Home - Syllabus - Seminar Section

U4735x Environmental Science for Decision Makers

Lectures 12 and 13: Origin of Hydrocarbons.

Walter Pitman

List of Projections:

 

Decomposition of Organic Matter in the Oceans.

Diagrammatic representation of the energy budget of the earth.

Photosynthesis and respiration.

Over 99% of the organic matter produced as and by living organisms is broken down by oxidation (respiration).

CH2O + O2 --> CO2 + H2O + energy

NH3 + 2O2 --> H+ + NO3- + H2O

Organic matter may be oxidized as it sinks through the water.

On the bottom, sulphate reducing and denitrifying bacteria maybe decompose the organic matter without oxygen.

Phytoplankton are the base of the food chain in the ocean. They are photosynthetic and live in the photic zone using sunlight, water, CO2 and raw nutrients which have been ultimately derived from the land.

Phytoplanktonic algae are the most important of the organic matter which gives rise to petroleum:

  1. diatoms.
  2. dinoflagellates.

Zooplankton feed on the phytoplankton (and on each other). They provide the (lipid rich) material from which petroleum is derived:

  1. radiolaria.
  2. foraminifera.
  3. pteropods.

Four main settings for anoxic environments favorable for preservation of organic matter in sediments.

Preservation is enhanced by:

  1. reduction in bioturbation.
  2. reduction in oxygen content of the water.
  3. increased sedimentation rate. (A very high sedimentation rate will dilute the organic content.)

Microbial breakdown is due to:

  1. bacteria.
  2. fungi.
  3. protozoa.

Oxygen may penetrate into the sediment to depths of 5-20 cm.

Formation of Hydrocarbons and Hydrocarbon Reservoirs/Traps.

  1. Diagenesis.

    Biogenic decomposition produces biogenic methane. At slightly higher temperatures and pressures the organic matter is converted to kerogen - an amorphous material of carbon, hydrogen, and oxygen.

  2. Catagenesis.

    At higher temperatures and pressures kerogen is altered and the majority of crude oil is formed. During this phase and the next, the larger molecules break down into simpler molecules (a process called cracking).

  3. Metagenesis.

    In the final stage of alteration (at higher temperatures and pressures) of kerogen and crude oil, natural gas (mostly methane) is produced and residual carbon is left in the source rock.

Products of the maturation of organic matter.

Petroleum fractions.

Temperature of the maturation stages and relative amounts of crude oil, wet gas and dry gas produced.

Correlation between hydrocarbon generation, temperature, and some paleothermometers.

Model showing possible relationship between time after burial of source rocks and the temperature for oil and gas formation.

Burial curve for the Central Graben of the North Sea.

Some Definitions:

Source rocks are mainly organic rich shales.

Primary migration - within and out of the source rock.

Secondary migration - to and within the rock where the oil and gas accumulate - called a reservoir rock.

Oil and gas must flow from the source rock (often assisted by pressure) through some permeable conduit or pathway to a reservoir which must consist of relatively porous and permeable rock.

The reservoir must be covered or sealed by a caprock, which is a rock layer of very low permeability, which prevents the oil and/or gas from migrating further upward and escaping.

Good caprocks:

  1. evaporites (salt).
  2. well compacted shale with perhaps carbonate cement.

Primary migration of petroleum out of source rocks, followed by secondary migration into reservoir rocks.

Summary of Requirements for Trap Formation:

  1. Suitable source rock must have been subjected to the maturation process.
  2. Primary migration possible.
  3. Suitable reservoir rock must exist.
  4. Secondary migration from source rocks into reservoir rocks possible.
  5. Large enough potential accumulation.
  6. Reservoir sealed.

Common geologic structures that trap oil and gas in the subsurface.

Migration of oil and gas into and out of an anticlinal trap.

Sketch of seismic lines across Goshute Valley, northeast Nevada.

Sketch showing illustration strike-slip faulting, listric normal faulting, and crustal attenuation.

Baltimore Canyon.

Western Canada Basin.

Cross section across Central Appalachian Basin.

Tertiary Tectonics - Western Cordillera.

Basin types of the world.

Coal.

Diagrammatic representation of the energy budget of the earth.

  1. Land plants first appeared 400my ago in Devonian.
  2. First rapid development of flora - 345 my ago.
  3. Gave rise to the first vast deposits of coal in the carboniferous.

Coal is fossil plant matter (mostly land plants)

It does not migrate as oil and gas do. Under a microscope it is often seen to consist of altered vegetative matter: wood, bark, leaves, roots, etc.

The place of accumulation is almost always swamps, because in swamps the dead plant matter may be submerged under water in a non-oxidizing environment.

In coastal swamps, lagoons, deltas, etc. there is massive production of organic matter in the form of plants and trees.

The residue of these forms of peat which, when more deeply buried, may be transformed to lignite and bituminous coal. A small fraction may form oil and gas.

Features of the Mississippi Delta.

Coal Formation.

  1. Growth of plant matter in a swamp.

  2. Submersion of this dead plant matter underwater without oxygen.

  3. Partial decomposition by anaerobic bacteria giving off hydrogen and oxygen in the combined form of various gases. The carbon becomes increasingly concentrated.

  4. The decomposition is terminated when the anaerobic bacteria are poisoned by the very acid compounds they liberate.

  5. As the peat is progressively buried to deeper levels and compressed and heated, volatile compounds continue to escape and the carbon concentration increases.

  6. The peat is converted successively, ie.

    lignite --> sub-bituminous coal --> bituminous coal.

Cyclic sequence in a Coal Measures deposit.

Fomation of coal from peat.

Sub-bituminous coal and bituminous coal are sedimentary rocks. The next phase - anthricite - is a metaphoric rock created from the above by the pressure and heat of tectonic processes.

Anthricite is hard to ignite but burns without smoke.

The other coals - bituminous, sub-bituminous, lignite, and peat - are increasingly easy to light but increasingly smoky.

Conversion of kerogen to oil, gas, and coal as a function of temperature.

Definitions:

Metamorphic rocks:

Any rock derived from pre-existing rocks by mineralogical, chemical, and structural changes, essentially in the solid state, in response to marked changes in temperature, pressure, shearing, stress and chemical environment at depth in the earth's crust, ie below the zones of weathering and cementation.

Metamorphism:

The mineralized and structural adjustment of solid rocks to physical and chemical conditions which have been imposed at depth below the surface zones of weathering and cementation which differ from conditions under which the rocks originated.

Plot of temperature vs heating time.

Proportions of constituents in various grades of coal.

World map showing the distribution of known coal deposits.

Heavy Oil and Tar Sands.

Bitumen:

a black viscous hydrocarbon material found where oil has lost its volatile light weight components through exposure to air:

  1. dark in color,
  2. quite viscous,
  3. high in sulfur (3-6%), nickel, and vanadium,
  4. rich in asphaltine.

They are residuals after oxidation and/or evaporation of the higher fraction.

Oil Shales:

Diverse group of fine grained rocks that contain significant amounts of kerogen. May be converted to oil at ~500°C.

Global distribution of major tar-sand deposits and location and size of heavy oil deposits in Alberta, Canada.

Table of shale oil resources of the world.

Map of oil shale deposits in the United States.

Table of heavy oils and tar sands per country.