Formational-Scale Differences in Styles of Deformation in the Pismo-Huasna Syncline and Implications for Petroleum Migration and Production

Abstract

The Pismo-Huasna Basin, California, is a petroliferous Neogene basin located in a fold and thrust belt between the Coastal Ranges and the Western Transverse Ranges. The Obispo, Monterey, and Pismo formations within the Pismo- Huasna Syncline deformed in a variety of styles and intensities due to their distinct lithologies and typical mechanical layer thicknesses. Some of the resulting structures play a crucial role in the migration and production of hydrocarbons by acting as conduits or barriers. For example, the Edna Member of the Pismo Formation of the Arroyo Grande oil field contains abundant deformation bands, many of which exhibit fault sealing characteristics. Some of the deformation bands act as permeability barriers that hinder the downward migration of oil from uplifted and exposed reservoir rocks and likely compartmentalize the subsurface reservoir. Initial work on these deformation band suggest that they formed after folding during the most recent convergent stage. Quantitative image analysis of the average grain size between the deformation band and host rock indicates there is up to a 41% decrease in average grain size due to cataclasis. Porosity within the deformation bands is reduced to 4-17% from 22-32% in the host rock. Scanning Electron Microscope imagery shows that sand grains within the bands are sheared, fractured, spalled, and locally coated in smectite clay. Fractures analyzed at other outcrops of the Obispo, Monterey, and the Pismo formations vary in orientation and frequency and in shear-sense compared to the deformation bands. In contrast to the sealing deformation bands in the bituminous Edna Member of the Pismo Formation, faults, fractures and injectites in the underlying Monterey Formation provide conduits for migration through and out of the formation. The thin-bedded Miguelito Member of the Pismo Formation is cut by hydrocarbon-bearing sand injectites. Orientations of the injectites are the same as fracture sets found in the same unit, suggesting that oil and sand may have been locally injected into preexisting fractures. Our future work will focus on the relative timing of folding, faults, fractures, injectities and deformation bands in the three different formations. These will be analyzed to see if they principally reflect a temporal evolution of stress, simultaneous deformation, local stress fields or mechanical differences, or a combination of these.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)