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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 43 (1959)

Issue: 1. (January)

First Page: 253

Last Page: 253

Title: Geology of Vallecitos Area: ABSTRACT

Author(s): Donald M. Davis

Article Type: Meeting abstract

Abstract:

The Vallecitos syncline is a part of the San Benito trough recognized as a Tertiary seaway which connected the San Joaquin Valley area with the Tertiary seas on the west. It encompasses an area approximately 20 miles in length and 4-8 miles in width with a trend of approximately north 70° west. It lies between the Ciervo anticline on the north and the remnants of the western part of the Coalinga nose on the south. It is a relatively small intermountain valley whose floor has an average elevation of 2,000 feet. This area has been of economic interest since the Civil War when cinnabar, chromite, and other subsidiary minerals were found in this region. The New Idria Mining Company is the world's largest producer of cinnabar ore and mercury. It has been studied as a pote tial oil producing area since 1912 as oil seeps had been recognized in various places on the flanks of the syncline. As a channel-type seaway, the stratigraphic relationships of the Eocene formations vary greatly over relatively short distances and the general time relation between the Vallecitos and the San Joaquin Valley is obscured by the apparent development of sand bar and pseudo-deltaic deposits which developed at the point of connection. The Vallecitos syncline contains a relatively stable sequence of Pliocene, Miocene, Eocene, and Cretaceous sediments. Facies changes are extreme and stratigraphic complications are great, but, despite these handicaps, oil was discovered in commercial quantities by the Long and Hedges 1A well in June, 1955. Normal, reverse, and lateral faulting is resent in all sizes and shapes with continuous movement being in evidence. Subsequently, several other operators have been successful in delineating oil pools of commercial significance. To-date, the primary reservoir is the Domengine-"Yokut" sands, with minor production from Kreyenhagen sand units. Drilling activity has continued since the discovery and probably will continue for several years to come since the complexities of the area require many bore holes with which to study the structural and stratigraphic problems.

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Copyright 1997 American Association of Petroleum Geologists