James M. Galloway, Geologist, Pacific OCS Region

As long as humans have recorded their history, the natural occurrence of oil and gas at the earth's surface has been of great curiosity and considerable economic interest. Oil, tar, and natural gas seeps are part of the natural environment, and geological and archeological evidence shows that seepages have occurred throughout California for thousands of years. Even today, natural hydrocarbon seeps along the California coast continue to be more than an idle curiosity. Two small underwater containment structures positioned near Goleta Point, placed to collect natural seepage, have alone captured over 4 billion cubic feet of natural gas since 1982: enough natural gas to supply the needs of over 25,000 residential natural gas users each year.

HISTORIC IMPORTANCE

The importance of petroleum predates written history, evidenced in many Old World archaeological sites. It is no surprise that the Native Americans of the coastal areas of California, like the inhabitants of the Old World, incorporated naturally occurring hydrocarbons into their cultures. The earliest accounts of oil and gas seepages in California come to us from the seventeenth century annals of the European explorers. Native Indians, including the Chumash, Yokuts, Achomawi, and Maidu used oil, tar, bitumen and other natural substances from the seeps for ceremonial and recreational purposes. Oil was often used as a base for paints and asphaltum, a brownish-black mixture used in paving, roofing, and waterproofing, was used as a mastic to inlay colorful stones and shell fragments.

Pedro Fages, a Spanish explorer and military commander of the Monterey Presidio, in his report to the Viceroy of New Spain recorded the use of tar and oil by the natives near Mission San Luis Obispo. Fages' account, written in 1775, mentions natives using tar for water- proofing baskets and pitchers and for caulking small boats. Fages also noted " ... pools of bitumen bubbling out of the ground" near the mouth of the Santa Clara River. In 1776, Spanish missionary Pedro Font recorded that "... much tar which the sea throws up is found on the shores, sticking to the stones and dry, little balls of tar are also found. Perhaps there are springs of it which flow out into the sea." In 1793, during the travels of English explorer James Cook, his navigator, George Vancouver, recorded in his journal that they had anchored off of Goleta. Vancouver reported that the sea was "... covered with a thick, slimy substance, which, when separated or disturbed by any little agitation, became very luminous, whilst the slightest breeze, that came principally from onshore, brought with it a very strong scent of burning tar." He continued that "... the sea had the appearance of dissolved tar floating on its surface, which covered the ocean in all directions within the limits of our view."

Early California pioneers (c. 1850) used the oil from natural seeps to grease their wagon wheels and settlers and ranchers, especially in the Santa Barbara, Ventura, Los Angeles, and Orange county areas, used seeped oil for lubricating farm machinery, for tarring roofs, and for illumination. Early on seeps were exploited by digging directly into them. Just as one would sink a shallow water well, diggers shoveled down until oil and tar began to ooze into the pit, or until the petroleum fumes and gas overcame them. In Ventura and Kern Counties, mining techniques were used to tunnel into the mountainsides, with elaborate lighting and ventilation systems. By the 1870's, technological advances allowed the development and use of specially designed oil drilling tools to sink wells deeper than any digger could excavate.

From the 1860's to the early 1900's, every oil or gas field discovered in California was located on the basis of nearby seeps. In California, the earliest oil exploration efforts began in areas where numerous seeps, such as Ojai and Santa Paula, occurred. The first commercial oil discoveries in Pennsylvania spurred both commercial and academic interest in coastal California oil, tar, and gas seeps.

SCIENTISTS INVESTIGATE

In the mid-nineteenth century, little scientific evidence existed concerning the origin and habitat of oil and natural gas. Geologic science was in its infancy and the branch known as petroleum geology was not yet conceived. It was during this time that geologists began to study the nature of oil seeps and how they related to the age, structure, stratigraphy, and lithology of rocks. It was determined that rocks like sandstone, shale, and limestone contained oil, gas, and tar, and that rocks like granite, marble, or lava did not. Indeed, those rocks that had been deposited in ancient oceans or great lakes were most likely to contain petroleum. It was also noted that coal-bearing rocks were likely formed in swampy environments.

Early geologists learned that seeps generally occur where bituminous, or oil-bearing rocks were exposed at the earth's surface by erosion, or where a fault was present. Almost every major petroleum producing province in the world has been discovered because of surface seepage. Some famous oil seeps, such as the La Brea Tar Pits or the Pitch Lake of Trinidad, formed accumulations so large that they trapped the people or animals that unwittingly stumbled into them. The Perpetual Fires of Baku, in a large gas seep area in Azerbaijan, were known to the ancients.

Oil, tar, and gas seepages are common along the California coastal areas because of the geologic history of the area over the last 70 million years. Scientists have studied this history as it is recorded in the rocks and fossils, and their research indicates that the paleogeography of the area was significantly different than it is today.

Anywhere from 70 to 30 million years ago the topography of the coastal areas of North America was similar to that of today's South American west coast. Mountainous areas flanked the coastline but the mountains were continually worn away by erosion and the great amounts of sand, silt, and clay sediments that washed down the rivers and dumped into offshore basins. Over millions of years, these sediments accumulated, lithified, and formed strata comprised of sandstone, siltstone, and shale. In some shallow water areas calcium carbonate-rich reefs formed and were preserved as limestone.

Geologists study these rocks, called the Great Valley Sequence, where they are now exposed. These rock formations are seen in the Santa Ynez, San Raphael, and Santa Susana Mountains as well as the Channel Islands and are the same rocks that have been observed in deep wells throughout the San Joaquin and Sacramento basins and much of the Coastal Ranges.

About 30 million years ago, the tectonic plates collided along the North America-East Pacific boundary causing dramatic alterations to the shape and form of the coastal areas. Great fault systems, such as the San Andreas fault, were formed. Massive blocks of land were rotated, translated, and jostled. Rock formations were folded and faulted to form mountain ranges. Deep depressions subsided, forming basins which caught eroded sediments. The oceans off the coast were rich in biological activity and organic detrital matter filtered down to the ocean floor, mixing with sediments to form the building blocks of hydrocarbon-rich petroleum source rocks.

According to petroleum geologists, good source rock contains about 1% total organic carbon (TOC). In the California coastal areas, the source rocks often contain 4-6%, with some zones measuring almost 20% TOC. It is no wonder then that, where the source rocks are exposed at the surface, they literally bleed petroleum. It is also no surprise that oil, tar, and gas seepages occur offshore.

Over the last 20,000 years, relative sea level has risen more than 300 feet. The erosion that today exposes oil seeps along the beaches and in the local hills did the same to areas now under water. A significant difference between offshore seeps and onshore seeps is that underwater seeps are subject to the weight of the seawater column and to shifting ocean floor sediments. Seeps are affected by tidal changes, and, like onshore seeps, offshore seeps are stimulated by earthquake activity. Because ocean temperatures are more constant than air and ground temperatures, underwater seeps do not speed up or slow down on hot or cold days. They do, however, show seasonality in seepage rates.

Today, especially on a warm summer day, a person driving through Santa Paula will smell petroleum fumes and notice thick, black crude oil running down the side of the road ... exactly what the Chumash smelled centuries ago. All over California, from Humboldt to Kern to San Diego Counties, place-names such as Oil Creek, Petrolia, Oildale, Brea, and Coal Oil Point testify to the widespread occurrence of petroleum seepages.

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