GEOL 115: The Earth’s Energy Resources

We finished talking about clastic petroleum reservoirs. We looked at some of the sand bodies that con be produced by turbidity currents (these are the undersea debris flows that we looked at last time).

Carbonate rocks may also make good reservoirs. These are rocks composed of calcium and magnesium carbonate minerals such as calcite and dolomite. Rocks consisting of calcite are called limestones whereas those consisting of dolomite are called dolomite. Some classic examples include the Ghawar(you may need to be at an on-campus computer to see this image) field (and many other Middle Eastern fields) as well as some large oil fields in the central portion of the Permian Basin of West Tx.

Carbonate rocks are chemical or biochemical precipitates. Thus the solubility of calcium carbonate in seawater controls where these rocks are found and they occur where calcium carbonate solubility is minimized. That will be in the warmest portions of the oceans which will be in equatorial regions. We can find carbonate rocks anywhere on the planet; remember that the continents move. So some of the most famous carbonates in the world are found in the Canadian Rockies just west of Calgary AB.

Clastic reservoirs derive almost all their porosity from intergranular porosity (open spaces that occur between individual grains). Carbonate grains may have very delicate structures that contain internal porosity (intraparticle porosity). Carbonate grains and rocks are also very reactive chemically; thus, entire grains may dissolve (moldic porosity) or caves may develop (cavernous porosity). This site has an excellent series of images displaying different types of carbonate porosity.

We then looked at traps. You need to know that there are anticlinal, stratigraphic, fault, and salt dome-related traps. Only anticlinal and stratigraphic traps can produce very large oil fields. Fault traps and salt dome-related traps can be very lucurative business ventures but are unlikely to be giant or super giant oil fields.

Remember that Ghawar (the world’s largest oil field) is an anticlinal trap and that Prudhoe Bay and East Texas (Two of the largest oil fields in the US) are stratigraphic traps.

Finally, we considered seals. Seals are the rocks that prevent oil and gas from migrating up to the surface. They need to be impermeable. Evaporites produced by the evaporation of seawater) make the best seals. Shales (produced by the lithification of mud) are also very good. Everything else can be pretty dicey.