Insights into the mechanical stratigraphy and vertical fracture patterns in tight oil sandstones: The Upper Triassic Yanchang Formation in the eastern Ordos Basin, China

Abstract

Vertical fracture patterns are mapped in the outcrops to analyze the impact of mechanical stratigraphy on vertical fractures in the relatively undeformed Upper Triassic tight oil sandstones of the Yanchang Formation along Yanhe River in the east Ordos Basin, China. The relationship between natural fractures and different bounding interfaces are analyzed based on rock relative strength measurement by N-type Schmidt Hammer, fracture and facies description in the outcrops. According to the probability of the bounding interface terminating natural fractures being more than 20%, seven types of mechanical interfaces terminating fracture longitudinal propagation are identified, namely 1) the interface of laminae-set or laminae co-sets, 2) bedding surface interface, 3) accretionary interface, 4) mudstone interbed, 5) mudstone barrier, 6) calcareous barrier and 7) depositional scour interface. According to fracture sizes and the mechanical interfaces which constrain fracture propagation, the vertical fractures are divided into micro-scale, small-scale, meso-scale and macro-scale ones. Three possible vertical fracture patterns in various sedimentary microfacies of the shallow water delta deposits are discussed based on field outcrop observation, the restriction capacities of different mechanical interfaces and the temporally changing tectonic stress. Finally, the application of vertical fracture patterns for subsurface fracture prediction is carried out in the Ansai Oilfield in the eastern Ordos Basin, China. The predicted fractures in wells are consistent with the fractures obtained from cores and conventional logs. This study links sedimentary stratigraphy, mechanical stratigraphy and natural fractures in the tight oil sandstones within a relatively undeformed setting, meanwhile unravels the vertical fracture patterns in different sedimentary microfacies of the shallow water delta deposits in the study area. The results will thus guide subsurface fracture prediction between wells in the tight oil sandstones of the eastern Ordos Basin.