Research paperGeochemical and tight reservoir characteristics of sedimentary organic-matter-bearing tuff from the Permian Tiaohu Formation in the Santanghu Basin, Northwest China

- Peculiar tight oil of sedimentary organic-matter-bearing tuff was investigated.
- Geochemical/physical characteristics of organic-matter-bearing tuff were analysed.
- Organic acids generated by organic matter favour the formation of reservoir pores.
- Porosity is controlled by tuff's original composition and degree of devitrification.

Tuff reservoirs have been found in petroliferous basins but have not been sufficiently studied. The discovery of the Permian Tiaohu Formation reservoir in the Santanghu Basin in Northwest China has offered an excellent opportunity to further enhance our knowledge of the tuff's geochemical and reservoir characteristics. The tuff reservoir has the peculiar property of being a sedimentary organic-matter-bearing tuff. In this study, an integrated analysis of the organic geochemistry, elemental and mineral compositions, quartz crystallinity, pore types, and tight reservoir characteristics was conducted based on samples from the 20-30 m-thick tuffs. The extracted tuff samples have total organic carbon (TOC) values of 0.5-1.0 wt.%, total hydrocarbon yield values of 2-6 mg/g, and hydrogen index values of 20-336 mg HC/g TOC. The organic matter consists predominantly of Type III and II2 kerogens, and the temperature of the maximum yield of pyrolysate varies from 420 to 450 °C, which reflects the oil-generating capacity of the rock. The physical properties of the tuff are characterised by high porosity (varying from 10% to 25%) and low permeability. The air permeability mainly ranges from 0.01 to 0.50 mD. The devitrification of vitreous textures within the tuff primarily dictates the reservoir's characteristics, i.e., the greater the degree of devitrification, the higher the porosity of the reservoir. Vitric tuffs have reservoirs with higher porosity than crystal-vitric tuffs, and the degree of devitrification is generally controlled by the burial depth (temperature) and organic matter (organic acid) content. Vitric tuffs with high porosity and permeability are usually located far from the crater and within the central-lower part of the reservoir. This research may influence geologists to pay more attention to the exploration value of this special type of reservoir in other regions.