Geology 111G                                                                        25 April 2002

Lecture 24.  Energy and Mineral Resources

 

Energy Resources

Coal

Petroleum

            Four Conditions in Origin

            Plate Tectonics and Petroleum

Other Sources of Fossil Fuel

Alternative Energy Sources

Mineral Deposits

Metallic

Non-metallic

Ore Deposits

Strategic Mineral

Geologic Processes that Produce Mineral Deposits

Plate Tectonics and Mineral Resources

 

 

I.  Energy Resources:  Commodities that occur unevenly distributed in crust, where concentration resulted from favorable geologic conditions.  Examples are locations of ancient deltas, which were sites for peat accumulation.

A.  Renewable Resources: 

B.  Non-renewable resources:  Resources not replenished on a human time scale. The way to extend the period over which we can use these is to find more or use them more efficiently (conserve them).

 

II.  Coal.  Formed in depositional environments where abundant organic debris formed by woody plants accumulates.

A.  Initial accumulation:  Organic material free of sand and mud, called peat , is deposited.

B.  Progressive Burial:  accompanied by increased temperature drives off hydrogen and oxygen, leaving a residue increasingly enriched in carbon.

1.  Rank:  Increase in percent carbon.

a.  Lignite.  About 70% carbon.

b.  Bituminous.  About 80% carbon.

c.  Anthracite.  About 80-95% carbon.

2.  Coal occurs naturally in beds or seams on the order of a few meters thick.  They are usually interbedded with sandstone and shale.

a.  Used as solid fuel or converted to methane (CH4) via gasification.

b.  One of the minor elements in coal inherited from its precursor plants is sulfur, which combines with oxygen during combustion to form sulfur dioxide (SO2).  This in turn combines with rainwater to form H2SO4, sulfuric acid, hence the term acid rain.

 

III.  Petroleum.  Collective term for oil and natural gas.  These are naturally occuring liquid and gaseous hydrocarbons typically associated with sedimentary rocks of marine origin.  They are primarily mixures of hydrogen and carbon with other minor elements (O, S, N).

            A.  Origin of hydrocarbons is a 4-step process.

1.  Deposition of organic rich source rock.  Typically a marine shale rich in organic carbon derived from micro-organisms that lived in water column.  These accumulate in bottom mud.

2.  Maturation of source rock.  Exposure to the sedimentary rock to heat during burial results in conversion of organic carbon to liquid hydrocarbons (60-115º C) and natural gas (115-200º C).  Above 200º all movable hydrocarbon is driven from source rock , which is then termed overmature.

3.  Migration of hydrocarbons to a suitable reservoir.

4.  Accumulation of hydrocarbons in a trap, which is a geologic condition that consists of a reservoir and a seal. 

5.  Primary job of a petroleum geologist is to identify traps, and to evaluate them in terms of the other three factors.

 

 

IV.  Other sources of fossil fuel.

A.  Oil Shale.  Fine-grained organic rich lake deposits in which hydrocarbons are dispersed throughout rock.  Shale must be heated or retorted to extract HC.

B.  Tar sands.  High-viscosity HC fills pore spaces of shallowly buried sandstone.  Extraction is a technical problem, which generally involves steam injection. Canada, Venezuela, Russia.

 

 

V.  Renewable alternatives.

A.  Hydropower.      

B.  Nuclear power: 

C.  Geothermal power.  Hot groundwater used to turn turbines.

D.  Wind.

E.  Solar energy.

 

VI.  Mineral deposits:  Typically classified as either metallic or non-metallic

A.  Metallic:  mined for metals such as iron, aluminum, uranium, gold

B.  Non-metallic:  Salt, building stone, sand/gravel, sulfur.

C.  Ore deposit:  mineral deposits that can be worked economically , or at a profit.  This is determined not only by the element's concentration in the deposit, but also by technology and politics.

1.  Copper is an example.  In the late 1970's, the price of copper fell from about $1/lb to 75¢.  Right now it is about 68¢.  The 1970's collapse was caused by a couple of factors which lead to the closure of some mines in the Southwestern United States:

a.  A proximal political factor:  Huge deposits in Chile and Zimbabwe came on line.  Very cheap labor.

b.  Long-term technological factor:  plastic  (PVC) had largely replaced copper in plumbing.

 

VII.  Strategic Mineral.  A commodity that is vital to the security and well being of the country but is in short supply within the national boundaries.  Access to these commodities could be drastically reduced as a result of embargo or war.  Tin, manganese, tungsten.

A.  Initial accumulation:  Organic material free of sand and mud, called peat , is deposited.

B.  Solutions:

1.  Devise a substitute.

2.  Search intensively for the mineral in national boundaries.

3.  Stockpile the commodity when price is low.

4.  Nonpeaceful acquisition.

 

VIII.  Geologic Processes and Mineral deposits.  Several types of processes concentrate mineral deposits that may ultimately become ore deposits.

A.  Magmatic processes:  settling of heavy mineral grains in magma chambers may concentrate useful elements.  Platinum and chromium are examples.

B.  Hydrothermal processes:  Fluids circulating within and beyond cooing plutons frequently carry metal ions that are deposited in the country rock or within the pluton itself.  Copper,  lead, zinc minerals  and gold and silver commonly deposited by these fluids in veins that fill cracks in country rocks.  Southern Arizona and southwest New Mexico (Gadsden Purchase).

C.  Depositional processes.  Some minerals are concentrated in sedimentary rocks and sediments.

1.  Banded iron formations of the Lake Superior region consist of interbedded chert and hematite deposited by Precambrian seas.

2.  Evaporite deposits.  Salt, potassium salt (sylvite, used in fertilizer), gypsum.

3.  Placers: Concentrations of heavy mineral elements deposited by wave and water action.  Diamonds on beaches of SW Africa and gold in the foothill streams of California.

D.  Weathering Processes:  leaching of certain elements from the soile zone may leave behind a residuum of important minerals.  Bauxite is aluminum oxide concentrated in tropical soils.

 

IX.  Plate Tectonics and Mineral Deposits. An understanding of plate tectonics may aid in the search from mineral deposits.  Example is porphyry copper deposits of SW US.

A.  Observations.  Formed by hydrothermal systems associated with plutons with ages ranging 70-50 Ma.

B.  Hypothesis.  Plutons are subduction-related.

C.  Test.  Distribution seems to confirm the relationship with late Cretaceous-early Tertiary Subduction system.