--> H2S Generation in the Montney Tight Gas Siltstone Reservoir: Petrographic and Isotope Geochemical Evidence

AAPG ACE 2018

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H2S Generation in the Montney Tight Gas Siltstone Reservoir: Petrographic and Isotope Geochemical Evidence

Abstract

The world-class Montney siltstone tight gas play of the Western Canadian Sedimentary Basin was deposited as a westward-dipping succession of lower shoreface to offshore deposits. Locally high H2S concentration in the Montney Formation gas-producing wells in western Alberta affect the economic value of the natural gas, and has environmental impact. This study presents petrographic observations and stable isotope data to address the potential source(s) of aqueous sulfate, represented by early to late diagenetic sulfate phases and H2S generation in the study area in western Alberta and northeast British Columbia.

The studied samples are mostly laminated dolomitic siltstone with dolomite, calcite, pyrite, anhydrite, and gypsum as major diagenetic minerals. The early diagenetic anhydrite is the dominant form of anhydrite in the low H2S concentration zone in northeast British Columbia. While the late-stage anhydrite and gypsum cements are more dominant in the high to intermediate H2S concentration zone in western Alberta. Anhydrite occurs as blocky and poikilotopic cement and replacive forms. Gypsum appears as poikilotopic and euhedral tabular crystals. Pyrite mainly occurs as both framboids and euhedral crystals of likely multiple generations. Total organic carbon (TOC) and thermal maturity in the Montney Formation generally increase southwestwards across the study area from western Alberta into northeast British Columbia. The d34S and d18O values of bulk sulfate fraction in the studied samples vary widely from 5.8 to 28.8‰ V-CDT and -6.2 to 15‰ V-SMOW, respectively. The d34S values of bulk sulfide fraction are also very variable ranging from -32.5‰ to 36.9‰ V-CDT.

This isotopic signature suggests two or multiple sulfate-bearing fluids contributed to sulfate mineral precipitation in the Montney Formation. Early anhydrite of low H2S zone displays more affinity to the coeval Triassic seawater. The similar isotopic signature of late-stage anhydrite in the high to intermediate H2S zone to the Devonian evaporites suggests involvement of fluids from dissolution of Devonian evaporites and mixing with pore waters of Triassic seawater signature. It appears that brine sourced from dissolution of underlying Devonian evaporites was introduced to the Montney Formation by deep-seated faults/fractures network, provided required sulfate for BSR/TSR, leading to formation of H2S. The surplus of sulfate resulted in formation of late anhydrite cement.