Fracture Characterization in Fine-Grained Sedimentary Systems: Miocene Monterey Formation, California

The middle to late Miocene Monterey Formation of coastal California is a source and reservoir for the majority of hydrocarbons found in the Santa Barbara - Ventura Basin. The reservoir is unique in that conduits for hydrocarbon migration are primarily systems of interconnected fractures, faults and breccias. Fracture related data was collected from wave-cut terraces and dip-slope, beach cliff exposures along Gaviota Beach, approximately 30 miles west of Santa Barbara, off of highway 101 in Santa Barbara County, California. Five systematic sets were identified with mean orientations of 13° ± 8° azimuth (set 1), 38° ± 7.5° (set IV), 78° ± 5° (set V), 115° ± 5° (set II) and 174° ± 7.5° azimuth (set III). Sets I, II and IV are most abundant in the area and their angular relationships correlate well with offshore core data from the Holly Platform. Outcrop data from this local matches well with data collected by previous researchers at Arroy Burro Beach suggesting that the observed geometries are a large scale, regional phenomenon and likely predictable. Fracture characteristics vary between lithologic packages or cycles; in that neighboring cycles may exhibit differing fracture set geometries. This fracture partitioning appears to occur at various scales from centimeters to meter scales. Data suggest that cycle boundaries act as mechanical boundaries enabling each cycle to behave separate from one another on a 1 - 1.5 meter scale. There likely exists a mechanical package on a scale of tens of meters controlling the propagation of large pervasive fractures which cut through numerous cycle boundaries before propagation is halted. This larger scale mechanical boundary may overprint the 1 - 1.5 meter mechanical packages previously described. The effects of weathering or subaerial processes on fractures, in fine-grained siliceous sediments, is generally restricted to the enhancement or widening of the fracture aperture. Data were collected along a stratigraphic horizon between minimally exposed wave-cut terraces and laterally equivalent, fully exposed dip-slopes. Generally the average aperture increased by 0.02 ± 0.002 mm/m from middle terrace to dip-slope exposure. The rate at which this occurs through time requires further study