Fracture spatial arrangement in the context of diagenesis
During the past decade, the need for meaningful documentation fracture spatial arrangement has become increasingly important as this is a key aspect of structural heterogeneity and anisotropy in the upper crust. Uncertainty about fundamental fault and fracture spatial patterns impacts engineering operations such as fluid injection underground, management of induced seismicity and the efficiency and success of fluid extraction. Spatial arrangement is an essential element in developing and managing unconventional and deep hydrocarbon reservoirs, CO2 sequestration, and geothermal systems. My dissertation research focuses on the challenge of describing and understanding fracture spatial arrangement in space. Specifically, my aim is to improve spatial characterization methods and use those methods together with other approaches to understand how the fracture patterns arise. My strategy explicitly links structure and diagenesis: the chemical/diagenetic aspects of fractures are an underutilized source of information on when and under what conditions fracture patterns arise. Fracture patterns comprise the spatial arrangements, sizes, and orientations of fractures. Moreover, cement deposits in fractures may themselves influence fracture arrangements. The central hypothesis guiding my research, is that the mechanical effects of progressive diagenesis—as manifest as deposits within fractures or as mechanical properties or property changes in the host rock—govern the types of patterns that arise. In this dissertation, Chapter 1 primarily explores and explains fracture spatial quantification methods and developing fracture pattern interpretation, but also corroborates that clustering can form independently of diagenesis (or cement deposits) within the fractures themselves. Chapter 2 explores fracture spatial patterns in a folded naturally fractured sandstone hydrocarbon reservoir using horizontal image logs and an outcrop analogue, leveraging the spatial quantification method explained in chapter 1. We discovered spatial patterns associated with high fracture intensities reflect shear on preexisting fractures, possibly resulting in more numerous but less spatially correlated open fractures. Furthermore, production data suggests that clustered but sparse quartz-lined open fractures are more effective fluid conduits than closely spaced partly damaged/sheared arrays. Chapter 3 show that due to different diagenetic histories and quartz accumulation amounts, rocks in outcrops with low thermal exposure and in the deep subsurface with extensive thermal exposure have fractures that differ in terms of their size, shape, and fracture porosity despite forming in the same sandstone host rock and in a common regional structural setting. Previous chapters set up the stage for Chapter 4, in which I address how diagenesis affects rock property evolution through time with diagenesis and rock physics modelling, in situations where timing of fractures is well attested. My results indicate that considerable magnitude of property changes can occur during fracture pattern development, and thus should be taken into consideration in fracture pattern interpretation and modeling processes.