- Let's start the discussion on fluid properties with a very important property, the API gravity of the oil. Oil gravity is a basic physical property of dead oil, not reservoir fluid, measured at standard surface conditions of pressure and temperature. It is measured in degrees API. API refers to American Petroleum Institute and is a measure of the density of the oil relative to water. Oil density at 60 degree Fahrenheit is the actual lab measured value, which then is used to calculate oil API gravity at 60 degree Fahrenheit using the shown formula. Oil API gravity is used for different classifications of crude oil, like the one below, and is one of the parameters that defines the quality and price of the oil. In addition, note that oil viscosity increases with decrease in oil API gravity. Crude oil with API gravity less than 10 degrees is a higher density fluid and is referred to as extra heavy oil. Extra heavy oils derived from oil sands deposits in Alberta, Canada, for example, have an API gravity of around eight degree. On the other end of the spectrum, light oils with an API gravity between 40 and 45 degree have lower density and command the highest oil prices, as we'll discuss in the next slide. Let's look now at the relationship between basic fluid properties and oil price, which defines the value of the crude oil and the importance of fluid properties to the business. The figure presents a plot of oil API gravity on the X-axis and sulfur content in the oil on the Y-axis. The data plotted represent oils produced from different parts of the world. You can see from the plot that the United States West Texas Intermediate oil is high quality oil, with high API gravity and low sulfur content and is often priced from five to seven dollars higher per barrel than the so-called OPEC Basket, and the West Texas Intermediate oil is on average one to two dollars higher per barrel than North Sea Brent Blend oils. Lower API gravity and higher Sulfur content oils are lower quality oils and carry lower oil price while higher API gravity-lower sulfur content oils are considered higher quality and carry higher price. Therefore, it is really valuable if we could predict the quality of potential oil accumulation during exploration stage and evaluate the actual oil fluid properties and quality of the discovered hydrocarbon accumulation during the appraisal stage. Determining oil API gravity and sulfur content as well as other basic fluid properties are routine analyses in a geochemistry analytical program during appraisal, as we discussed earlier, and are commonly interpreted in conjunction with the PVT results of the reservoir fluids from downhole fluid sampling and the primary and secondary control processes on fluid properties. So, what are the main control factors on fluid properties? These factors could be grouped in two categories, primary control factors and secondary control factors. The primary control factors include thermal maturity of the source rock, source rock type and facies, which define the type of organic matter, and migration processes. The secondary control factors include in-reservoir processes like biodegradation, water-washing, trap leakage. It is important to know that understanding the controls on fluid quality and fluid properties is a powerful approach to increase predictive capability, decrease uncertainties, and bring value and profit to the business.