Application of incident light microscopy techniques for organic petrology in
high temperature thermogenic shale gas systems demonstrates that solid bitumen is the
dominant organic matter. Solid bitumen is retained as a residual conversion product as
oil-prone kerogen cracks to hydrocarbons or occurs from the cracking of once liquid
oil. Oil-prone Type I/II kerogens are not present in shale gas reservoirs, already having
converted to hydrocarbons. Type III/IV kerogens (vitrinite and inertinite) are refractory
and persist in shale gas reservoirs to high maturity with little morphological change
apart from condensation and aromatization causing higher reflectance. Organic
petrology applications are most useful for thermal maturity determination and
delineation of hydrocarbon windows through measurement of vitrinite reflectance and
vitrinite reflectance equivalents from other organic matter (zooclasts and/or solid
bitumen). Depositional organo-facies determination generally is not possible in the gas
window of thermal maturity; fluorescence microscopy is not useful as organic matter is
no longer autofluorescent. Application of scanning electron microscopy (SEM) allows
observation of an interconnected nano-scale organic porosity in shale gas systems but
suffers from inability to identify organic matter types. SEM approaches to shale gas
reservoir characterization therefore should not attempt differentiation of kerogen types
or kerogen vs. solid bitumen identification unless correlative organic microscopy is
performed. Herein are reviewed organic petrology results as used in the shale gas
systems of North America, Europe and China, including SEM applications, citing
recent examples from the literature.
Keywords: Gas window, Organic petrology, Shale gas, Solid bitumen, Thermal
maturity, Vitrinite reflectance
