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GEOS 251 Physical Geology Spring 2012

Energy Resources from the Earth(Chapter 23)

Overview Earth resources are of many types. Beyond water and air (and the biological realm), yet

central to civilization are materials that we use for fuel, for making things, for growing things These constitute energy resources (this lecture) and mineral resources (next lecture)

Ultimate sources of energy (just like for all Earth processes) have two origins: in the Earthsinternal energy (original heat / radioactive elements generated in supernovae / light elements(H, Li) that could support fusion) and from external (solar) sources

World energy consumption has increased exponentially in the last 200 years, even faster thanthe exponential increase in population this is obviously a reflection of industrialization,mechanization of many processes, and the ubiquity of powered transportation

During this period, energy sources have moved from renewable resources (those that

continuously regenerate on a human time scale animal / wood / wind / hydro) to a mixthat is dominated by non-renewable resources (these may continue to form, but only ongeologic time scales coal, petroleum, natural gas, uranium, geothermal)

These resources are governed by geological principles many of which have been coveredin earlier lectures. Here we focus on fossil fuels, their distribution and origins (why are theyso irregularly distributed in time and space?); other types (nuclear fuels, geothermal, andhydro power) are considered briefly relevant background for them is in other lectures

Fossil fuels Fossil fuels are the product of trapping a small fraction of the suns energy in biological

(organic) material in the sedimentary record these are simple accumulations in the caseof coal, much more complicated in the case of petroleum and natural gas

Resources economic vs. subeconomic, discovered vs. hypothetical are important conceptsand underlie much debate about future supplies, land use, geopolitical concerns

Coal >> oil (liquid ~ oil shale) >(?) natural gas (but all less than uranium)

Hydrocarbons Petroleum/Natural Gas ( Hydrocarbons because they are comprised of many compounds that consist mainly of H andC)

Petroleum (oil) and natural gas (mainly methane, CH 4) form from the burial and heating of organic-bearing marine and lake sediments during burial, trapped organic material breaksdown (effectively, it cooks) to give up the lighter, more hydrogen-rich components (oilsand gases), leaving behind the less reactive material (analogous to grilling a burger liquidfraction comes off, vapors are released, and the residual material becomes closer to graphite(okay, not so appetizing)

Required sedimentary geological environment can be deltas, reefs, deeper basin fills, etc.,in areas with

(1) abundant organic debris (generally planktonic forms),(2) preservation and burial of this in the sediments,

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(3) modification by biological activity (near surface), and by(4) diagenesis (heat, pressure, time) to generate oil and gas (generally 75-150 C)

Oil and gas distributed through these organic-bearing source rocks are generally not of economic interest, thus migration of fluid hydrocarbons (similar to movement of other fluidsthat are lower density than their surrounding / cf., magmas) to areas of accumulation

(geologic traps, such as structures, stratigraphic traps, and salt domes) The key elements are thus: a source rock, a process to generate the fluids, a drive for

migration, and an appropriate reservoir rock (one that is porous) where the fluids can betrapped by a much less permeable cap rock

Oil (tar) sands, oil shales, and gas clathrates (methane ices on continental slope and rises)represent enormous resources of hydrocarbons [Canada producing; US has large resources]

Coal Coal is a rock consisting of mainly woody material that accumulated in swamps and bogs;

burial by continued sedimentation leads to diagenetic and metamorphic changes increasingthe rank (or grade) of the coal in contrast to oil and gas, it is the residual (cf. burger)part of the coal that is preserved coal is thus much higher in C but lower in H than others

Geologic environment:(1) abundant terrestrial plant matter in swamps or bogs can be estuaries, bogs, swamps

(this setting forms sulfide minerals such as pyrite, which present an environmental challenge)(2) near marine setting (cf. SE US coast) or it can be interior; temperate or tropical basins(3) the key requirement is preservation by burial, followed by:(4) diagenesis (heat, pressure, time) to compact and drive off water and gas

Types of coal: There is a progression from unmodified (perhaps partly decayed) woodymaterial through peat to lignite (brown coal) to bituminous (soft coal) to anthracite (hardcoal)

Distribution of fossil fuels in time and space Fossil fuels have a highly irregular distribution

Nearly all are Phanerozoic, most oil is Mesozoic or younger Coals reflect the evolution of wood land plants, but are pretty widespread overall Oil is more of a special case: although it is widespread, it is very easily destroyed (or nottrapped) in active continental margins; furthermore, high biological productivity (and thustraps) are uncommon (even in the modern world) thus, the concentration of oil in certainareas (e.g., the Middle East, around the Gulf of Mexico) is the result of favorable initialgeology and the lack (thus far) of active tectonics to destroy the oil fields

Other primaryenergysourcesand environmentalissues Geothermal localized in space, considerable engineering issues; possible byproducts Nuclear (U, Th, Li, etc.) fundamentally mineral resources, can be enhanced by

engineering far more potential than exploited Solar / wind energy / hydropower large, but have material and land use issues


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Many geology-based environmental issues arise with energy production and use: theseinclude land use / competitive pressures, consequences of strip mining, acid mine water, oilspills, greenhouse gases, air-water pollution, waste disposal

Technology of energy utilization has similar trade-offs: For example, what is required forthe hydrogen economy so-much promoted over the last few years? (Where does energy

come from? What is required to transform it into useful forms and get it to areas where it isused?)


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