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ABSTRACT: Fracture Density in the Deep Subsurface of the Point Arguello Oil Field

Wayne Narr, John Suppe

Monterey Formation reservoirs rely on fractures (joints) for permeability; therefore, quantitative information on fracture spacing is essential to exploration strategies and for understanding reservoir behavior. We measured density of joints in core of four wells from the Point Arguello reservoir using a new, probability-based method, and compared with joints at nearby outcrops. Our measure of joint density is fracture spacing index (FSI), the slope of the layer thickness/joint spacing trend.

A single set of extension joints parallel to bedding dip predominates at outcrops. Likewise, core contains 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 a binary distinction between nonjointing mudstone and harder, jointed rocks. Within each well the FSI of hard rocks is equal (thought it varies between wells). At outcrops FSI in chert, dolostone, porcelanite, and siliceous shale is equal, approximating 1.25. Chert looks more fractured because of thin bedding.

In the reservoir joint density relates to structural position. The most densely jointed strata (FSI = 0.45) are in core from the hinge of a minor anticline, where its plunge steepens. Core from a less steeply plunging anticlinal nose has lower joint density (FSI = 0.30). The lowest joint density (FSI = 0.095 and 0.12) is in core from gently dipping areas.

FSI is used to compute parameters such as fracture porosity and volume of fractures that directly contact the well bore.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990