Fracture development characteristics and main controlling factors of the volcanic reservoir in Xujiaweizi fault depression

Reservoir physical properties of volcanic rocks in Xujiaweizi fault depression are influenced obviously by the fracture developing extent. According to the survey of similar outcrops, the core observation, casting thin slice and imaging logging analysis data, in combination with stress field numerical simulation technology, the authors analyzed fracture types, trends, dip angles, lengths, densities, fillings and the factors influencing the fracture development of the volcanic reservoir. The result shows that fractures in the study area are mainly tectonic shear fractures, followed by diagenetic fractures. Four nearly WE-, NS-, NE-and NW-trending fracture groups are developed, mainly high angle fractures and inclined fractures, and the lengths and densities of these fractures change significantly. The main factors influencing the development of the volcanic rock fractures include ancient tectonic stress, lithology, volcanic edifice and fault. Therefore, the high value area of the ancient tectonic stress has good relationship with fracture developed area. In a variety of volcanic rocks, fractures in trachyte, agglomerate and rhyolite are relatively well developed. Fractures are mainly developed in the facies of volcanic crater and near-crater. Fractures are also developed on the upper wall of the fault with obvious stress disturbance and, with the increase of the distance from the fault, the extent of the fracture development decreases.