Origin Mechanism of Anomalous Tightness of Middle and Lower Jurassic Sandstone Reservoirs in Central Sichuan Basin

Key words: 
• Sichuan basin /
• tight sandstone reservoir /
• compaction /
• dynamic diagenesis theory

Abstract: As unique tight reservoirs in China, the middleupper Jurassic sandstones in central Sichuan basin occur at presentday burial depth of 1 500~3 000 m and they are characterized by ultralow porosity (average below 5%) and ultra-low permeability (average below 1×10-3 μm2) that fail to match their burial depth. Microscopic observation and statistics of relative contribution of compaction and cementation to porosity loss indicate that intensive compaction is responsible for the anomalous tightness of these sandstones. Furthermore，utilizing the research method of sandstone dynamical diagenesis theory, based on the analysis of original sandstone components and basin diagenetic dynamic factors, the mechanism of intensive compaction of Middle and Lower Jurassic sandstones in the study area has been discussed. It is suggested that fine grain size and high ductile fragments content are the internal cause, while the basin diagenetic dynamics characteristics of deep paleoburial, high paleogeothermal field and late oil emplacement are considered as the external cause. 
〓〓Mechanisms of diagenesis and porosity evolution of some sandstone reservoirs are quite complex in superimposed basins in western China. In one of these complex cases, this study of origin mechanism of anomalous tightness of Middle and Lower Jurassic sandstone reservoirs in central Sichuan Basin shows that the sandstone dynamical diagenesis theory and method can integrally and systematically take original sandstone components and all basin diagenetic dynamics factors (structure, thermal flow, pore fluid, etc.) into account, and which can well resolve such difficult questions in complex sandstone reservoirs.