Formation and distribution of tuffaceous tight reservoirs in the Permian Tiaohu Formation in the Malang sag, Santanghu Basin, NW China

Based on analysis of major and trace elements, observation of thin sections, cathodoluminescence and scanning electron microscope of tuff samples, in combination with the restoration of paleotopography in the Malang sag, the formation and distribution characteristics of the Permian Tiaohu Formation tuff tight reservoirs in the Santanghu Basin are examined. The analysis shows that the tuff samples of the Tiaohu Formation are acid-intermediate, and they are the product of the end of the volcanic eruption cycle. The tuff reservoir rock includes vitric tuff, crystal-vitric tuff, and pelitic tuff, among which, vitric tuff has the best physical properties, followed by crystal-vitric tuff, and pelitic tuff is the poorest. Vitric tuff is usually distributed further away from the crater zone laterally and vertically in the central-lower part of the tuff. Crystal-vitric tuff, mostly interbedded with vitric tuff, is usually distributed near the crater zone laterally and also vertically in the central-lower part of the tuff. Pelitic tuff is generally distributed far away from the volcanic crater on the plane, and in the upper part of the tuff vertically. Types of tuffs are affected by the distance from the volcanic activity belt, and the thickness of tuff is controlled by both volcanic activity belt and sedimentary paleotopography. Thus, depositional depressions on both sides of volcanic activity belt are the favorable locations for the development of tuff. It is predicted that there are five thickness centers of tuff in the Tiaohu Formation of Malang sag (Wellarea M1, L1, M56, M7 and southwest of M7), of which the biggest thickness of tuff is up to approximately 40 m.