致密砂岩中碳酸盐胶结物形成机制——以川西坳陷中段龙门山前沙溪庙组砂岩为例

Formation Mechanism of Carbonate Cement in Tight Sandstone: A Case from the Sandstone of Shaximiao Formation in the Piedmont of Longmen Mountain in Middle Section of Western Sichuan Depression

碳酸盐胶结物是川西坳陷中侏罗统沙溪庙组储集层最主要的胶结物,也是其储集层致密化主要因素之一,其中以龙门山前地区最为发育,其平均含量约为12%.以龙门山前沙溪庙组砂岩为研究对象,在样品微观观察的基础上,结合碳酸盐胶结物碳氧同位素分析、电子探针分析等手段,对其碳酸盐胶结物形成机制进行研究。通过分析,研究区沙溪庙组碳酸盐胶结物以连晶和微晶方解石为主,δ^(13)CPDB为-7.69‰^-2.13‰,以无机碳源为主,δ18OPDB为-15.34‰^-9.60‰,其形成温度为56.1~101.3℃,总体上形成于早成岩阶段。其中连晶方解石FeO和MnO含量均较低,少量微晶方解石富含FeO和MnO,表明其形成以早成岩阶段为主,但也发育有晚成岩阶段的方解石胶结物,其形成与来自深部富Fe和Mn的流体有关。

Carbonate cement is the most important cement in the reservoir of the Middle Jurassic Shaximiao formation in the western Sichuan depression and is also one of the key factors of reservoir densification.Carbonate cement is very developed in the piedmont of Longmen mountain and its average content is about 12%in the area.Taking the sandstone of Shaximiao formation in the piedmont of Longmen mountain as the research object,the paper studies the formation mechanism of the carbonate cement based on the microscopic observation on samples and the analysis of carbon and oxygen isotopes in the carbonate cement and electronic probe.The study shows that the carbonate cement in Shaximiao formation in the area is dominated by intergrown and microcrystalline calcites with theδ13CPDB of-7.69‰^-2.13‰and theδ18OPDB ranging from-15.34‰to-9.60‰.The inorganic carbon acts as the main source of the carbonate cement.The cement formed at the temperature of 56.1~101.3℃,indicating an early diagenesis stage.Relatively low FeO and MnO contents in the intergrown calcites and relatively high FeO and MnO contents in a few microcrystalline calcites indicate that the carbonate cement was mainly formed at the early diagenesis stage.There are also some calcite cements developed at the late diagenesis stage whose formation is related to Fe and Mn rich fluids in the deep strata.