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Drilling Down on Oil: The Case of California’s Complex Midway Sunset Field

Given the state’s future oil prospects along with large volumes currently being produced, refined, and sold, it is incumbent that elected officials and the public better understand California’s oils.

Published on March 15, 2017


California has vast, complicated, and extremely varied carbon-packed oils about which little is known. The state’s Monterey oil formation is one of the nation’s largest. Exact recoverable reserve amounts vary as technology advances and oil prices fluctuate, but billions of barrels are buried there. A century ago, long before climate change was a concern, California recognized the striking differences between its fields and launched historic efforts to understand its heaviest, most difficult to process oil by charting Midway Sunset, its largest and most productive oilfield. Yet even as the state has moved aggressively and decisively to become a climate leader, these kinds of efforts to better understand its oils have lapsed. Today, Midway Sunset is still California’s most productive field, with oils that have grown heavier and more complex as it has aged, while air quality in the surrounding region constitutes the worst in the nation. What resources actually comprise this super-giant oil field that has produced billions of barrels is still largely unknown and must be ascertained and publicly disclosed.

To that end, a new open-source model—the Oil Climate Index (OCI)—estimates that Midway Sunset is one of the world’s most climate intensive oils tested to date. Barrel for barrel, Midway Sunset, which produces 70,000 barrels per day, has greenhouse gas (GHG) emissions that rival Canadian oil sands.

Given the state’s future oil prospects along with large volumes currently being produced, refined, and sold, it is incumbent that elected officials and the public better understand California’s oils. Information is key, including routine collection of standardized oil assays and steam required in production. This would not only protect public health and safety, the local environment, and global climate—it would also establish a global precedent for assessing the GHG emissions associated with the increasingly diversified array of oil resources worldwide.

Targeting Midway Sunset

There is no better place to start this quest of drilling down deeper to evaluate oil and inform decision making than Midway Sunset oil field.  One of the state’s oldest, is located in Kern County at the southern end of the San Joaquin Valley about 40 miles southwest of Bakersfield. Asphalt deposits were first sighted in 1858, the field was tapped in 1894, and within decades Midway Sunset turned into a gusher that has since produced over 3 billion barrels from its tens of thousands of wells. This mature field in the midst of depletion continues to experience significant reserve growth due to enhanced recovery methods that flood and cycle steam as well as reinject water to keep oil flowing despite the growing list of production challenges.

As early as 1919, the federal government surveyed Midway Sunset’s striking difference compared to other California oils, which themselves are considered unusual in many important respects from oil produced elsewhere in the United States. Much of the oil found in California is extremely heavy, with characteristics akin to oil sands. Some 5 trillion barrels of heavy oil is estimated to be in place worldwide. While these low-quality reserves reside in every global hemisphere—from California and Canada to Venezuela and Russia—it is estimated that California contains nearly one-half of the country’s heavy oil that requires complex technology to produce and refine.

Midway Sunset oil is uneven, unstable, and waterlogged. Geologically, its oil-bearing beds range from relatively young to very old. Ten times more water is pumped out of Midway Sunset than oil. Its quality and weight fluctuates markedly—from as little as 8° API gravity near the outcrop up to 32° at greater depths—with consistencies ranging from the thickest peanut butter to gooey molasses to runny maple syrup.  

Assessing Midway Sunset GHGs Through the Oil Supply Chain

Using the Oil Climate Index (OCI), a collection of three models discussed below, the climate impacts of oils can be compared—barrel for barrel—from their upstream production, midstream refining, and downstream end use. What is noteworthy is how wide-ranging different oil’s GHGs can be. In a test of 75 global oils—25 percent of current production—there is a 60 to 90 percent difference in climate impacts between the least carbon intensive and the most. Midway Sunset is included in the OCI analysis. As stated above, the OCI estimates that Midway Sunset, at current production levels, is one of the highest emitters modeled in the OCI, nearly as climate intensive barrel for barrel as oil sands. But this highly complex field may contain oils that, depending on their particular characteristics, are even higher than the OCI initially estimated.

When it comes to estimated upstream production GHGs, Midway Sunset has been provisionally modeled along with 153 other California oils in the Oil Production Greenhouse Gas Estimator Model. Extracting a barrel of Midway Sunset is estimated to release 180 kilograms of carbon dioxide equivalent emissions (kg CO2 eq./bbl crude), making this field one of the highest-emitting oils produced in the state. Over the past fifty years, enhanced recovery techniques have been employed as Midway Sunset oil has aged, leading to a four-fold increase in upstream production emissions. If these trends continue and steam injection rates rise, upstream emissions could possibly increase further to over 200 kg CO2 eq./bbl crude. And beyond elevated climate risks, operating in this densely developed, highly fractured formation is a dangerous enterprise. Continuing to collect data on steam usage would facilitate routine updating of this field’s production emissions profile, enhancing policymaker oversight of Midway Sunset’s 40 operators, including Aera, Chevron, and Linn (an investor partnership).

Midway Sunset also presents climate challenges during refining that are assessed with the Petroleum Refinery Life Cycle Inventory Model, PRELIM. This analysis is limited by the surprising lack of information about this field’s varied nature. Despite its known complexity, there is only a single, questionable publicly available oil assay for Midway Sunset. Even under a best-case scenario, refining Midway Sunset is estimated to emit 81 kg CO2 eq./bbl crude, ranking it as one of the highest GHG emitters compared to the 75 global oils modeled in the OCI. The use of proxy assays to better represent the varying nature of this heavy oil suggest that refining emissions may be at least 10 percent higher.

The lack of clear characterization of Midway Sunset due to missing assays creates critical uncertainties in modeling downstream end use emissions. The OCI employs the Oil Production Emissions Module, OPEM, to obtain the volumes of petroleum products reported out in PRELIM and calculate their GHG emissions in end use. The only oil assay that is publicly available dates back to 1978 and is published by Knovel. This assay characterizes Midway Sunset as a medium-weight oil (22.6° API gravity) with commensurate GHG emissions estimated at 464 kg CO2 eq./bbl crude. Yet there is sufficient evidence to the contrary; Midway Sunset is actually quite heavy. Chevron markets this benchmark crude at 13° API gravity and USGS records indicate gravities below 11° API, although no open-source assays are available for these different Midway Sunset oils.

Absent actual assay data, we can model Midway Sunset using proxy assays from other oils at 19°, 12°, 10°, and 8° API gravities and run them through complex coking refineries. End use emissions that are modeled through OPEM are as high as 500 kg CO2 eq./bbl crude. Increased emissions are associated with increasing volumes (as much as one-fourth total product yield) of petroleum coke, or petcoke. Petcoke is a solid, residual fuel that is too dirty to use in California and is exported to India and other countries where its combustion may adversely impact the local environment.

Updated, verifiable oil assay data and steam-to-oil ratios (SOR) are lacking for California Midway Sunset. The three Midway Sunset scenarios labeled in this figure were modeled using different OCI input assumptions in order to estimate the significant range in this complex oil’s greenhouse gas emissions. Certain scenarios estimated Midway Sunset total GHG emissions may be higher than some Canada Oil Sands.

  • Scenario A – Publicly available Assay: Midway-Sunset supplied by Knovel, API: 22.6°, Steam to oil ratio (SOR): 5.79. While Scenario A uses an actual Midway Sunset assay, it is not thought to be entirely representative of this complex field.
  • Scenario B – Public proxy Assay: Venezuela Tia Juana supplied by Stratiev, API: 12.1°, SOR: 5.79. While Scenario B uses a proxy assay, it is thought to be more representative of the 13° API gravity Midway Sunset that is posted monthly as a benchmark crude by the Oil & Gas Journal.
  • Scenario C – Proxy Assay: Athabasca Bitumen Thermal supplied by Alberta Energy, API: 8.1°, SOR: 7.50. While Scenario C uses a proxy assay, it may provide a better approximation of the higher end of GHGs for lower quality Midway Sunset oil that is similar to Canadian oil sands.

Note: Emissions may not add up due to rounding; the sulfur content of different Midway Sunset crudes, like many other California oils, is not consistently reported. Proxy assay sulfur content may not be representative of Midway Sunset crudes. But this should not affect appreciably alter GHG estimates in the OCI.

Protecting Air Quality Too

The challenges posed by Midway Sunset extend beyond climate change. Production of heavy oils, including the oil sands found in Canada, are known to produce secondary organic aerosols (SOAs) that make up fine particulate pollution (PM 2.5). PM 2.5 is a dangerous air quality criteria pollutant that has been tied to increased risk for cancer, diabetes, and various lung and heart problems. A recent study found that production of Albertan oil sands is the leading source of air pollution in North America, emitting twice as much SOAs as car and truck exhaust.

While the pollution impacts from Midway Sunset oil production have not been as carefully examined, similarities between the composition of oil sands and Midway Sunset’s complex oil raise local air quality concerns. Nearby Bakersfield is home to the highest rates of particulate pollution in the nation, according to a 2016 American Lung Association report.

Refining heavy oils like Midway Sunset use coking processes to reject the excess carbon and turn these oils into more gasoline and diesel. These deep conversion refineries have high particulate emissions that are directly released into the surrounding atmosphere. The California Air Resource Board has recently proposed emission reduction goals aimed in part to reduce pollution in nearby communities over health concerns.

Air quality problems trail Midway Sunset oil to petroleum product consumption, where fuel grade petcoke that is too dirty to burn in California is shipped to Asia and elsewhere. And pollution from exported residuals may return to California in prevailing winds and intercontinental transport.

Staying Informed in the Next Age of Oil

Once upon a time, oil gushed when poked, flowed easily with little prodding, and was readily distilled into gasoline with minimal treatment and reforming. Today, oil conditions have changed. Increasing effort is going into producing, transporting, refining oil, and consuming its growing array of petroleum byproducts. California is a testament to these difficulties. Regulatory oversight is an ongoing concern. And managing oil in a warming world will be an ongoing challenge along with protecting local environmental health and safety.

California has the opportunity to lead the world in the responsible management of its heavy oil resources. This path must be illuminated by the routine collection of standardized, open-source oil data. And there’s no better place to start than with Midway Sunset.

Carnegie does not take institutional positions on public policy issues; the views represented herein are those of the author(s) and do not necessarily reflect the views of Carnegie, its staff, or its trustees.