Abstract: 

 

Middle and upper Miocene hemipelagic sediments are the primary oil source in California’s prolific oil producing basins.  They are collectively referred to as the “Monterey Shale”.  However, if you look for “Monterey” on stratigraphic charts and outcrop maps around Los Angeles basin, you won’t find much north of Newport Bay.  Northwest of the Los Angeles River there is the Modelo Formation.  East from the San Gabriel River is the Puente Formation.  Middle Miocene is all called the Topanga Group/Formation depending on where you are.  On the isolated Palos Verdes Peninsula you find the Altamira Shale, the Valmonte Diatomite and the Malaga Mudstone.  Yet the Los Angeles Basin is the most prolific oil basin per unit volume ever found.  So, where did all the oil come from?

 

Subsurface stratigraphy has developed along different lines than traditional outcrop techniques.  Characteristic weathering profiles and coloring are not helpful when trying to evaluate drilling cuttings.  Cores, cuttings logs and wireline log data, have been correlated using micropaleontology, primarily with benthic formaminifera.  Rock unit correlations have been created using log “markers” locally within each oil field.  “Zones” have been delineated within individual oil fields mostly corresponding to intervals that produce in certain areas.  Despite this concentration on identifying contiguous reservoir sand bodies petroleum geologist have documented several unique lithosomes and startling lateral variability within fine grained deep marine sediments of the middle and upper Miocene.  Terms like “Nodular Shale” have taken on stratigraphic standing equivalent to outcrop formation names on the west side of the L.A. Basin. 

 

With the recognition that “shales” can be effective reservoirs as well as source rocks, interest has (once again) peaked in the California “Monterey Play”.  So how is a geologist supposed to answer the question, “where do I drill for Monterey around here?”  It is the task of the geologist to relate traditional outcrop stratigraphy to the very different data set available from the subsurface.

 

Under the old Long Beach Oil Field Signal Hill Petroleum has discovered a new producing zone beneath the established turbidite sand bodies.  The “Signal Zone” is a very thick section of naturally fractured shale facies with intermittent dolomitic and volcanic ash beds, separated into “lower” and “upper” sections by an as yet poorly understood Middle Miocene clastic interval, possibly equivalent to the San Onofre Breccia.  Four wells have been completed in the “Signal Zone” to date.  We are I the process of reviewing a rich subsurface dataset in an attempt to characterize the new zone and understand how it relates to the rest of the Los Angeles Basin Miocene section.  We hope the integrated data will yield the subsurface equivalent of a “type section” that will allow us to predict the extent and economic potential of this Monterey-like section.