RE: ..8 Feb 2019 20:44
BlackHopper this is the best explanation I found on the subject!
The Hugoton gas field has a near uniform stable isotopic composition of C1-C3 hydrocarbons. In contrast to earlier reports, we find these to be consistent with oil-associated gases produced from a moderately mature, marine source rock with a vitrinite reflectance of approximately 0.8%. There is no observed correlation between the hydrocarbon phase and the noble gases.
The mantle He/N2 and groundwater-recharge Ar/N2 ratios enable us to rule out significant magmatic or atmosphere con- tributions to the gas field N2, which is dominantly crustal in origin. We have used a combination of noble gas and stable isotopes to identify two crustal nitrogen components in the Hugoton-Panhandle gas field. One N2 component accounts for up to 60% of the N2 in the Texas section of the field, dropping to as little as 25% in the Kansas/Oklahoma section. This “He-associated” N2 dominantly originates from the devolatilization of low-grade metamorphic rocks and is transported and focused into the Texas Panhandle portion of the field dissolved in the regional groundwater system.
The hydrocarbons and related gases were already in place, and provide the mechanism with which to quantitatively partition the dissolved gas species into the reservoir gas phase. Magmatic He is also identified within the reservoir in very high concentrations and from reasonably uniform 3He/ 4He is therefore linked with the He source. A combination of magmatic 3He-source requirement and 4He mass balance limit the source of the 4He-, 3He-, and He- associated N2 to the great plains W-E flowing groundwater system to the west of the Hugoton-Panhandle gas field.
The second N2 component accounts for between 40% of the N2 in the Texas Panhandle and as much as 75% of the N2 in the northernmost Kansas-Hugoton. This N2 component is characterised by ??15NN
4 = +13‰, but is associated with neither resolvable 4He nor 20Ne. The isotopic composition of this N2 is consistent with denitrification of a relatively mature marine source rock. The location of the highest concentrations of this nitrogen, both isolated from the regional groundwater system and furthest away from the hydrocarbon producing areas, sug- gests that formation of this nitrogen component predates the dominant hydrocarbon filling phase.
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