Diagenesis P-T Space

The transition from alkali zeolite and analcite to albite-bearing rocks appears to occur near 180°C at depths of several hundred meters and 120°C at 5 Km. This information is known only at two P-T points and therefore might be considered as open to question to a certain extent, especially in the zone between the two data points. 

The production of significant quantities of natural gas plus a petroleum-like substance from organic material in shales apparently occurs between 60 and 120°C. This, as we have seen, could as well be a function of time as of depth given the data at hand. The reason is that all hydrocarbons above CH weight are inherently unstable in a thermodynamic sense, even at 25°C and 1 atmosphere (Thorstenson, 1970). However, if depth of burial and time elapsed since deposition can be considered as one variable (basin subsidence therefore must occur at similar rates for similar geothermal gradients) a depth-temperature plot will be useful. 

as has been previously stated, the shale layers become compacted to near 0% porosite at lKm depth, the petroleum substances will use the released constitutional water held in the montmorillonites in order to migrate to any great extent. This does not normally occur until 

90-30% expanding illite-montmorillonite mixtures All = 30% mixed layered mineral with ordering reflection (allevardite-type structure) I = illite Chi = chlorite Kaol = kaolinite Py = pyrophyllite Q = quartz Anal = analcite Ab = albite. 

In zone II, that of normal mixed layered dioctahedral minerals, there are few characteristic mineral reactions. However, the change of the interstratified material as it becomes allevardite-type mineral, i.e., showing a discrete super-lattice reflection, is undoubtedly complex. 

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