MIDDLE MIOCENE FORAMINIFEROUS DEPOSITS IN THE SOUTHERN SAN JOAQUIN VALLEY

The middle Miocene Foraminite unit, found locally in the southwestern San Joaquin Valley of California, is a thinly interbedded deposit of shale and microcoquina of foraminifera. The Relizian-Luisian Foraminite is defined paleontologically by the appearance of Siphogenerina branneri and locally comprises the interval between the McDonald and Media Shales. Hydrocarbon production from Foraminite comprised oil and minor gas, and occurred primarily in the Railroad Gap Field, Section 15, T. 30S, R22E, though the unit may extend west into the McKittrick field and southeast to the Asphalto Field. The Foraminite unit ranges in thickness from 40-150 feet as interpreted from well logs and mud logs, has porosity of 20-30%, and has permeability of 2mD.

In the Railroad Gap field, the quality of the Foraminite as a hydrocarbon reservoir depends on the percentage of rock consisting of foraminifera, because the fossils enhance porosity and permeability of the host shale and siltstone. Locating good Foraminite reservoir rock elsewhere requires predicting where deposition of foraminifera dominated deposition of silt and clay. The Foraminite unit has been found in regions where the lower Relizian, shallow-water Button Bed Sandstone is absent, and where the bathyal Gould and Devilwater Shales directly overlie lower Miocene rocks. Published and unpublished field mapping, and subsurface well log analysis indicate that Relizian sedimentary rocks locally unconformably overlie the Saucesian, and in places Zemorrian, section below it; this relationship implies uplift and erosion of the lower Miocene prior to deposition of Foraminite. Well log character suggests that Foraminite in the Railroad Gap field is best-developed where the section is thinnest from the Relizian basal unconformity to the base of the Mohnian Antelope Shale, resulting from limited deposition of clay and silt. These data indicate that the Foraminite unit was deposited on paleotopographic highs, where the amount of foraminifera deposited relative to the amount of clay is greater than in the paleotopographic lows where deposition of clay and silt predominated.