Magnetic-Reversal Stratigraphy of Pliocene-Pleistocene Producing Section of Saticoy Oil Field, Ventura Basin, California

Abstract

The Saticoy field produces oil from deep-marine turbidites of Pliocene-Pleistocene age in the Ventura basin, an area characterized by high rates of late Cenozoic subsidence and sedimentation. The turbidites are folded and cut by the Oak Ridge high-angle reverse fault which bounds the field on the southeast. Seventy-three samples of siltstone from two wells continuously cored through the producing interval were analyzed for NRM, then subjected to stepwise af demagnetization in fields ranging from 50 to 700 oersteds and thermal cleaning up to 300° C. The sampled sequence is at 2 to 2.4 km below sea level at original reservoir temperatures of 70 to 85° C, and dips 44 to 60° northwest. The fission-track age of zircons from an ash bed midway in the sequence is 1 2± 0.2 m.y. The largest group of samples has declinations of 180 to 200° and inclinations of -50 to -60°, and a smaller group has declinations of 340 to 360° and inclinations of +30 to +50°, suggesting removal of viscous components in enough samples to permit determination of polarity sign, despite low intensities of 10-4 to 10-7 emu. The sequence is mainly of reversed polarity with a normal event at the top marked by 4 samples and a second normal event below the ash bed marked by 8 samples. If the fission-track age is correct, the normally magnetized samples represent the Jaramillo and Olduvai events. The sequence can be correlated to the center of the basin which is now at maximum burial near the coast at Ventura. The paleomagnetic data in icate sedimentation rates at Ventura of 1 mm/year increasing upsection to nearly 3 mm/year.