Because Monterey Formation reservoirs rely on fractures (joints) for permeability, quantitative information on fracture spacing is important to exploration strategies and for understanding reservoir behavior. Density of joints in cores of four wells from the Point Arguello reservoir has been determined with a new, probability-based method, and these subsurface joint densities are compared with joint densities measured at nearby outcrops. My measure of joint density is the fracture-spacing index, which is the slope of the trend of layer thickness to joint spacing.

A single set of extension joints that strike parallel to bedding dip predominates at outcrops. Likewise, the cores contain a single set of extension joints perpendicular to the anticline axis, and parallel to present-day maximum horizontal compressive stress.

In core and at outcrop, the only lithologic control on joint density is between nonjointed mudstone and harder (more brittle), jointed rocks. Within each well, the fracture-spacing index is the same for all hard rocks (though it varies between wells). In the reservoir, joint density relates to structural position. The most densely jointed strata (fracture-spacing index = 0.45) are from the hinge of a minor anticline, where its plunge steepens. A less steeply plunging anticlinal nose has lower joint density (fracture-spacing index = 0.30). The lowest joint density (fracture-spacing index = 0.095 and 0.12) is in cores from gently dipping areas.

At outcrops in various structural settings, the fracture-spacing index is the same (approximately 1.29) in chert, dolostone, and porcelanite and siliceous shale. These rocks may be "saturated" with joints, so that differences in brittle strain due to local structural variations have been overwhelmed as joints continued to form during unroofing of these strata. Chert looks more fractured than other lithologies because of thin bedding.

Fracture-spacing index is used to compute such parameters as fracture porosity and volume of fractures that directly contact the well bore. These parameters may be important when trying to model the behavior of a petroleum reservoir, or when trying to assess the feasibility of strongly deviating wells to improve the performance of a fracture reservoir.