Characteristics and controlling factors of tuff tight reservoir in Tiaohu formation of Malang sag, Santanghu basin, China

Abstract

Santanghu basin in Northwest China is a petroliferous basin. In recent years, researchers have discovered tuff tight reservoirs in the Tiaohu formation of Malang sag. Thin section, scanning electron microscopy, electron probe, X-ray diffraction, and CT scanning analysis are applied to reveal the petrological characteristics, physical properties, and pore texture of tuff tight reservoirs. Major controlling factors of the formation of high-quality tuff tight reservoirs are also discussed. The results show that the tight reservoir is intermediate-acidic crystal-vitric tuff. The main component of tuff reservoir is felsic vitroclastic, followed by crystal fragment and a small amount of bioclast, and trace clay content. Mercury injection and CT scanning show that the tight oil reservoir is characteristic of medium-high porosity, ultra-low permeability, and high oil saturation, which is controlled by its pore texture. There are four types of micropores in tuff reservoir, among which the principal reservoir space is devitrified micropores and crystal fragment dissolution micropores. Micropores are distributed universally in the tight oil reservoir, and the throat is nano-scale throat. It is considered that the tuff in the study area is formed by the deposition of volcanic ash in stable water after long-distance drifting in the air. The formation of high-quality tuff tight oil reservoir derives from 3 key factors: (1) shallow-semi-deep lacustrine environment, which is less affected by terrigenous input; (2) intermediate-acidic volcanic ash which forms the tuff reservoir; (3) devitrification and dissolution during the diagenesis.