油发育带,扇缘细粒重力流沉积物是页岩油发育有利相带。湖盆细粒重力流沉积和富有机质层的发育受与气候变化相关的湖平面变化的影响。湖平面上升有利于发育富有机质泥岩,湖平面下降更趋向于发育细粒重力流沉积。细粒重力流沉积物能形成页岩油的甜点层。

The sedimentation of fine-grained gravity flows is an important sedimentation style.It can transport fine-grained clasts and organic matter from shallow water to deep lake to form“sweet spots”of shale oil and good source rocks.Research on sedimentary features of fine-grained gravity flows has great significance for evaluation of shale oil in lacustrine basins.During the Fengcheng Formation period(P1f),a sub-lacustrine fan system was deposited in the salinized deep lake sub-environment of the Mahu depression.Eight kinds of lithofacies associations(LA)are identified in fan fringe sub-environment based on core observations.LA1:the LA of fine-grained concentrated density flow,fine-grained transitional flow,and fine-grained debris flow deposits;LA2:the LA of fine-grained hyperpycnal flow deposits;LA3:the LA of fine-grained concentrated density flow and fine-grained debris flow deposits;LA4:the LA of fine-grained concentrated density flow,mud flow and fallout deposits in quiet water;LA5:the LA of finegrained transitional flows and fine-grained debris flow deposits;LA6:the LA of fine-grained debris flow and mud flow deposits;LA7:the LA of fine-grained debris flow and turbidity wake flow deposits;LA8:LA of fine-grained lower transitional plug flow,fine-grained upper transitional plug flow,and quasilaminar plug flow deposits.The LA of coarse-grained concentrated density flow and fine/coarse-grained debris flow deposits are in proximal sub-lacustrine fan sub-environment.These LAs are the result of transformation of sediment gravity flows.They belong to incomplete hybrid event beds and can constitute a complete hybrid event bed.The salinized lacustrine basin and addition of deep hot liquid and tuffaceous made the sub-lacustrine fan system with dolomite,shortite,sodalite and searlesite.The dolomite is penecontemporaneous,generated by chemically precipitated calcite during the penecontemporaneous and shallow burial stages.Shortite,sodalite and searlesite may come from deep hot brines.The coarse-grained gravity flow deposits in proximal sublacustrine fan are in tight oil zone,while the fine-grained gravity flow deposits in fan fringe are shale oil zone.The fine-grained gravity flow deposition and the development of the organic-rich layer in the lake basins are affected by lake level changes.The rise of lake level is favorable for the development of organic-rich mudstone,while the fall of lake level is more conducive to the development of fine-grained gravity flow deposits.Fine-grained gravity flow deposits can form“sweet spots”for shale oil.