Characteristics and controlling factors of tuff reservoirs of Huoshiling Formation in Dehui Fault Depression, Songliao Basin, NE China

This study takes the tuff reservoir of the Huoshiling Formation in the Dehui Fault Depression, Songliao Basin, as an example to clarify the characteristics and primary controlling factors of tuff reservoirs. 3D seismic, core, image log, thin section, production dynamic and other data were used. Mineral x-ray diffraction, core computerized tomography scanning, high-pressure mercury injection, porosity, permeability, scanning electron microscopy, cathodoluminescence thin section, element testing and other experiments were conducted. The volcanic rocks in the study area are divided into andesite, volcanic breccia, lithic tuff and crystal tuff. The experiment results show that the mineral composition of tuff is primarily quartz, feldspar and clay minerals. The reservoir space comprises secondary pores and fractures, whereas primary pores are not developed. Average pore-throat radius of most samples ranges from 0.017 to 0.131 mu m, and the pore-throat is primarily fine. The average porosity is 12.21% (a typical medium-high porosity reservoir), and the average permeability is 0.072 mD (an extra-low permeability reservoir). Tectonic fractures generated by tectonic movement are one of the reservoir spaces of tuff. Faults and tectonic fractures constitute a crucial migration channel of the fluid in the deep stratum. The tuff formed in the underwater environment, and the reservoir is affected by alteration. Devitrification is a crucial cause of medium-high porosity and ultra-low permeability in reservoirs. The alteration caused by hydrothermal fluid in deep fluids creates a considerable number of pores and fractures filled with clay minerals. Furthermore, acid fluid dissolves tuff along faults and fractures. In terms of time, the filling action is earlier than the dissolution action. The weathering indexes of CIA, CIW, PIA and WIP indicate that the tuff has suffered intense weathering, and the reservoir's physical properties have been improved to some extent.