Physical Geology lecture - Geologic resources
and economic geology. Answers to questions.
What are major groups of
geologic resources?
- metallic resources; e.g. aluminum, iron, chromium,
gold.
- energy resources.
- ground water resources.
- industrial rock and mineral resource; e.g. limestone
aggregate, gravel, kaolinite.
- soils.
- sites for waste burial.
What are concentration processes
that produce ore anomalies?
- hydrothermal processes: hydrothermal circulation cells,
important factors = rock chemistry, water chemistry, P and T conditions,
flux and time
- sedimentary sorting and placer deposits: panning for
gold as one example.
- intense chemical weathering: aluminum as an important
example
- magmatic differentiation: remember the Bushveld complex
in S. Africa
- many others processes. This forms the basis for the
classification of types of ore deposits.
What are the various types
of geologic energy resources?
- conventional fossil fuels: oil, gas, coal.
- unconventional fossil fuels: tar sands, oil shales,
gashydrates (permafrost and sea bottom).
- geothermal.
- nuclear, isotopes of Uranium as primary fissionable
material.
- tidal
What are the geologic requirements
for formation of oil and gas deposits?
- organic rich source rocks, waxy complex material called
kerogen.
- source rocks need to pass through the thermal window
= right temperatures to cook out the oil and gas and mobilize it. This
is time of oil/gas migration.
- structural and stratigraphic traps. Reservoirs to
hold it.
- the right history of development of these elements.
For example, if a structural trap develops after migration in the area,
then we are out of luck.
How do we explore for oil
and gas?
- drill where oil is leaking out of the ground.
- simple surface extrapolation from geologic mapping
to appropriate traps at depth.
- geophysical techniques to image the interior, especially
seismic refraction (now 3-D).
- conceptual models combined with experience very important.
How much water can the ground
hold?
- porosity - % of void space = % that can be water.
- intergrain porosity:
- function of grain shapes, range in size, and arrangement.
- of a sand typically 30-40%, of a very well cemented
sandstone several percent or less.
- of a clay can be as high as 60%.
- fracture porosity:
- function of fracture sizes and frequencies. Can be
highly variable within a rock body.
What are the physics of
fluid flow through rocks?
- for intergrain porosity - Darcy's law.
How does groundwater connect
to the rest of the hydrologic cycle?
- seepage of water into the ground.
- springs.
- direct exchange between surface water bodies and groundwater.
- trapped as formation waters.
- reintroduced as hydrothermal fluids.
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