HYDROCARBON ORIGINS AND OCCURRENCES

Hydrocarbon generation is the natural result of the maturation of buried organic matter.

Organic matter (organic carbon) in sediments underlying the oceans is derived from different sources (Emeis and Kvenvolden, 1986), including the following:

  1. Marine phytoplankton,
  2. Phytobentos in shallow water with sufficient light,
  3. Bacteria, and
  4. Allochtonous (i.e., land derived) material.

The organic carbon produced in the water column varies from ~0.1% to 5%, depending on various factors such as the following:

  1. Oxygen depletion in bottom waters or in sediment as a result of high organic input,
  2. Adsorption of certain compounds to mineral particles,
  3. Preservation of organic compounds as shell constituents,
  4. Changes in the rate of deposition of sediment organic matter,
  5. High input of terrigenous organic compounds, which are more stable than organic matter, and
  6. Dominant input of argillaceous sediments where oxygenation of pore water is restricted.

Organic matter undergoes changes in composition with increasing burial depth and temperature. The three steps in the transformation of organic matter to petroleum hydrocarbons are termed diagenesis, catagenesis, and metagenesis. The general scheme of evolution of the organic fraction and the hydrocarbons produced is depicted in Figure F1 (Tissot and Welte, 1984). Petroleum hydrocarbons exist as gaseous, liquid, and solid phases, depending on temperature, pressure, burial time, and composition of the system.

C1-C4 Hydrocarbons

C1-C4 hydrocarbons (methane, ethane, propane, and butane) are found predominantly in the gaseous phase at surface conditions. These hydrocarbon gases, largely methane (C1), may be generated in significant quantities in sediment, either under near-surface conditions by bacterial action (Claypool and Kaplan, 1974) or at greater depths by thermochemical action (Schoell, 1988).

Biogenic gas (microbial methane) is produced in sulfate-depleted marine sediment where accumulation rates exceed ~50 m/m.y. and organic matter is preserved. Microbial methane production occurs by reduction of dissolved bicarbonate (CO2), and the process competes with sulfate reduction for electrons (hydrogen) generated by the anaerobic oxidation of organic matter. At sedimentation rates slower than ~50 m/m.y., sulfate is continually replenished by diffusion from overlying seawater until metabolizable organic matter is completely consumed, leaving none for methane generation.

Thermogenic gases (C1-C4) are produced in sediments at rates that are proportional to temperature. In most ODP holes with normal geothermal gradients (20-50C/km), sediment temperatures are insufficient to produce more than trace amounts of thermogenic gases. High concentrations of thermogenic gases in sediments at shallow depths and low temperatures generally indicate the existence of a hydrocarbon migration pathway. However, it is becoming increasingly recognized that the C2-C4 gases can be produced bacterially along with C1, although not in high concentrations (Vogel et al., 1982; Wiesenburg et al., 1985; Oremland et al., 1988; Feary, Hine, Malone et al., 2000). Either biogenic or thermogenic gas can be hazardous. Either can cause a blowout and catch fire. Biogenic and thermogenic gases usually (but not always) can be distinguished on the basis of chemical and carbon isotopic composition (not available on the JOIDES Resolution). However, it is amount of the gas and the possibility of high-pressure accumulation that poses the hazard, not the mechanism of origin.

C5 and Heavier Hydrocarbons

C5 and heavier hydrocarbons (oil), predominantly liquid, are almost exclusively the product of thermal generation from hydrogen-rich organic matter in deeply buried sediments (oil of microbial origin is unknown). This generation occurs at rates that become quantitatively important only as temperatures reach 90-150C (typically at burial depths of 2500-5000 m for average geothermal gradients). Hydrocarbon gases are generated with the oil, and although they consist largely of methane, they usually also include heavier hydrocarbons. Thermogenic conversion of organic matter to hydrocarbons continues at accelerating rates with increasing depth and temperature until all organic matter, including the oil itself, has been converted largely to methane and carbon-rich residues (Ocean Drilling Program, 1992).

The sources of organic matter can sometimes be inferred on board the JOIDES Resolution from results of geochemical analyses, of which Rock Eval pyrolysis, petrographic investigation of kerogens, and gas chromatography/mass spectrometry (GC/MSD) of solvent extracts are examples (see "Hydrocarbon Monitoring Procedures").

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