羌塘盆地南坳陷中侏罗统布曲组白云岩储层成因流体同位素地球化学示踪

在岩石学分类基础上,通过获取不同世代碳酸盐矿物的C,O和Sr同位素,对白云岩成因流体进行示踪,结果表明:①研究区布曲组白云岩经历了多期白云石化流体的叠加改造,保留先驱灰岩原始组构的白云岩和晶粒白云岩的成因流体为同期海水,C,O和Sr同位素组成具有对先驱灰岩具有一定的继承性;②随着埋藏深度的增加,白云石晶体结构逐渐变化,δ¹⁸O(PDB)值相应逐步负偏:微粉晶白云岩(-3.81‰)—细晶、自形白云岩(-4.34‰)—细晶、半自形白云岩(-6.10‰)—中-粗晶、它形白云岩(-7.36‰);③鞍形白云石的成因流体为大气淡水深循环至深部地层,再流经下伏地层碎屑岩地层进入布曲组中,78～64 Ma的构造热时间提供热源;④细-中晶、自形白云石胶结物、白云岩晶间孔胶结的方解石,形成于浅埋藏末期,成因流体为埋藏的同期海水,裂缝中充填方解石脉发育在鞍形白云石之后,其成因流体主要为大气淡水,且与青藏高原隆升有关。多期次白云石化流体的叠加改造及大气淡水淋滤、溶蚀,影响布曲组白云岩储层的形成与分布,并制约着白云岩储层的质量。

Geochemical tracing of isotopic fluid of dolomite reservoir in the Middle Jurassic Buqu Formation in southern depression of Qiangtang Basin

This paper presents study on the properties and evolution of genetic fluids in the dolomite reservoir of the Buqu Formation in the southern depression of Qiangtang Basin. Petrological classification is performed on the dolomite and genetic fluids tracing is carried out through isotopic analysis of carbon, oxygen, and strontium (C, O, Sr) in carbonate minerals of various ages. The results show that:(1) the Buqu dolomite in the study area experienced multiple-stage superimposed modification by dolomitization fluids, the genetic fluids of dolomite and crystalline-grained dolomite with original structure of precursor limestone are contemporaneous seawater. The isotopic (C, O, Sr) composition has a certain inheritance to the precursor limestone. (2) With the increase of burial depth, the crystalline structure of dolomite changed gradually. The oxygen-isotope (δ¹⁸O_PDB) gradually became more negative:from micro-fine-crystalline dolomite (-3.81‰), to fine-crystalline and automorphic dolomite (-4.34‰), to fine-crystalline and hypautomorphic dolomite (-6.10‰), and to medium-to-coarse-crystalline and xenomorphic dolomite (-7.36‰). (3) Genetic fluids of saddle-shaped dolomite is meteoric water which flows through the underlying clastic formations into the Buqu Formation. The primary heat source was related to tectonic hydrothermal events and lasts 78-64 Ma. (4) Fine-to-medium-crystalline and automorphic dolomite cements, intercrystalline dolomite pore-filled calcites may have been developed at the end of shallow burial stage. The genetic fluid is contemporaneous seawater. The bright calcite veins are developed after the saddle-shaped dolomite, and its genetic fluid may be meteoric water related with the uplifting of Qinghai-Tibet Plateau. The superposition of multi-stage dolomitization fluids, meteoric water leaching and dissolution affected the formation and distribution of dolomite reservoirs in the Buqu Formation, and further controlled the quality of dolomite reservoirs herein.