ASBOG Economic Geology and Energy 3 — Questions and Answers
Question 1: Which geologic structure is MOST favorable for trapping oil and gas in a conventional reservoir?
- Syncline adjacent to a normal fault
- Anticline sealed by an impermeable caprock (Correct answer)
- Graben bounded by reverse faults
- Monocline dipping toward a basin center
Correct answer: Anticline sealed by an impermeable caprock
An anticline capped by impermeable rock is the classic structural trap for hydrocarbons, as buoyant fluids migrate upward and accumulate.
Question 2: In petroleum geology, 'source rock' refers to:
- Permeable reservoir rock that stores hydrocarbons
- Fine-grained rock rich in organic matter that generates hydrocarbons upon burial and heating (Correct answer)
- The caprock that prevents upward fluid migration
- The basement rock from which magmatic fluids originate
Correct answer: Fine-grained rock rich in organic matter that generates hydrocarbons upon burial and heating
Source rock is organic-rich fine-grained rock (typically shale) that generates oil and gas when thermally mature.
Question 3: The Bingham Canyon copper mine in Utah is the world's largest man-made excavation and an example of which deposit type?
- Mississippi Valley-type (MVT)
- Porphyry copper (Correct answer)
- SEDEX (sedimentary exhalative)
- Epithermal gold-silver
Correct answer: Porphyry copper
Bingham Canyon is a classic porphyry copper deposit formed by hydrothermal fluids from a Eocene intrusion.
Question 4: Which parameter is used to describe the thermal maturity of organic matter in potential source rocks?
- Vitrinite reflectance (%Ro) (Correct answer)
- Total organic carbon (TOC)
- Hydrogen index (HI)
- Gamma ray API value
Correct answer: Vitrinite reflectance (%Ro)
Vitrinite reflectance (%Ro) measures how much incident light is reflected by vitrinite particles, indicating the degree of thermal maturation.
Question 5: A 'placer deposit' forms primarily through which process?
- Magmatic segregation during cooling of mafic melts
- Mechanical concentration of dense minerals by gravity in stream or beach sediments (Correct answer)
- Chemical precipitation from hydrothermal solutions in veins
- Residual weathering and in-situ concentration of insoluble minerals
Correct answer: Mechanical concentration of dense minerals by gravity in stream or beach sediments
Placer deposits form when dense, resistant minerals (e.g., gold, cassiterite, ilmenite) are concentrated by flowing water or wave action.
Question 6: Mississippi Valley-type (MVT) lead-zinc deposits are typically hosted in:
- Felsic volcanic rocks associated with caldera collapse
- Platform carbonate rocks (limestone and dolomite) in tectonically stable cratons (Correct answer)
- Greenschist-facies metamorphic belts
- Mafic intrusions in Archean greenstone belts
Correct answer: Platform carbonate rocks (limestone and dolomite) in tectonically stable cratons
MVT deposits form in platform carbonate sequences when basinal brines migrate into structural or diagenetic traps in stable cratonic settings.
Question 7: Coalbed methane (CBM) differs from conventional natural gas reservoirs primarily because:
- It contains more CO2 than methane
- The gas is adsorbed onto coal matrix rather than occupying pore space (Correct answer)
- It requires strip mining rather than drilling to produce
- It only occurs at depths greater than 5,000 feet
Correct answer: The gas is adsorbed onto coal matrix rather than occupying pore space
CBM gas is stored by adsorption onto coal macerals rather than as free gas in pores, requiring dewatering to reduce pressure before gas desorbs.
ASBOG Economic Geology and Energy 3
Which geologic structure is MOST favorable for trapping oil and gas in a conventional reservoir?