The Point Arguello field is located in federal waters at the western end of the Santa Barbara Channel, offshore California. The reservoir consists of highly fractured fine-grained siliceous rocks of the Miocene Monterey Formation. Nine wells were examined, using cores and Formation MicroScanner (FMS) images, to determine the relationship of fractures to lithology and structure.

The Point Arguello structure is a doubly plunging, slightly asymmetric anticline created by horizontal compressive stresses oriented N36°E. Monterey Formation rock types in the Point Arguello field consist of interbedded shale/mudstone, marl, porcelanite, chert, and dolostone. The siliceous rocks are in the quartz phase of diagenesis. Core and outcrop studies show that fracture development is highest in the more siliceous rocks (chert and porcelanite) and is lower in more argillaceous rocks. Fracture development is also higher in thin beds.

The Monterey Formation is fractured into three distinct sets of joints oriented relative to the principal horizontal compressive stress direction: extensional, longitudinal, and shear. Extensional fractures have dips ranging from 70° to 90° and strike directions ranging + or -15° perpendicular to the fold axis. Longitudinal fractures have dips that are variable but are often normal to bedding and oriented subparallel (+ or - 15°) to the fold axis. Shear fractures cut the other fractures at an angle and occur as conjugate pairs, with the development of one half of the pair dominating over the other. The orientations of fracture sets are similar to those described in outcrop in the western Santa Ynez Mountains.

Fractures are grouped according to their order of frequency for each well, with first order fractures having the highest frequency, and second, third, and fourth order fractures having lesser frequencies. All the wells have one dominant set of fractures (first order) with second and third order fractures occurring at comparative frequency ratios of 2:1 to 25:1. No one fracture type dominates in the order of development, and the development appears to be random across the structure.

Extensional fractures are common to all the wells, whereas longitudinal and shear fractures are not. Extensional fractures probably provide a mechanism for connecting other fractures that have developed at all scales, from the microscopic to the megascopic. The result is that extensional fractures may be responsible for draining the reservoir.

Fracture analyses were conducted on both core and Formation MicroScanner (FMS) images. Two wells had both types of data recorded from the same intervals. The fracture analyses and bed orientations derived from FMS images compare very well with cores and dipmeter data.