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Using Borehole Images To Characterize A Major Fault In The Ventura Avenue Field


An updated 3D model of the complexly faulted Ventura Avenue Field reveals the geometric relations of the major north and south verging thrust faults that compartmentalize the reservoir. Wells penetrate these thrust faults in a variety of ramp and flat positions. Borehole image logs were obtained for a number of these wells, enabling us to characterize the fault zones in the subsurface.

The focus of this study is the Taylor 73 Fault (named for the well where the fault was originally identified). This major south verging thrust acts as an effective seal, separating most of the productive reservoir intervals from shallower zones above. The reservoir facies are turbidite sandstones that include thickly bedded amalgamated units, thinner inter-bedded sandstone and mudstone units, and intervals that are predominantly mudstone. The facies act as contrasting mechanical units that can impact the character of the fault zone. The position along the fault plane is also a primary factor. Not surprisingly, it is often difficult to identify specific fault planes in the flat-on-flat fault position. Identifying the fault zone in ramp positions is much more obvious, although the character and fault expression are highly variable. Fault cores and damage zones range in thickness from tens to hundreds of feet, and can consist of sheared and fractured intervals, intact rotated blocks, and cemented intervals. Our ongoing work investigates position along the fault plane, fault separation, and mechanical stratigraphy as factors that impact fault expression. Refining fault interpretations with image logs constrains the overall subsurface interpretation. We can more confidently distinguish structural from stratigraphic deformation, and identify potentially isolated compartments and barriers in the reservoir.