In this lesson, we learned about the origin of crude oil and natural gas and their occurrence in subsurface reservoirs. In particular, we learned that subsurface reservoirs are composed of porous and permeable rocks and that there are five requirements for a crude oil or natural gas reservoir:
- the presence of a source rock where organic material accumulates and, over time, is converted to kerogen (a precursor to crude oil and natural gas) due to the subsurface heat and pressure;
- the presence of a migration path where hydrocarbons can migrate from the source rock to the reservoir;
- the presence of a cap rock to prevent the lighter oil and gas molecules from escaping from the reservoir to the surface because of their buoyancy in water;
- the presence of a reservoir rock to allow for (A) the accumulation of oil and gas in commercial quantities and (B) the producibility of fluids at commercial rates;
- the presence of a trap to provide a mechanism to contain the oil and gas over geologic time until discovery.
In addition, we discussed the two major classifications of hydrocarbon traps:
- stratigraphic traps where the trapping mechanism is dominated by the layering of the rock;
- structural traps where the trapping mechanism is dominated by the folding, faulting, and deformation of the reservoir rock.
We also saw that crude oil and natural gas is composed of hydrocarbon molecules of organic origin. These hydrocarbon molecules are categorized into four types:
- paraffinic (or alkane) hydrocarbons which are straight chain molecules (or branched-chain molecules call isomers);
- naphthenic (or saturated cyclic) hydrocarbons which are ringed molecules where all of the bonds between carbon atoms are single bonds;
- aromatic (or unsaturated cyclic) hydrocarbons which are ringed molecules where double bonds between carbon atoms are present;
- asphaltene hydrocarbons which are complex mixtures of chain and ring molecules.
We also saw that the dominant hydrocarbon type can be determined by the Watson Characterization Factor.
Finally, we discussed the basics of the phase behavior of the hydrocarbon accumulations. We saw that this phase behavior can be visualized in a Phase Diagram and can be quantified with tools, such as the Gibbs Phase Rule, among others (like Cubic Equations of State). We also saw that the phase envelopes shown in the Phase Diagrams, the initial reservoir pressure and temperature at discovery, the pressure-temperature path of the reservoir during production, and the pressure-temperature path of the fluids going up the well and through the surface equipment results in several types of hydrocarbon reservoirs. The hydrocarbon reservoirs that we discussed were:
- undersaturated and saturated black oil, or low-shrinkage, crude oil reservoirs;
- undersaturated and saturated volatile oil, or high shrinkage, crude oil reservoirs;
- gas condensate reservoirs (sometimes referred to as retrograde condensate reservoirs) where liquid hydrocarbons condense from the gaseous phase;
- wet gas reservoirs;
- dry gas reservoirs.
Reminder - Complete all of the Lesson 2 tasks!
You have reached the end of Lesson 2! Double-check the to-do list on the Lesson 2 Overview page to make sure you have completed all of the activities listed there before you begin Lesson 3.