Energy Excursions

# Cymric Oil Field—Heavy Oil in California

The Cymric oil field is located in Kern County, about 35 miles west of Bakersfield and 3.5 miles from the community of McKittrick. The oil field measures roughly 34 square miles (almost 14,000 acres).1CalGEM. (n.d.). High-energy surface expressions. A Story Map. https://cadoc.maps.arcgis.com/apps/MapJournal/index.html?appid=4f6b21eb4e224cab9e86fbb763a41307

## Heavy Crude

The Nacirema Oil Company discovered the Cymric field in the San Joaquin Valley in 1909. Oil produced at Cymric is predominantly heavy crude (oil) with specific gravity around 11-15 °API.2CalGEM. (n.d.). High-energy surface expressions. A Story Map. https://cadoc.maps.arcgis.com/apps/MapJournal/index.html?appid=4f6b21eb4e224cab9e86fbb763a41307 Let’s stop right here to examine what that means.

### API gravity units

When engineers and scientists use the term °API gravity they are referring to a specific gravity scale developed by the American Petroleum Institute (API) for measuring the relative density of various petroleum liquids. API gravity is graduated in degrees on a hydrometer instrument and was designed so that most values would fall between 10° and 70° API gravity.

The equation for API Gravity is as follows:

${}^{\circ }\text{API} =\frac{141.5}{\text{SG}} -131.5$

where

Let’s remind ourselves that specific gravity (SG) is the ratio of the density of an oil to the density of water.

A specific gravity of 1 means that the liquid being compared has the same density as water. If the specific gravity is greater than 1, the liquid is more dense than water. If it is less than 1, the liquid is less dense than water (and will float on it).

It also is possible that we might already know the °API and want to calculate specific gravity (SG). To do that, we can algebraically rearrange our API equation above so that SG is a function of  °API:

$\text{SG} =\frac{141.5}{{}^{\circ }\text{API} +131.5}$

## Practice Problem

The Cymric oils range from 11–15 °API. How do they relate to water as far as their specific gravity? Grab a piece of paper or a calculator and figure out the answer.

The range of specific gravity is 0.99–0.97, so just slightly less than water, and will float on water.

An oil with 30 °API (SG relative to water = 0.88) we would consider a medium oil, whereas, an oil with 45 °API (SG relative to water = 0.80) we would consider a light oil.

## Viscosity

The heavy crude from Cymric flows poorly and that is not an ideal condition. This inability to flow well is related to its viscosity, the property of a liquid to resist flow. Viscosity is the evidence of cohesion between particles within the oil, which causes it to offer resistance, similar to friction.

Synapticrelay, CC BY-SA 4.0, via Wikimedia Commons

Simulation of liquids with different viscosities. The liquid on the right has higher viscosity than the liquid on the left.

Why is this oil so viscous? The chemistry of the oil is the secret to its viscosity, as well as its density. Cymric oil is “heavy” (dense and viscous) because it has larger hydrocarbon molecules than lighter oils. In addition, the Cymric hydrocarbon compounds tend to have carbon ring structures rather than simpler carbon chain structures. Intuitively, you can envision how this would increase the resistance to flow. Specifically, heavy oils have a higher proportion of aromatics (one or more rings) and naphthenes (single rings) than linear alkanes (simple chains); and they contain high amounts of nitrogen, sulfur, oxygen and heavy metals.3Heavy crude oil. (2021, June 9). In Wikipedia. https://en.wikipedia.org/wiki/Heavy_crude_oil The chemistry of an oil today is a result of the composition of the original organic material (deposited in the source rock) as well as the subsequent history of temperature and pressure conditions, contact with other fluids, and bacterial degradation.