Landslides cover about 345 acres, or 45 percent, of the eastern producing area of the Ventura Avenue oil field, California. In operations which extended over about 45 years to the end of 1969, 61 oil wells were destroyed by landslide activity, and 53 were returned to production. In all, 132 oil wells lie within, or are endangered by, 11 landslides, but have been kept in production notwithstanding continued landslide activity.

Slide movement is ordinarily slow, being measured in inches or a few feet per year. However, unbalanced forces culminate at times in massive movements. A major slide in January 1968 involved more than 1 million cu yd of material and traveled 100 ft within a few minutes. Abnormal rainfall in January and February 1969 reactivated the upper part of one of the older slide areas. In general, however, slides have been controlled and movement has been slow.

The oil field landslides occur, for the most part, in massive granular sandstones of the middle Pico Formation (Pliocene) which are intercalated at irregular intervals with clay-bearing strata. The sandstone is a granular and poorly cemented subangular, porous arkose. Clay strata are ordinarily thin but contain substantial amounts of <2µ montmorillonite with some illite and a minor content of kaolinite. The clay-water mixture probably is the most important factor in landslide mobility at Ventura, although slope, structure, and compaction require proper consideration.

Every reasonable suggestion for slide control within economical limits has been tried, including avoidance of slide-prone terrane, bridging over slides, complete removal, destruction of vegetation, encouragement of vegetation, compaction, terracing, and oiling the slide surface. Attempts to control moisture include horizontal drains, vertical drains, water wells, mine shafts for deep horizontal drains, and even a half-mile drainage tunnel, plus miles of lined ditches. For the most part, control of moisture has proved to be the most rewarding measure.