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

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，MgO和SrO质量分数呈正相关。微区同位素测试显示，文石胶结物δ¹³C_PDB值为3.5‰~3.98‰，δ¹⁸O_PDB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。

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

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), mineralogical 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 analysis indicates that δ13C values range from 3.5‰PDB to 3.98‰PDB and δ18O 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 δ18O is mainly controlled 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.