陆相混合细粒岩沉积微相及其对甜点的控制作用——以吉木萨尔凹陷芦草沟组为例

Continental Sedimentary Microfacies Distribution of Mixed Fine-Grained Rocks and Its Controlling Effect on Sweet Spots

【目的】吉木萨尔凹陷二叠系芦草沟组发育了一套陆相混积沉积体系,混积细粒岩沉积微相类型及甜点储层分布规律等研究的不足制约了该区页岩油高效开发。【方法】综合岩心、露头、测井、X射线荧光光谱(X-Ray Fluorescence,XRF)元素录井、分析化验和试油试采等资料,采用岩相构型、沉积微相耦合分析和动静结合等方法,开展了沉积微环境及微相类型划分、沉积模式及其对甜点的控制作用等研究。【结果】(1)研究区主要发育浪成交错层理、水平层理、同生变形构造、黄铁矿、方解石条带、鸟眼构造和结核、缝合线构造、含生物化石和白云岩溶蚀孔8类沉积构造。研究区分为半深湖—深湖、混合浅湖和碎屑浅湖亚相,细分为半深湖泥、云坪、混合滩坝、混合浅湖泥、浅湖砂坝和碎屑浅湖泥等6种微相,不同微相沉积特征及其沉积机制差异明显。(2)建立了咸化湖泊相混积细粒岩沉积微相模式。芦一段自下而上从深湖—半深湖—浅湖演化过程,发育了浅湖砂坝、混合滩坝与云坪微相。其中,云坪呈连片状大范围分布,混合滩坝呈土豆状分布,发育程度较高,不同层规模有差异,浅湖砂坝发育程度一般,呈断续状局限分布,垂向上呈薄储集层与厚层泥岩叠置状。芦二段以浅湖为主,微相类型与芦一段类似,但形态、规模与叠置样式差异较大。云坪和混合滩坝发育数量和规模大大减小,浅湖砂坝则发育程度较高,呈大规模分散土豆状发育,垂向上呈厚层叠置状。不同位置剖面微相的叠置样式不同。(3)甜点段XRF元素分析显示,目的层沉积时为干燥炎热的半咸水环境到咸水过渡环境,古生产力水平较高;下甜点段为富氧环境,上甜点段则为厌氧亚还原到还原环境。(4)沉积微相控制了甜点分布和开发对策。混合滩坝微相Ⅰ类甜点发育程度高,非均质性弱,开发效果最好,采用大规模部署水平井平台化作业进行效益开发。云坪微相Ⅲ类甜点发育,非均质性强,开发效果一般,是后续接替区域。浅湖砂坝Ⅱ类甜点发育,整体分散状,连续性较差,非均质性强,开发效果较好。【结论】后续应在沉积微相空间分布精细研究基础上,参考核磁曲线参数部署水平井开发。

[Objective]The Permian Luhcaoao Formation in the Jimusaer Sag has developed a continental mixed sedimentary system.Insufficient research on the sedimentary microfacies types of mixed fine-grained rocks and the distribution pattern of sweet spots has hindered the development of shale oil in this area.[Methods]Using core,outcrop,logging,and X-ray fluorescence(XRF)element logging data,analytical testing,oil testing,and production are integrated by lithofacies,sedimentary facies coupling analysis,and dynamic static combination.The sedimentary micro-environment,microfacies classification,model,and its controlling factors on deserts were studied.[Results](1)From the core,eight types of sedimentary structures were identified:wave cross bedding,horizontal bedding,syngenetic deformation structure,pyrite,calcite strip,birds-eye structure and nodule,suture structure,biological fossil,and dolomite dissolution hole.The sub-facies are divided into semi-deep to deep lacustrine,mixed shallow lacustrine,and clastic shallow lacustrine.These are further subdivided into six microfacies:semi deep lacustrine mud,dolomitic flat,mixed beach bar,mixed shallow lacustrine mud,shallow lacustrine sand bar,and clastic shallow lacustrine mud.The sedimentary characteristics and mechanism of different microfacies are different.(2)A sedimentary microfacies model of mixed fine-grained rocks in saline lakes has been established.No.1 member of the Lucaogou Formation evolved from deep to semi-deep to shallow lacustrine from bottom to top.Typical reservoir microfacies are developed in different layers,including shallow lacustrine sand bar,mixed beach bar,and dolomitic flat.Among them,dolomitic flat has a more continuous and large-scale sheet shape,and mixed beach bar has potato shape,with highly developed and different scales.The shallow lacustrine sand bar is intermittent and scale limited;vertically,it is located in the superposition of thin reservoir microfacies and thick mudstone.The Lucaogou Formation No.2 member is dominated by shallow lacustrine facies.The reservoir microfacies are similar to No.1,but the architecture style is different.The development quantity and scale of dolomitic flat and mixed beach bars are greatly reduced,and shallow lacustrine sand bars were developed,showing large-scale scattered potato shapes and thick layer superposition vertically.The profile shows that the stacking style of microfacies changed at different positions of the plane.(3)The deposition of the target layer is a transition environment from dry and hot brackish water to salt water,with a high level of paleo-productivity.The lower desert section was deposited in an oxygen enriched environment,but the upper section was deposited in an anaerobic sub reduction to reduction environment.(4)Sedimentary microfacies control desert distribution and oilfield development strategies.The mixed beach bar microfacies is a type I sweet spot reservoir with high development degree,weak heterogeneity and the best production effect.Large scale deployment of horizontal well platform operation is adopted for development.The dolomitic flat microfacies is a type III sweet spot reservoir,with strong heterogeneity and general development effect.It is a follow-up replacement oil and gas resource.The shallow lacustrine sand bar is a type II sweet spot reservoir being,with overall dispersion,poor continuity,strong heterogeneity,and good production effect[.Conclusions]After the detailed study of the spatial distribution of sedimentary microfacies,the horizontal well development is deployed with reference to the parameters of the nuclear magnetic resonance curve.