Diagenesis and reservoir characteristics analysis of the Late Triassic Xujiahe Formation tight gas sandstone in the northern Sichuan Basin, China

The Late Triassic Xujiahe Formation is a key target for tight gas in the northern Sichuan Basin. Thin section, scanning electron microscopy, X-ray diffraction, porosity and permeability analyses have been performed to delineate the diagenesis and reservoir characteristics of Xujiahe sandstone. The results show that the Xujiahe Formation contains feldspathic litharenite, litharenite, sublitharenite and quartzarenite sandstone. Sandstones of the Xujiahe Formation are characterized by low feldspar content and both secondary and micro-fracture porosity. Porosity and permeability analyses of 185 core samples show a broad but low porosity range from 0.79 to 10.43% (average 4.55%) and wide but low permeability range (0.0021-26.001 mD, average 0.449 mD). The higher permeabilities resultfrom micro-fracturing. Strong mechanical compaction plays a more important role in reducing primary porosity of sandstone than cementation during eodiagenesis. Carbonate cement is detrimental to reservoir porosity. Early carbonate cement precipitated from depositional water during eodiagenesis can block primary pores while late carbonate cement formed during mesodiagenesis can fill secondary pores. Quartz cement shows a slight relationship with porosity and permeability. There is a positive relationship between grain-coating chlorite and porosity and permeability. The effect of diagenesis on the reservoir quality of Xujiahe tight gas sandstone is greater that depositional environment during deep burial.