Impact of Inorganically Bound Sulfur on Late Shale Gas Generation

Nonisothermal,
confined pyrolysis was applied to a mature shale
sample from the Ordovician Salgan Formation in Tarim Basin, northwest
China. Experiments were conducted using gold-tubes with added water
at a very slow heating rate (2 °C/h) and end temperatures between
336 and 600 °C. To investigate the influence of inorganically
bound sulfur on the generation of gases and to consider the geological
occurrence of sulfur-containing minerals, such as prevalent pyrite
in shales, the experiments were carried out with and without admixtures
of MgSO4, CaSO4, and pyrite. High amounts of
methane along with lower amounts of wet gases were formed from highly
mature shale without minerals added, demonstrating a huge late gas
generation potential at post peak-oil window maturities. In the experiments
with added sulfates and pyrite, all organic gases were consumed in
varying proportions, resulting in different chemical and stable carbon
isotopic compositions. Pyrite treatment affects wet gas (C2–C5) evolution directly, but it affects methane
(C1) evolution indirectly. In contrast, sulfate treatments
affect C1–C5 evolution directly. The
cumulative yield ratio of CO2/H2S indicates
that pyrite impacts on the hydrocarbon gas generation through low
valence sulfur such as S0 or others, which are associated
with H2S generation. In the pyrite series, the smooth increase
in ethane yield at temperatures exceeding 504 °C, together with
a concomitant stable carbon isotope reversal, demonstrates a new origin
for ethane at high temperatures. The isotopic reversal may come from
reactions between water and solid kerogen/coke/pyrobitumen. Isotopic
reversal of ethane occurs only in the control and pyrite series but
not in the sulfate treatments. This provides evidence that anoxic
conditions are required. Thus, one can expect to encounter isotopic
reversals in high maturity, unconventional gas shale environments
in the presence of pyrite.