N
- PDF / 5,595,052 Bytes
- 24 Pages / 595.276 x 841.89 pts (A4) Page_size
- 63 Downloads / 163 Views
NATURAL CAS Sedimentary accumulations of gas contain a limited number of compounds: saturated light hydrocarbons from methane to pentane, C0 2 , H 2 S, N 2 (Durand, 1993) and traces of noble gases, mercury and hydrogen (Sokolov, 1974). All of these gases, except mercury and the noble gases, may come from organic matter, either from bacterial activity, or by thermal degradation. Inert gases may also have mineral origins: N 2 from ammonium of shales or from the deeper crust, H 2 S from thermo-reduction of sulfates (with hydrocarbon as a reducing agent), and C0 2 from thermal alteration of carbonates or mantle contamination. Nevertheless, they are clearly identified in some fields as generated from organic matter.
Hydrocarbon gas generation and accumulation Part of the organic matter buried during sedimentation undergoes several transformations. In the shallower sediments, organic diagenesis induces biochemical changes, transforming the organic matter into kerogen, with the possible production of C0 2 and 12 C-rich methane. Two distinct paths may produce bacterial methane, through acetate fermentation or through C0 2 reduction (Whiticar et al., 1986). Deeper in the sedimentary column, the kerogen will undergo thermal cracking, controlled mainly by kinetic chemical reactions, producing a first generation of gas associated with oil and a later generation of gas from the residual kerogen. Another source of gas may result from the secondary cracking of oil, due to an increase of thermal stress, or a longer duration of cracking. Most gases migrate from their source to a reservoir, and often suffer partial leakage through the seal of the reservoir, bacterial oxidation and other reactions. Each of these post-genetic processes may in turn induce substantial fractionation between the generated and accumulated gas. Gas in the reservoir may be oil associated, or present as a single gas phase, depending on its genetic and migration history. Some unconventional gas deposits may however be found directly within the source, such as coal bed methane, where the gas is adsorbed on coal layers from where it originated.
During gas generation, methane is progressively enriched with increasing maturation. Some ratios, such as the ethane/ propane ratio, or the iso/normal butane ratio, stay at a constant value during primary generation, and increase dramatically during secondary cracking. The carbon isotopic ratios vary widely in the course of gas history, and are used to better characterize such parameters as the transformation ratio, the extent of secondary cracking, the quality of the accumulation, and the amount of gas lost by leakage or oxidation (Galimov, 1985).
Inert gases: organic generation N 2 : Although the total amount of nitrogen in a kerogen is quite low, cracking at extremely severe thermal stress may generate large quantities of molecular nitrogen (Krooss et al., 1995). A segregative migration from a deep overmature source rock may then feed almost pure nitrogen into some reservoirs. This late nitrogen genesis is also asso
