Pattern of Vertical Fracture Development Controlled by Mechanical Stratigraphy in Carbonate Layer in Xikeer Outcrop Area of Xinjiang Autonomous Region

It is supposed that we can effectively predict the fracture density in carbonate layer by studying the pattern of vertical fracture development controlled by mechanical stratigraphy. The latitude of stratum is smooth in Ordovician carbonate in Xikeer outcrop area of Xinjiang autonomous region. Opening-mode fractures (joints) that are perpendicular to bedding are typically controlled by mechanical stratigraphy. After computing amounts and analyzing characteristics of vertical fractures in combination with the results of dividing sedimentary cycle in stratum section of Number 21 (latitude 39°50'17.34″N, longitude 77°28'36.12″E), number 32 (latitude 39°50'18.18″N, longitude 77°28'25.62″E) and number 43 (latitude 39°50'10.88″N, longitude 77°28'26.1″E) in researching outcrop area, we identify the distribution characteristics of different mechanical interfaces and based on which we calculate the fracture density controlled by different mechanical units in each section. The average of fracture density of section 21 is 7.1 per meter which is larger than 2.9 per meter of section 43 and 2.2 per meter of section 32. At the same time, we study the dominating factors of fracture density differences in different section from lithological characteristics, thickness of mechanical unit and stress state. It shows that fracture density increases as the content of mudstone is higher in limestone in carbonate stratigraphy and fracture density is sharply lower in grain limestone than that in mudstone. On the other hand, because of the influence of stress shadow, the thickness of mechanical unit has negative correlation with fracture density: the thicker the mechanical unit is, the less the fracture density is. Fracture saturation in section 43 and section 32 is larger than 0.8, which shows they have approached fracture saturated state. However, the section 21 that has the largest fracture density is of unsaturated state, with a fracture saturation of mere 0.6. Therefore, the average fracture has negative correlation with fracture saturation.