鄂尔多斯盆地南部奥陶系马五段去白云石化成因及模式  

Dedolomitization Origin and Model for the Ordovician Majiagou Formation(5th Member)in the Southern Ordos Basin

作　　者：雷涵 黄文辉[1,2,3] 孙启隆 车青松 LEI Han;HUANG Wenhui;SUN Qilong;CHE Qingsong(School of Energy Resources,China University of Geosciences,Beijing 100083,China;Key Laboratory for Marine Reservoir Evolution and Hydrocarbon Abundance Mechanism,Ministry of Education,China University of Geosciences,Beijing 100083,China;Beijing Key Laboratory of Unconventional National Gas Geology Evaluation and Development Engineering,China University of Geosciences,Beijing 100083,China)

机构地区：[1]中国地质大学(北京)能源学院,北京100083 [2]中国地质大学海相储集层演化与油气富集机理教育部重点实验室,北京100083 [3]中国地质大学(北京)非常规天然气地质评价及开发工程北京市重点实验室,北京100083

出　　处：《现代地质》2021年第2期378-387,共10页Geoscience

基　　金：国家重点基础研究发展计划(973)项目(2014CB238900)。

摘　　要：通过研究马五段去白云石化作用的成因及发生模式,可进一步了解鄂尔多斯盆地南部奥陶系碳酸盐岩储集层成岩和物性的演化。通过岩心与薄片的观察,结合地球化学方法,和对白云岩的碳、氧同位素与微量元素的测定,分析了去白云石化作用成因及成岩演化过程,并结合物性资料分析去白云石化作用对储集层的影响,建立了研究区去白云石化的作用模式。结合样品的碳氧同位素和微量元素特征显示:M51+2含石膏结核的白云岩层段普遍受淡水淋滤作用影响,但只有部分层段发生淡水淋滤成因的去白云石化作用,Th/U、Th/Cr、Cr/Zr与Y/Ho特征表明由于淡水淋滤作用,大气淡水溶解陆源花岗岩类物质并携带陆源泥质灌入岩层。淡水淋滤去白云石化作用流体Ca^(2+)源来自溶解蒸发岩(膏盐层)和石膏结核,CO_(3)^(2-)源来自表生期白云石化作用方解石分解,流体主要运移通道为溶蚀缝和缝合线;作用过程分为表生—浅埋藏期的交代过程和浅埋藏期的胶结过程,富Ca^(2+)和CO_(3)^(2-)大气淡水流至运移通道末端,沿晶间缝下渗,造成白云石的选择性溶蚀和方解石对其的等体积交代,方解石胶结物填充晶间孔、缝和裂隙,使白云岩物性变差。To constrain the diagenesis and property evolution of the Ordovician carbonate reservoir in southern Ordos basin,it is important to understand the origin and mechanism of dedolomitization there.Based on core and thin-section observations,and geochemical analysis,the dedolomitization mechanism and diagenetic evolution were determined by oxygen isotopes and trace elements.Subsequently,combined with the property data,the influence of different dedolomitization styles on the reservoirs was analyzed.Finally,the dedolomitization model of the study area was constructed.Combined with the carbon-oxygen isotopes and trace element data of the samples,we show that the dolomite with gypsum core in M51+2 is generally affected by freshwater leaching,yet some of them also underwent dedolomitization caused by freshwater leaching.The Th/U,Th/Cr,Cr/Zr,and Y/Ho ratios indicate that meteoric water may have dissolved granitoids and carried terrestrial argillaceous sediments into the strata,again due to freshwater leaching.Ca^(2+)source for the freshwater-leaching-related dedolomitization was likely from the dissolved evaporite(gypsum)and gypsum cores,whilst the CO_(3)^(2-)may have sourced from calcite decomposition during the supergene dolomitization.And the main fluid migration pathways may have been dissolution fractures and suture zones.This process can be divided into 2 stages:replacement(supergene to shallow-burial stage)and cementation(shallow-burial stage).In this process,freshwater rich in Ca^(2+)and CO_(3)^(2-)may have flowed along the migration pathways,and infiltrated along intra-crystal cracks,resulting in the selective dissolution and dolomite replacement with equal volume of calcite,together with the infilling of intra-crystal pores,cracks and fractures by calcite cement,which is destructive to the reservoir.

关 键 词：去白云石化作用 淡水淋滤作用 成因模式 鄂尔多斯盆地 马家沟组 

分 类 号：TE122.2[石油与天然气工程—油气勘探]