羌塘盆地南部布曲组含油白云岩中自生文石胶结物的鉴定及其成因探讨  

Mineral and genesis study of authigenic aragonite in sucrosic dolomites from Middle Jurassic Buqu Formation in southern Qiangtang Basin,Tibet

作　　者：张帅[1] 伊海生[2,3] 夏国清[3] 梁定勇 

机构地区：[1]成都理工大学地球科学学院,成都610059 [2]油气藏地质及开发工程国家重点实验室,成都610059 [3]成都理工大学沉积地质研究院,成都610059 [4]海南省地质调查院,海口570226

出　　处：《石油实验地质》2016年第6期772-778,共7页Petroleum Geology & Experiment

基　　金：国家自然科学基金(41572089);国土资源部沉积盆地与油气资源重点实验室开放基金(zdsys2014002)资助

摘　　要：在羌塘盆地南部隆鄂尼地区中侏罗统布曲组砂糖状白云岩中首次发现针柱状胶结物,分布于沥青充填的溶蚀孔隙中。显微结构、微区X射线衍射以及电子探针分析表明,胶结物类型为自生文石,呈针柱状晶形,簇状、放射状集合体,Mg O和Sr O质量分数呈正相关。微区同位素测试显示,文石胶结物δ13CPDB值为3.5‰~3.98‰,δ18OPDB值介于-11.63‰^-9.98‰。文石在地质历史时期形成的碳酸盐岩中很少发现,常通过新生变形作用或溶解作用转变为低镁方解石。砂糖状白云岩中自生文石碳氧同位素组成与现代海洋沉积和大气淡水成因的文石存在较大差异,其形成过程中淡水淋滤作用影响较小,δ18O值的负偏主要受埋藏期地温梯度控制。储层成岩序列显示文石胶结物形成于埋藏白云石化作用之后,有机酸溶蚀伴随着碳酸钙自生矿物沉淀,烃类充注抑制了文石向方解石的转化。文石胶结物形成于晚成岩阶段,属于埋藏环境下碳酸盐矿物溶解再沉淀的产物,烃类充注可能对文石的保存起重要的控制作用。Columnar-acicular cement has been discovered in the Middle Jurassic Buqu Formation of Longeni area in the southern Qiangtang Basin of Tibet, distributed in saccharoidal dolomite in dissolution pores filled with bitumen. As demonstrated by microstructure, in situ X-ray diffraction （XRD） and electron microprobe analysis （EMPA）, minera-logical investigations have confirmed the emergence of authigenic aragonite. It occurs as bundles and radiating clusters consisting of needle crystals. Major elements show that MgO and SrO have a positive correlation. In situ isotopic analy-sis indicates thatδ13 C values range from 3.5‰PDB to 3.98‰PDB andδ18 O ranges from-9.98‰PDB to-11.63‰PDB. Aragonite is rarely found in carbonate rocks formed during geological times because of the conversion of aragonite to low-Mg-calcite through neomorphism or dissolution. Oxygen and carbon isotopic composition of aragonite cements in saccharoidal dolomite has greater variability compared with aragonite formed in modern marine or meteoric diagenesis. In the formation process meteoric water leaching has less influence and a negative excursion of δ18 O is mainly con-trolled by geothermal gradient during burial. The reservoir diagenetic sequence shows that aragonite cements formed after burial dolomitization. Authigenic mineral precipitates with organic acid dissolution, and hydrocarbon filling has largely inhibited the conversion of aragonite to calcite. The comprehensive analysis suggests that aragonite cements were precipitated from a dissolution-reprecipitation process of carbonate minerals during late diagenesis. Hydrocarbon filling played an important role controlling the preservation of aragonite cements.

关 键 词：文石 微区X射线衍射 碳氧同位素 中侏罗统 羌塘盆地 

分 类 号：TE122.2[石油与天然气工程—油气勘探]