221 / 2019-06-25 22:04:45

Characteristics and Controlling Factors of Tuff Tight Reservoir in Tiaohu Formation of Malang Sag, Santanghu Basin

tight oil; tuff; pore structure; physical properties; Santanghu Basin

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. In this paper, polarizing microscopy, scanning electron microscopy, electron probe, X-ray diffraction and CT scanning are applied to analyze the petrological characteristics, physical properties and pore structure of tuff tight reservoirs, as well as the main controlling factors of the formation of high-quality tuff tight reservoirs. 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, with very low clay content. Laboratory data show that the tight oil reservoir is characteristic of medium-high porosity, ultra-low permeability and high oil saturation. There are four types of micropores in tuff reservoir among which the principal reservoir space is devitrified micropores and crystal fragment dissolution micropores. A large number of micro-pores are distributed evenly in the tight oil reservoir, and the throat is nanometer throat. The characteristics of medium-high porosity and ultra-low permeability are caused by the pore structure. It is considered that the tuff in the study area is formed by the deposition of volcanic ash in quiet water after drift of certain distance in the air. The formation of high-quality tuff tight oil reservoir is influenced by many factors:1)Shallow-semi-deep lake area, which is less affected by provenance and stable water body, is a favorable place for the formation of high-quality reservoirs;2)Intermediate-acidic volcanic ash is the material basis for the formation of high-quality reservoirs;3)Devitrification and dissolution in the later stage are necessary conditions for the formation of high-quality reservoirs.