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Nitrogen gas is commonly found in natural gas accumulations within Cretaceous and younger rocks in selected areas of the Sacramento Valley. Considerations of a variety of data and concepts suggest that the nitrogen originates independently from and earlier than the methane with which the nitrogen is now commingled and that it does not originate within the sediments in which the natural gas accumulations are found today. The proposed answer is that the Sacramento Valley nitrogen originates from low-grade metamorphism of sedimentary rocks containing organic matter. By elimination, the enigmatic Franciscan rocks of the Coast Range province appear to be the most probable source of this nitrogen. Abnormally high fluid pressures also exist within the Cretaceous sediments of the Sacramento Valley and may play a critical role in the origin of the methane within this dry-gas province. The existing fluid-potential distribution strongly suggests that the abnormally high-fluid potentials are the result of tectonic compaction--stemming from continuous uplift of the Coast Ranges at least from late Tertiary into Recent time.
The general fluid-potential distribution within the Sacramento Valley is such that vertically upward flow is commonly present. Water flowing upward through shales serving as methane sources would contain in solution different quantities of methane per unit volume of water depending upon the fluid pressures. The high solubilities of simple paraffin hydrocarbons in water as opposed to those of more complicated hydrocarbons and the exponential variation of these solubilities with pressure provide a mechanism for selectively transporting in aqueous solution essentially only simple paraffins--particularly methane--from a shale source at high pressure and discharging them as free gas at lower pressures in a reservoir rock. The operation of such a mechanism would be dependent upon significan vertical fluid-potential differences and would be independent of the age and degree of compaction of the shale. The methane for the Sacramento Valley dry-gas province thus may have evolved and accumulated fairly recently--subsequent to the presumed initiation of the regional high fluid potentials in late Tertiary time.
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