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 degrees C/h) and end temperatures between 336 and 600 degrees 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 (C-2-C-5) evolution directly, but it affects methane (C-1) evolution indirectly. In contrast, sulfate treatments affect C-1-C-5 evolution directly. The cumulative yield ratio of CO2/H2S indicates that pyrite impacts on the hydrocarbon gas generation through low valence sulfur such as S-0 or others, which are associated with H2S generation. In the pyrite series, the smooth increase in ethane yield at temperatures exceeding 504 degrees 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.