ASBOG - Association of State Boards of Geology Economic Geology and Energy Questions and Answers — Questions and Answers
Question 1: An exploration geologist discovers a large, stratabound ore body hosted within a Paleozoic dolomite reef complex. The primary ore minerals are sphalerite and galena, with minor fluorite and barite. Fluid inclusion studies indicate formation temperatures of 100-150°C from highly saline fluids. This ore deposit is best classified as which of the following types?
- Volcanogenic Massive Sulfide (VMS)
- Porphyry Copper
- Mississippi Valley Type (MVT) (Correct answer)
- Sedimentary Exhalative (SEDEX)
Correct answer: Mississippi Valley Type (MVT)
Mississippi Valley Type (MVT) deposits are characterized by their epigenetic, carbonate-hosted (limestone/dolomite) nature. They are typically dominated by lead (galena) and zinc (sphalerite) and form at relatively low temperatures (typically 90-200°C) from migrating basinal brines, far from any igneous activity. The features described in the scenario perfectly match the MVT model.
Question 2: Which of the following lists the five essential elements required for the formation and preservation of a conventional hydrocarbon accumulation?
- Kerogen, Porosity, Permeability, Anticline, and Overburden
- Shale, Sandstone, Salt dome, Oil window, and Basement rock
- Organic matter, Heat, Pressure, Time, and a Basin
- Source rock, Reservoir rock, Seal, Trap, and Migration (Correct answer)
Correct answer: Source rock, Reservoir rock, Seal, Trap, and Migration
A viable petroleum system requires all five components: a source rock with sufficient organic matter, a migration pathway for hydrocarbons to move, a reservoir rock with porosity and permeability to store them, a seal or cap rock to prevent their escape, and a trap to concentrate them. The other options list related concepts or examples, but not the complete set of five fundamental elements.
Question 3: A geologist is logging core from a porphyry copper system and observes a central zone rich in secondary biotite and K-feldspar. This is immediately surrounded by an extensive zone of quartz-sericite-pyrite alteration. What are the correct names for these two alteration zones, from the core outwards?
- Propylitic, Argillic
- Phyllic, Potassic
- Potassic, Phyllic (Correct answer)
- Skarn, Greisen
Correct answer: Potassic, Phyllic
Porphyry copper deposits have characteristic, large-scale alteration zoning. The core is typically the highest temperature potassic zone, characterized by secondary K-feldspar and biotite. This is commonly surrounded and overprinted by the phyllic (or quartz-sericite-pyrite) zone. Further out are the argillic and propylitic zones.
Question 4: A seismic survey reveals a potential hydrocarbon accumulation where a porous sandstone reservoir pinches out updip, becoming encased in impermeable marine shale. There is no evidence of faulting or folding in the immediate area. What type of trapping mechanism does this describe?
- Salt dome trap
- Anticlinal trap
- Fault trap
- Stratigraphic trap (Correct answer)
Correct answer: Stratigraphic trap
This scenario describes a stratigraphic trap. These traps are formed by changes in lithology or depositional features, such as a reservoir rock terminating, or 'pinching out', against a sealing rock. This is distinct from structural traps, which are formed by deformation like folding (anticlines) or faulting.
Question 5: Which of the following correctly lists the ranks of coal in order of increasing heat content, carbon content, and metamorphic grade?
- Bituminous, Anthracite, Lignite, Sub-bituminous
- Lignite, Anthracite, Sub-bituminous, Bituminous
- Anthracite, Bituminous, Sub-bituminous, Lignite
- Lignite, Sub-bituminous, Bituminous, Anthracite (Correct answer)
Correct answer: Lignite, Sub-bituminous, Bituminous, Anthracite
Coalification is the process where peat is converted into coal through burial, increased pressure, and temperature. The sequence represents increasing metamorphic grade, carbon content, and energy content. It begins with Lignite (lowest rank), then Sub-bituminous, then Bituminous, and finally Anthracite (highest rank).
Question 6: Roll-front uranium deposits are a significant type of sandstone-hosted uranium deposit. What is the fundamental geochemical process responsible for their formation and crescent shape?
- Gravitational settling of dense uraninite crystals in a stream channel.
- Precipitation at the interface between oxidizing and reducing groundwater. (Correct answer)
- Intrusion of a uranium-rich pegmatite into the sandstone host rock.
- Hydrothermal precipitation from high-temperature magmatic fluids.
Correct answer: Precipitation at the interface between oxidizing and reducing groundwater.
Roll-front uranium deposits form when oxidizing groundwater, carrying dissolved uranium in its soluble U6+ state, migrates through a permeable sandstone. When this oxidizing water encounters a reducing barrier (often caused by organic matter or sulfide minerals), the uranium is reduced to its insoluble U4+ state and precipitates as uraninite (UO2), forming a crescent-shaped ore body at the redox interface.
