Pacific Section AAPG Convention

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Two Small-Scale Deformation Features In The Monterey Formation And Their Significance For Reservoir Characterization


The Monterey Formation is often highly fractured and deformed due to its brittle nature and occurrence along an active transform margin. The degree of brittle deformation is a crucial factor in the quality of fractured Monterey Formation reservoirs. Small-scale features often record multiple episodes of structural deformation and their presence can impact reservoir properties such as porosity, permeability and hydrocarbon saturation. Incorporating detailed core description, optical petrography, SEM with EDS, FTIR and XRD, we are able to characterize two distinct features, which are often grouped together as small scale fractures, but reflect very different timing and mechanisms of formation. Intrastratal microfractured zones (IMZ) are small, strata-bound, en echelon tear fractures that are compositionally the same as the matrix but have a higher grain density, and form in the early stages of burial diagenesis. The IMZ have a lower porosity and permeability than the matrix rock, which can affect hydrocarbon saturation and fluid movement. Vertical pressure solution seams, on the other hand, are features that are compositionally different than the matrix and form due to horizontal stresses, causing pressure solution at grain contacts. The pressure solution seams form perpendicular to sigma 1 and are important indicators of paleo stress directions. These typically form during later stages of burial, related to tectonic stresses. They also serve as pre-existing planes of weakness that can be reactivated in opening mode or shear with further tectonic strain or induced well stimulations.