Methane-dominated thermochemical sulphate reduction in the Triassic Feixianguan Formation East Sichuan Basin, China: towards prediction of fatal H₂S concentrations

Abstract

New sour pools have recently found in the Lower Triassic Feixianguan Fm carbonate reservoirs in the East Sichuan Basin in China with H₂S up to 17.4% by volume. A recent blowout from a well drilled into this formation killed hundreds of people as a result of the percentage concentrations of H₂S. In order to assess the origin of fatal H₂S as well as the cause of petroleum alteration, H₂S concentrations and the isotopes, δ³⁴S and δ¹³C have been collected and measured in gas samples from reservoirs. Anhydrite, pyrite and elemental sulphur δ³⁴S values have been measured for comparison. The high concentrations of H₂S gas are found to occur at depths >3000 m (temperature now at ∼100 °C) in evaporated platform facies oolitic dolomite or limestone that contains anhydrite nodule occurrence within the reservoirs. Where H₂S concentrations are greater than 10% its δ³⁴S values lie between +12.0 and +13.2‰ CDT. This is within the range of anhydrite δ³⁴S values found within the Feixianguan Fm (+11.0 to +21.7‰; average 15.5±3.5‰ CDT). Thus H₂S must have been generated by thermochemical sulphate reduction (TSR) locally within the reservoirs. Burial history analysis and fluid inclusion data reveal that the temperature at which TSR occurred was greater than about 130–140 °C, suggesting that the present depth-temperature minimum is an artifact of post-TSR uplift. Both methane and ethane were actively involved in TSR since the petroleum became almost totally dry (no alkanes except methane) and methane δ¹³C values become significantly heavier as TSR proceeded. Methane δ¹³C difference thus reflects the extent of TSR. While it is tempting to use a present-day depth control (>3000 m) to predict the distribution of H₂S in the Feixianguan Fm, this is an invalid approach since TSR occurred when the formation was buried some 1000–2000 m deeper than it is at present. The likelihood of differential uplift across the basin means that it is important to develop a basinal understanding of the thermal history of the Feixianguan Fm so that it is possible to determine which parts of the basin have been hotter than 130–140 °C.

Introduction

The Sichuan Basin is one of China's largest natural gas provinces with gas discovered in the Sinian and Carboniferous through to the Jurassic parts of the stratigraphy. Recently, gas pools have been found in the Lower Triassic Feixianguan Fm carbonate reservoirs in Northeast Sichuan (Chen, 2001) with a proven gas reserve of 850×10⁸ m³ and an estimated recoverable reserve of 327×10⁸ m³ in the Tieshanpo area (Chen, 2001, Xie, 2003, Yang and Wen, 2002). The reservoirs for the sour pools are oolitic grainstones and dolomitic carbonate (Wang, 1996) covering an area of about 4000 km². They have an average porosity of about 8% and an average thickness of 33.7 m (Yang & Wen, 2002). The gas contains up to 17.4% H₂S by volume (Jiang et al., 2002, Wang et al., 2002, Yang et al., 2002).

A major blowout from a sour (H₂S-rich) gas well being drilled in the Feixianguan Fm occurred when a well (well LJ16H) was drilled in the Luojiazhai gas field in the East Sichuan to the depth of 4049.68 m on December 23, 2003 and led to the death of 243 persons (Gang, 2004). Deaths were mostly caused directly by the accidentally emitted H₂S and ‘turned 10 square miles into a death zone’ (Goodman, 2003). The well had a formation pressure of about 40 MPa and was approximately 23% overpressured. H₂S gas is not only corrosive and an economic problem for asset managers and petroleum processing facilities, it is environmentally damaging and, as the recent tragedy demonstrates, extremely dangerous to human beings. Understanding the origin, spatial distribution and relationship of the H₂S with hydrocarbon gases are of importance to petroleum exploration and development and are a paramount consideration for human health.

It has been suggested, but not proven, that H₂S in the Feixianguan Fm may have been derived by thermochemical sulphate reduction (TSR) within the reservoirs (Jiang et al., 2002, Wang et al., 2002, Yang et al., 2002). Thermochemical sulphate reduction (TSR) occurs when sulphate, from whatever source, is chemically reduced by petroleum fluids at elevated temperature. The minimum temperature for TSR has been the subject of lively debate (Baric and Jungwirth, 1997, Machel, 1998, Worden et al., 1995, Worden et al., 1997, Worden et al., 1998) although it is conventionally assumed to be approximately 120 °C. The products of TSR include reduced forms of sulphur (H₂S, elemental sulphur) and oxidized forms of carbon (calcite, CO₂), as well as water and a variety of organo-sulphur compounds.

It is still not clear whether TSR occurs between light hydrocarbon and sulphate compounds or whether TSR affects light hydrocarbon gas carbon isotopes. It has been suggested that methane, reputedly the least reactive petroleum compound, does not undergo TSR, that being the preserve of liquid phase petroleum compounds (Machel, 2001). The location for the optimum rate of TSR in petroleum reservoirs has also been questioned. TSR has been shown to occur both at a gas–water transition zone (Machel, Krouse, & Sassen, 1995) and in a gas zone (Worden, Smalley, & Cross, 2000).

In this paper, we present gas chemistry, δ¹³C and δ³⁴S, anhydrite, pyrite and elemental sulphur δ³⁴S along with fluid inclusion data. We seek to confirm that TSR was responsible for the fatal occurrence of H₂S in the Feixianguan Fm. We also seek to address the questions of where TSR occurred in the petroleum system (gas zone or gas-water transition zone) and whether methane was involved in TSR in the Feixianguan Fm. Anhydrite is present in the low permeability caprock to the Feixianguan Fm reservoirs as well as being disseminated within the Feixianguan reservoirs. We will address whether one or both of these types of anhydrite are involved in TSR. We will also assess what will be required to build a model capable of predicting the distribution of H₂S in the Feixianguan Fm in the Sichuan Basin.