Growing up in Manhattan Beach resulted in plenty of beach time. My family lived on 36th Street in the sand and within easy walking distance of the beach. One thing I remember was that I often had to remove tar from my feet with gasoline when I got home. I now live in Redondo Beach within walking distance of the beach. I still have to occasionally remove tar from my feet and from my grand kid’s feet after a day at the beach. We used to blame it on tanker leakage from offloading operations at the El Segundo refinery, but the real reason was natural oil seeps offshore California.
Oil seeps have been a part of California daily life since prehistoric times. Archeologists know the native population used tar from seeps to waterproof baskets and caulk canoes at least 7,000 years ago. Seeps forming the La Brea Tar Pits have been in existence for over a million years. Since there are oil seeps onshore Southern California, it is not surprising that they exist offshore too.
At least 54 offshore oil seeps have been catalogued between Point Conception and Huntington Beach. The largest and most studied concentration of offshore seeps is in the Santa Barbara Channel area. Of these, the most active offshore seeps are at the Coal Oil Point offshore oilfield near Isla Vista. These seeps were releasing an estimated 50-70 barrels of oil per day in the early 1970’s.
In 1917, the National Research Council reported the San Pedro Channel oil seep 10 miles offshore between Pt. Fermin and Santa Catalina Island. The seep at Redondo Beach was the noted in the publication “Geologic Journeys in Southern California” (1939) which reported “On a clear day, petroleum and gas may be seen bubbling to the surface.” The Redondo Beach seep is the southern most of a series of six seeps along the offshore extension of the Palos Verdes Fault. This system of offshore seeps includes the Venice, El Segundo, Manhattan (two seeps), Hermosa and Redondo seeps. These are the most active seeps south of Coal Oil Point.
The Redondo seep is approximately 2 miles offshore at the head of the Redondo submarine canyon in water depths of around 800 feet. Moving north, the Hermosa seep is 3 to 4 miles offshore, Manhattan seeps some 4 to 5 miles offshore, El Segundo 6 miles and the Venice seep is nearly 8 miles offshore. Activity varies with some seeps dormant for extended periods. In 1972, observers studied the Redondo seep and estimated flow rates based on the physical size and sheen of the slicks. While imprecise, rates were placed at 12 to 15 barrels of oil per day along with an unknown volume of gas.
An early 1970’s study of the oil washed up on South Bay beaches determined that some 86% was from natural seeps with the rest the result of contamination from ships or rain water runoff.
Oil is contained in reservoir rocks beneath the surface at pressures approximating the hydrostatic gradient of 0.45 psi/foot of depth. Thus, for an oil reservoir at 3,000 feet, the oil would be pressured to 1,350 psi. Should a pathway to the surface develop through a fault or fissure, the oil, being lighter than water could work its way to the surface as a seep. The rate of flow is directly proportional to the total permeability of the pathway to flow and the pressure “pushing” the oil out of the reservoir. Oil production would naturally reduce reservoir pressure. As reservoir pressure decreases, seepage will decrease as long as the permeability of the pathway remains unchanged – half as much pressure will result in half as much seepage. If there was an earthquake with fault movement, the permeability of the pathway could change and the rate of seepage might also change, for example.
If oil production operations caused an increase in reservoir pressure, as with water injection, the rate of seepage could also change. Water injection to increase oil recovery could be undertaken without exceeding original pressure to mitigate any increase in seepage rate, however.
The hydrocarbon reservoirs offshore Hermosa Beach and Redondo Beach are extensions of the Torrance field and are in pressure communication. The productive pay sands are approximately 2,200 feet to 3,400 feet deep. The Redondo Beach offshore field extension was discovered in 1956, developed and finally abandoned in 1991 after producing nearly 8 million barrels of oil, perhaps 10% of the original oil in place. Since the offshore Hermosa Beach portion of the Torrance Field is in communication with the Redondo offshore portion, it can be expected that reservoir pressure is somewhat depleted, reducing natural offshore seepage.
The Marina Del Rey based Ocean Conservation Society noted in 2011 that the Manhattan Beach and Redondo Beach seeps average about 10 barrels of oil per day, somewhat less than the amount reported in in the early 1970’s.
A study funded by the University of California at Santa Barbara concluded that between 1973 and 1995 seepage in a 13 square kilometer area around platform Holly (which produces oil from Coal Point and South Ellwood Anticlines) decreased over 50% due to oil production. The study was reported in the November 1999 issue of Geology.
That seepage is sensitive to pressure was further demonstrated in a study of oil and gas emissions from abandoned offshore well A-10 at Summerland, California. Subsea tents were placed over the well and oil and gas seepage was captured and measured. At low tide seepage increased, but was reduced at high tide with increasing water pressure against the well head.
So while oil seeps result in that nasty tar you get on your feet at the beach, they are natural. And one way to reduce the rate of seepage is to reduce the pressure in the oil reservoir by oil production.Richard D. Finken, PE