The orientation and relative positions of subsurface faults are inferred from analysis of macro- and microfractures in cores from the Saticoy field, Ventura County, California. The conclusions are reached from study of 1,044 macrofractures (joints) in 4,168 ft of core from 28 wells and from the orientations and relative abundances of microfractures (unhealed fractures in quartz grains) studied in thin sections from cores at several depths in each of six wells. The conclusions are checked by comparison with the known structure of the Saticoy field.

In the main, four macrofracture sets exist along the length of the field. Their "real" rather than observed relative development is assessed upon consideration of the problems of determining the orientation of macrofractures in core samples. In the absence of offset criteria, an interpretation of the fracture data is made solely on geometric grounds and with the geologic knowledge available before the field discovery well was drilled. It is concluded that two of the fracture sets parallel, and therefore define, the orientations of two reverse faults.

The microfractures are shown statistically to be parallel with the macrofractures. In addition, information on the abundance of microfractures is presented for 31 sample locations in six holes that are deviated both toward and away from known faults. The abundance of microfractures increases with proximity to the faults and is essentially independent of the depth of burial.

The strike, dip, and position relative to drill holes of the faults predicted from the fracture data agree with the known subsurface structure.