Ventura Basin Study Group Maps & Cross Sections


There are currently several options for accessing the VBSG information:


VBSG Guide

Guide to Ventura Basin Maps & Cross Sections 



Introduction

The Ventura Basin is one of the more active tectonic regions of the world. In the basin, the Oak Ridge fault accommodates high rates of oblique crustal strain and, along with several other major faults, is considered a significant seismic hazard to a large urban population. The 1994 M6.7 Northridge earthquake occurred on a blind, south-dipping fault beneath the San Fernando Valley that is considered part of the same active fault and fold system that extends westward into the central Ventura Basin. Assessing the nature, geometry, and seismic potential of these active subsurface faults is difficult because (1) many of these structures are blind or buried and do not crop out where they can be easily characterized; and (2) many of these structures have experienced a complicated history of tectonic deformation.

To better understand the earthquake hazard and subsurface structure of the Ventura Basin, ICS and the Southern California Earthquake Center (SCEC) acquired access through the National Earthquake Hazards Reduction Program (NEHRP) to a unique 3D dataset of structure contour maps, cross sections and well information files produced by the Ventura Basin Study Group (VBSG). The VBSG study is based on the detailed analysis of nearly 1200 deep-penetration wells drilled for the purposes of petroleum exploration and hydrocarbon development. Many of the wells drill active fault and fold structures associated with major fault systems, including the San Cayetano, Oak Ridge, and Santa Susana faults. The data used in this study include wire-line logs, mud logs, well correlations, core analyses, core descriptions, paleontological reports, surface maps, and reports from producing fields. The wire-line logs (including resistivity, dipmeter, gamma-ray, sonic-velocity, self-potential, and neutron-density) provide information on lithology, bed thickness and continuity, dip magnitude and direction, characteristic rock properties (including porosity, permeability, and interval velocity) and fluid content. The data from these wells thus provide important and extensive subsurface structural, stratigraphic and petrophysical information to depths of 1 to 5 km, and--in areas where coherent seismic data can not be acquired-- this information may be the only subsurface data available.

The VBSG study and dataset includes 17 subsurface structure contour maps of various formation horizons that tie together in a grid of 21 structure cross sections consisting of 84 interlocking correlation cross section data panels. Each panel typically ties in 4 directions to define the sides of a 3D data volume or cell. Section panels jog from well to well so that data are not distorted by projection into the line of the section. Furthermore, interpretations are not projected below data depths or constrained by any specific structural model. The result is a 3D presentation of an enormous quantity of high-quality subsurface data that have been assembled and reconciled into a coherent geological interpretation.

It should be noted that the VBSG dataset fills a critical data gap in understanding subsurface structure between the shallow near surface accessible by trenches, and the depths at which most earthquakes occur (~5 to 20 km). It should also be noted that the VBSG correlation cross sections are not typical 2D cross sections, but are structure data panels that tie into a 3D grid, and thus provide a precise image in 3D of subsurface structure at specific subsurface control points (i.e., wells). Any 2D or 3D kinematic model of how the basin and its associated fault and fold structures developed, or any estimate of the regional seismic hazard must incorporate and accommodate the VBSG subsurface data, if it is to be successful. Any model that fails to adequately explain the VBSG data at mid-crustal levels is unlikely to correctly infer deep fault and fold geometry at seismogenic depths where large earthquakes originate.

Instructions

A Cross Section Index Map allows users to view individual cross sections by clicking on the line of the cross section from the index map. The cross sections and maps are also associated into one of four areas: Area 1, Area 2, Area 3, and Area 4.  Users can browse graphical thumbnails of the map and cross-section images by selecting an area index. Each area is organized into three sub-categories: Maps, North-South Cross Sections, and East-West Cross Sections. The Text Catalog provides a simple listing of all of the available images without graphical thumbnails for easy and fast down-loading.

Requesting Copies

Full-sized hard copies of the VBSG maps and cross sections are available upon request for research studies related to the National Earthquake Hazards Reduction Program from the Institute for Crustal Studies, 1140 Girvetz Hall, University of California, Santa Barbara, CA 93106-1100 (now part of the Earth Research Institute). Contact Craig Nicholson at craig@crustal.ucsb.edu (Phone: 805-893-8384) for further details. An on-line digital database of well information files for the Ventura basin is available from the California Division of Oil, Gas & Geothermal Resources (DOGGR) District 2 region.

Copies of the VBSG study and dataset for commercial and other non-NEHRP research-related purposes are available from Tom E. Hopps, Rancho Energy Consultants, Inc., Ventural, CA 93001. Phone: (805) 652-0066. tom@ranchoenergyinc.com

Comments and Suggestions

Comments regarding the utility of this web page are welcome. If you have suggestions on improving the layout, or if you need additional information not currently available, please email Craig Nicholson at: craig@crustal.ucsb.edu with your comments or suggestions. This web page was originally designed and constructed by David Valentine. We gratefully acknowledge the support of the U.S. Geological Survey External Grants Program, the National Science Foundation, and the advice and suggestions of our colleagues Geoff Ely, Ralph Archuleta, and Art Sylvester.