Abstract: Structural Interpretation of the Vallecitos Region

ERSKINE, MEL, Consulting Geologist, El Cerito, CA

The Vallecitos region has produced over 5 million barrels of oil and 5 bcf of gas from a structurally complex syncline. The hydrocarbon source was the Eocene Kreyenhagen Formation (Magoon, 1999, per. com.). The Central Diablo Range Antiform is interpreted as a fault bend fold that initially developed at least as early as latest Cretaceous-Paleocene time. The fold developed over a gently southwest dipping ramp of a major regional blind-thrust system. The Vallecitos syncline - New Idria anticline pair are interpreted as an out of sequence imbricate of this regional thrust system and the Coalinga earthquakes suggest the location of the basal ramp-run transition of the system. Erosional unconformities within the Cenozoic sedimentary section indicate that thrusting on this ramp has continued episodically throughout the Cenozoic and that the system is presently active. Deformation of Pliocene-Pleistocene sedimentary rocks suggests current N20E vergence, oblique to the strike of the San Joaquin Valley - Sierra Nevada system. The strike of the Vallecitos syncline -New Idria anticline also suggests oblique northeastward convergence for the earlier regime. This interpretation extends concern for Coalinga type earthquakes to the entire Diablo Range. The Livermore Valley and San Luis Reservoir seem at particular risk.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California