Impact of Inorganically Bound Sulfur on Late Shale Gas Generation

Abstract

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 MgSO 4 , CaSO 4 , 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 CO 2 /H 2 S indicates that pyrite impacts on the hydrocarbon gas generation through low valence sulfur such as S 0 or others, which are associated with H 2 S 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.

Experimental Investigation of Fracture Propagation Behavior Induced by Hydraulic Fracturing in Anisotropic Shale Cores

“…In the recent decades, unconventional reservoirs, such as shale gas [1,2], coalbed methane [3,4] and tight sandstone reservoir [5,6], have been developed rapidly. Hydraulic fracture network (HFN) induced by fracturing treatment provides migration pathways for unconventional oil and gas recovery [7,8].…”

Stable Carbon Isotopic Fractionation and Hydrocarbon Generation Mechanism of CO2 Fischer–Tropsch-Type Synthesis under Hydrothermal Conditions

“…After the experiments, the gas components in the capsules were collected in a special glass device connected to an Agilent 6890N GC (30 m × 0.25 mm × 0.25 μm) modified by the Wasson ECE instrumentation, as described in detail elsewhere. 28 The gas collector was connected to the GC through two valves for chemical composition analysis. Once an aliquot of gas entered the GC, the valve between the gas collector and GC was closed and the remaining gas in the gas collector was kept for stable carbon isotope analysis.…”

“…The internal pressure in the vessels was kept stable by pumping water in or out during the experiments. The experimental apparatus has been described previously . Prior to the experiments, one end of each gold tube was sealed by argon-arc welding, and then the tube was heated at 800 °C for 3 h in air to remove potential organic contaminants.…”

Hydrolysis of Pyrite: a Possible Pathway for Generation of High Concentrations of H2S Gas in Deep-buried Reservoirs?

“…Xie et al [37] found that large amounts of H2S were released from the reaction between n-hexane and pyrite at elevated temperatures. Recently, Wang et al [38] reported that large amounts of H2S were produced during the confined hydrous experiments using pyrite and a mature shale. Their study indicated that organic-inorganic interactions between pyrite and hydrocarbons may have implications for H2S generation under geologic conditions as well as bacterial and thermochemical sulfate reduction (BSR & TSR).…”

“…Their study indicated that organic-inorganic interactions between pyrite and hydrocarbons may have implications for H2S generation under geologic conditions as well as bacterial and thermochemical sulfate reduction (BSR & TSR). Nevertheless, there is a considerable question for the study of Wang et al [38] whether water may act as another hydrogen source for H2S generation besides organic matter. According to previous closed system model of thermodynamics, water dissolves sulfur and other components [39], leading to consumption of pyrite at moderate www.seipub.org/des Development in Earth Science (DES), Volume 5, 2017 temperatures to maintain phase equilibrium [40].…”