四川盆地中二叠统热液白云岩化作用及其储层改造意义

四川盆地中二叠统热液白云岩类型多、分布广,对油气勘探具有重要意义,但是关于热液白云岩化作用的机理和模式及其对储层的改造意义存在较多争议。在系统收集前期研究资料的基础上,通过对典型地区热液白云岩的矿物学、岩相学和地球化学等详细分析,总结了热液白云岩化作用的机理及其储层改造意义。研究认为:四川盆地中二叠统热液白云岩化作用与围岩的岩性和岩相没有明显的相关性,主要与同期断裂活动有关。根据热液白云岩与围岩的关系,可以将热液白云岩化模式分为3类:(1)滩相改造型热液白云岩化,主要分布在川西和川中地区的栖霞组中,热液白云岩化作用对储层的建设性改造作用不明显;(2)岩溶灰岩改造型热液白云岩化,主要分布在川中地区的茅口组中,热液白云岩化作用对该类储层的建设性改造作用也不明显;(3)致密灰岩改造型热液白云岩化,主要发育在川东地区的茅口组中,热液白云岩化作用对该类储层的建设性改造作用比较明显。断裂裂缝、充填残余的溶蚀孔和白云石晶间孔是热液改造型储层的主要储集空间,相比于原岩物性较好的储层,原始储集物性较差的储层受热液及其相关构造作用改造后,物性改善更加明显。

Hydrothermal dolomitization and its role in improving Middle Permian reservoirs for hydrocarbon accumulation,Sichuan Basin

The Middle Permian in the Sichuan Basin hosts various types of hydrothermal dolomites that are important to oil and gas exploration. Their mechanisms and occurring models as well as their contribution to reservoir quality enhancement have been the focus of study for years but a consensus is yet to be reached. This paper summarizes the mechanisms and their role in shaping reservoirs through detailed mineralogical, petrographic and geochemical analyses of hydrothermal dolomites from typical locations in the Sichuan Basin. The results show that the hydrothermal dolomitization in the Middle Permian of the Sichuan Basin is mainly related to contemporaneous fault activities and has no obvious correlation with the lithology and lithofacies of surrounding rocks. According to the relationship between hydrothermal dolomites and surrounding rocks, the hydrothermal dolomitization possibly occurs in three models: (1) hydrothermal dolomitization from modification of grain-beach facies, mainly occurring in the Qixia Formation of western and central Sichuan Basin and has no significant contribution to the improvement of reservoirs; (2) hydrothermal dolomitization from modification of karstified limestone, mainly occurring in the Maokou Formation of central Sichuan Basin and is insignificant in contributing to reservoir quality enhancement; and (3) hydrothermal dolomitization from modification of tight limestone, mainly occurring in the Maokou Formation of eastern Sichuan Basin and has contributed greatly to reservoir property improvement. Fault-related fractures, residual dissolution pores and dolomite intercrystalline pores constitute most storage space in reservoirs with hydrothermal dolomitization. Compared with reservoirs with relatively good initial physical properties, the reservoirs with poor initial physical properties are even more significant in terms of quality enhancement after hydrothermal dolomitization and relevant structural activities.