Samples examined thus far can be divided into two broad groups. One group is the higher thermal maturity samples, those currently buried in the range of 1,500 to 2,200 m and having vitrinite reflectances of 1.1% to 2.0%. The other category is relatively low thermal maturity samples, those currently buried less than 500 m. Vitrinite reflectance is less well constrained for these samples, but given regional trends, it should be less than 0.7% and possibly less than 0.5%.
More mature samples show well-developed nanopores concentrated in micron-scale carbonaceous grains. Large numbers of subelliptical to rectangular nanopores are present, and porosities within individual grains of as much as 20% have been observed. Shallowly buried, lower thermal maturity samples, in contrast, show few or no pores within carbonaceous grains.
These observations are consistent with decomposition of organic matter during hydrocarbon maturation being responsible for the intragranular nanopores found in carbonaceous grains of higher maturity samples. As organic matter (kerogen) is converted to hydrocarbons, nanopores are created to contain the liquids and gases. With continued thermal maturation, pores grow and may form into networks. The specific thermal maturity level at which nanopore development begins has not been determined. However, current observations support nanopore formation being tied to the onset of conversion of kerogen to hydrocarbons.
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