四川盆地东部中二叠统茅口组白云岩及云质硅岩储层特征与发育规律

Characteristics and development mechanism of dolomite and dolomitic quartzite reservoirs of the Middle Permian Maokou Formation in eastern Sichuan Basin

四川盆地东部中二叠统茅口组发育了独具特色的白云岩及云质硅岩类储层,主流观点是"断控热液"成因,但勘探多有落空,说明需要重新认识。通过盆地周缘露头剖面和盆内钻孔资料的系统对比分析,研究了储层特征与发育规律。结果表明,储层岩石类型主要包括晶粒白云岩、硅质白云岩和云质硅岩,它们的分布与岩溶系统有关,在纵横向上展布不稳定。其中,晶粒白云岩主要位于岩溶系统周缘,储集性能差,相比而言,硅质云岩及云质硅岩主要位于岩溶系统内部,储集性能好。储层发育遵循"相控岩溶"的规律,颗粒滩相为储层形成提供了物质基础,早成岩期层控型风化壳岩溶是优质储层形成的关键。热液白云岩化作用有利于储层的保存,硅质热液活动与后期的方解石胶结充填了部分孔隙,最终形成了独具特色的硅质白云岩和云质硅岩储层。据此建议在前期注重"断控热液"型储层的基础上,进一步加强对"相控岩溶"的分析,找准目标。研究认识加深了对四川盆地茅口组碳酸盐岩储层的理解,也可供相似地质背景地区参考。

Unique dolomite and dolomitic quartzite reservoirs occur in the Middle Permian Maokou Formation in eastern Sichuan Basin.They are commonly believed to be of ＂fault-controlled hydrothermal＂origin.However,most of the explorary wells targeting at these reservoirs are unsuccessful,thus further study needs to be performed.The characteristics and development mechanism of these reservoirs are studied by means of comparative analysis of the outcrops on the periphery of the basin and cores inside the basin.The results indicate the main reservoir rocks in the Maokou Formation are crystalline dolomite,siliceous dolomite and dolomitic quartzite,whose distributions are mainly related to the karst systems,thus are uneven both laterally and vertically.Most of the crystalline dolomite occurs on the periphery of the karst systems,and their reservoir properties are poor.On the contrary,the siliceous dolomite and dolomitic quartzite are mostly distributed within the karst systems and have favorable reservoir properties.The development of reservoirs obeys the law of ＂facies-controlled karst＂.Specifically,grain-beach facies provided the material basis for the reservoirs,and facies-controlled eogenetic karst was the key for the formation of high-quality reservoirs.The following hydrothermal dolomitzation made it possible for the preservation of reservoir pores.The siliceous hydrothermal activities and later calcite cementation resulted in the filling of some pores.The unique siliceous dolomite and dolomitic quartzite finally came into being.According to these results,it is proposed to further strength study on ＂facies-controlled karst＂ reservoirs in addition to ＂fault-controlled hydrotherm＂reservoirs,so as to improve the accuracy of reservoior prediction.