An Overview of Fractures in the Monterey Formation
Gross, Michael R.
Outcrops of the Monterey Formation provide a world class laboratory to study the effects of mechanical stratigraphy on fracture development in layered rocks. The emergence of the Monterey Formation as a light tight oil (LTO) or liquids rich shale (LRS) play, with an estimated 15 billion barrels of technically recoverable oil (US Energy Information Administration, 2011), has brought into focus the prominent role played by fractures, both in terms of natural fractures that formed by tectonic processes, as well as induced fractures generated by hydraulic fracturing. The objective of this presentation is to describe the geologic factors that control fracture and fault architecture in the Monterey Formation through a series of field-based vignettes. Mechanical stratigraphy refers to the subdivision of a rock section into distinct intervals based on the structures, deformation style or mechanical properties found in those intervals. The two main elements are the mechanical layer (or unit) and the mechanical layer boundary. The former are often represented by a lithologic bed or group of beds, whereas the latter can be a bedding contact or pre-existing fracture. One example of mechanical stratigraphy is the effect of lithology on the mode of brittle failure, whereby conjugate normal faults predominate in mudstone units and opening-mode veins develop in adjacent porcellanites and dolostones. Kinematic and strain analyses demonstrate that this difference in mechanical behavior occurs coevally in response to the same magnitude of applied strain. A second example is the thin to medium bedded nature of the laminated rocks, which leads to abundant, closely spaced fractures restricted to individual beds. These bed-confined fractures, in turn, control the geometry of larger throughgoing fractures that propagate across multiple mechanical layers and may serve as important conduits in the subsurface. Relationships established through the analysis of mechanical stratigraphy and structural deformation help to constrain conceptual models of Monterey reservoirs.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013