Quantification and timing of porosity evolution in tight sand gas reservoirs:an example from the Middle Jurassic Shaximiao Formation,western Sichuan,China

摘要

The diagenesis and porosity evolution of the Middle Jurassic Shaximiao sandstones were analyzed based on petrographic observations.X-ray diffractometry.scanning electron microscopy observations,carbon and oxygen stable isotope geochemistry,fluid inclusion microthermometry,and thermal and burial history modeling results.The point count data show that secondary pores(av.5.5%) are more abundant than primary pores(av.3.7%) and are thus the dominant pore type in the Shaximiao sandstones.Analysis of porosity evolution indicates that alteration of sandstones mainly occurred during two paragenetic stages.Mechanical compaction and cementation by early chlorite,calcite,and quartz typically decrease the depositional porosity(40.9%) by an average of37.2%,leaving porosity of 3.7%after stage Ⅰ(120 ℃,125-0 Ma),the precipitation of late chlorite,calcite,quartz,and kaolinite destroyed 3.3%porosity,leaving porosity of 9.2%in the rock today.