Natural Hydrocarbon Seeps Along The Northern Shelf of the Santa Barbara Basin, California

Abstract

ABSTRACT During 1972 new data on seeps were acquired using a towed 3.5kHz sub-bottom profiler system. The profiler records show individual gas and oil seeps as discrete reflections which have a distinctive signature. These seeps are located along faults and fractures paralleling the crest of the western extension of the Rincon anticlinal trend. Nearly all hydrocarbon occurrences closely parallel the east-west structural trends of this area. Over 900 individual occurrences of hydrocarbons have been mapped within a seven-square mite area off Coal Oil Point on the northern shelf of the Santa Barbara basin. Of this total, about 250 are active gas or gas-oil seeps. The other hydrocarbon occurrences are inactive seeps, shallow (near surface) zones of oil and tar saturated sands in Late Quaternary unconsolidated sediments or fractured shale of Late Neogene bedrock units. Most of these hydrocarbon occurrences were mapped utilizing data collected during the period between 1946 and 1958 by various major oil companies. In addition to structural control, the thickness of the Late Quaternary unconsolidated sediment veneer which overlies the bedrock units influences the location of the hydro- carbon occurrences. Of the 900 occurrences mapped, fully 50% are found in area covered by less than five feet of the unconsolidated sediment. When the sediment thickness exceeds 20 feet the seep frequency decreases to less than five percent. INTRODUCTION Historically, the offshore hydrocarbon seeps of the Santa Barbara basin were first recorded by Vancouver in 1792 (Imray, 1). Vancouver, who was Captain Cook's navigator, noted "...the sea had the appearance of dissolved tar floating on its surface, which covered the sea in all directions within the limits of our view? " In 1776, near the present site of Goleta (Figure 1), Father Pedro Font wrote " ...much tar which the sea throws up is found on the shores ..." The implication that the source of this tar was from submarine seeps has been borne out by later studies: Vernon and Slater (2), Allen and others (3), and Fischer and Kolpack (4). Data utilized for this study span a period of nearly 30 years. The various types of core data released by the oil industry, were gathered during two periods, 1946 to 1947 and 1953 to 1954. Acoustic profiles were taken by Moore (5) in 1959 and the present writers in 1972. EQUIPMENT AND METHODS High resolution sub-bottom profiling records were obtained during July 1972. An Edo transceiver, power booster and 3.5Hz transducer, components of the Edo, Model 415 system, were used in combination with a Gifft recorder. The transducer head was mounted in an inexpensive sled designed and constructed at California State University at Northridge. - (Fischer, Stevenson, and White, in preparation).