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

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 of 37.2 %, leaving porosity of 3.7 % after stage I (<85 degrees C, 175-145 Ma). The original intergranular porosity loss due to compaction is calculated to be 29.3 %, suggesting that mechanical compaction is the most significant diagenetic process in primary porosity destruction. Stage II can be further divided into two sub-stages (Stage IIa and Stage IIb). Stage IIa (85-120 degrees C, 145-125 Ma) is characterized by late dissolution, which enhanced porosity by 8.8 %, and the porosity increased from 3.7 % to 12.5 %. During stage IIb (>120 degrees C, 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.