Characteristics and formation mechanism of the tight tuff reservoirs of the Upper Triassic Chang 7 member in the southern Ordos Basin, China

Organic-rich shales are commonly associated with tuff layers. Previous studies have mainly focused on the influence of water-laid tuffs on the primary productivity and enrichment of organic matter. Tuffs can also act as tight or low permeability reservoirs or as sweet spots in shales. However, the characteristics and formation mechanism of tight tuff reservoirs are poorly explored. The tuffs in the study area are divided into five lithologies: vitric tuff, crystal-vitric tuff, crystal tuff, tuffite, and tuffaceous sandstone. The pores in these tuffs have various types and sizes, and mainly include (1) intergranular pores, (2) intragranular pores, (3) dissolution pores, (4) seams around grains, (5) organic matter related pores, and (6) fractures. The tuffs form tight reservoir rocks with low porosity and permeability, ranging from 2.33% to 12.62% (average 7.2%) and from 0.000518 mD to 0.281mD (average 0.0783 mD), respectively. The average porosity and permeability of the tuffs are better than those of shales, and the pores in the tuff reservoirs are often filled with oil residues, which indicate that the tuff reservoirs may be favorable for oil and gas accumulations. The tuff reservoir qualities have strong heterogeneity, which are significantly controlled by lithology, grain size, components, devitrification, dissolution, and fluid pathway systems. Tuffs with large grain sizes commonly have high primary porosity with relatively more cracks and seams within and around grains, which can be conducive to the entry of late diagenetic fluids, which cause tuff reservoirs to be more prone to devitrification and dissolution. The devitrification of volcanic glass can produce abundant microcrystalline quartz, feldspars and intergranular pores, and can greatly improve the reservoir quality. Devitrification has generally occurred in the vitric tuffs and crystal-vitric tuffs, and the porosities have a positive covariation with the quartz contents in the two lithologies. The devitrification pores can be further enlarged by dissolution. Devitrification and dissolution were controlled by the tuff compositions and fluid actions. The fluid pathway systems, including scouring surfaces, bedding, fractures, and well-developed primary pores, controlled the fluid movement. Tuffs that contain high volcanic glass contents and coarse particles with abundant beddings and fractures are prone to intense devitrification and dissolution, which results in high reservoir quality.