准噶尔盆地玛湖凹陷风城组页岩自生长英质矿物的成因机理及其储层改造意义  

Formation of authigenic quartz and feldspars in the Fengcheng Formation of the Mahu sag,Junggar basin,and their reservoir modification significance

作　　者：郭佩 柏淑英[3] 李长志 雷海艳 徐文礼[1,2] 张锡婷 文华国[1,2] GUO Pei;BAI Shuying;LI Changzhi;LEI Haiyan;XU Wenli;ZHANG Xiting;WEN Huaguo(State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Chengdu University of Technology,Chengdu,Sichuan 610059,China;Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu,Sichuan 610059,China;Luliang Oilfield Operation,PetroChina Xinjiang Oilfield Company,Karamay,Xinjiang 834000,China;Mahu Exploration and Development Project Department,Petro China Xinjiang Oilfield Company,Karamay,Xinjiang 834000,China)

机构地区：[1]成都理工大学油气藏地质及开发工程国家重点实验室,四川成都610059 [2]成都理工大学沉积地质研究院,四川成都610059 [3]中国石油新疆油田分公司陆梁油田作业区,新疆克拉玛依834000 [4]中国石油新疆油田分公司玛湖勘探开发项目部,新疆克拉玛依834000

出　　处：《地质学报》2023年第7期2311-2331,共21页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号42272117,42002116);中国博士后项目(编号2018M640903)联合资助的成果。

摘　　要：玛湖凹陷二叠系风城组是近几年准噶尔盆地页岩油接替的主攻领域,但其页岩成岩改造强烈,原始岩性归属尚不明确。X射线粉晶衍射测得风城组页岩黏土矿物含量普遍较低(<10%),长英质矿物含量较高(60%~80%)。薄片分析发现风城组长英质页岩存在三种类型:(1)粉砂级(4~63μm)长石和石英碎屑含量丰富,为典型的粉砂岩;(2)燧石条带和团块发育,为富硅页岩;(3)长英质基质发育,呈非碎屑状,且火山尘及火山玻璃少见,其矿物组成和形态与典型的黏土岩、粉砂岩及沉凝灰岩明显不同。背散射图像和高精度扫描电镜测得该长英质基质由碎屑和自生石英、钾长石和钠长石共同组成,可见黏土矿物、碎屑钾长石和钠长石向自生石英转化,碎屑钾长石向自生钠长石转化。通过调研全球中新生代碱湖沉积中自生硅酸盐矿物组合特征和成岩演化规律,探讨了二叠系风城组“贫”黏土矿物和“富”自生长英质矿物的原因,并提出风城组发育一类“改造长英质页岩”,主要由原始黏土和长石碎屑矿物和火山物质经过多期成岩改造而成。早期高碱(pH>9)沉积和成岩环境增加了SiO_(2)溶解度和元素铝的活性,导致黏土矿物、泥级—细粉砂级长英质碎屑、火山灰等在入湖后发生“溶解、转化”,形成次稳定的硅酸盐矿物,如沸石和含镁蒙皂石,在经历漫长埋藏成岩后进一步转化为更为稳定的石英、钾长石和钠长石。碱湖页岩的成岩改造过程消耗了黏土矿物和火山灰,极大增加了页岩脆性,同时伴生了大量基质溶孔和晶间孔,是一类优质的页岩储层。In recent years,the Upper Paleozoic Fengcheng Formation of the Mahu sag has been the new exploration target of shale oils in the Junggar basin,NW China.However,due to its highly alkaline depositional condition,the shales of the Fengcheng Formation have suffered intense diagenetic modification and the primary lithology of its shales remains uncertain.XRD results reveal that the shales of the Fengcheng Formation are characterized by low contents of clay minerals(<10%)and high contents of felsic minerals(60%~80%).Thin sections show that there are three types of felsic shales in the Fengcheng Formation.The first type is composed of detrital silt-sized(4~63μm)quartz and feldspar grains,typically defined as siltstone.The second type of felsic shales contain abundant cherts,which are siliceous shales.The third type of felsic shales mainly consist of cryptocrystalline felsic matrix.The rare existence of vitric pyroclasts and clays make it difficult to classify the third type of felsic shales of Fengcheng Formation as either mudstone,siltstone or tuffite.Backscattered electron imaging and high-resolution scanning electron microscopy reveal that the matric felsic minerals are mainly composed of authigenic and detrital quartz,albite,and K-feldspar.Detrital K-feldspar and albite have been intensively replaced by authigenic quartz and albite.Authigenic euhedral K-feldspar,albite,and quartz crystals,range from 2μm to more than 10μm in size.These phenomena together suggest that parts of quartz and feldspar minerals in the shales of the Fengcheng Formation are authigenic,not detrital in origin,which is different from the felsic shales in other lacustrine basins in China.After reviewing the mineral compositions and diagenetic modifications of sediments in modern and Cenozoic alkaline lakes over the world,this study proposes that the felsic shales of the Fengcheng Formation resulted from multiple diagenetic alteration of original clayey shales.Detrital clays and silt-sized quartz,as well as occasionally ashes,reacted extensivel

关 键 词：碱湖 蒙脱石转化 自生石英 自生长石 基质溶孔 晶间孔 

分 类 号：P618.13[天文地球—矿床学]