Sulfur-rich (Type II-S) source rocks and associated hydrocarbons
By Laura Garner
Sulfur-rich source intervals are associated with organic sulfur contents greater than 1% (Waldo et al., 1990) and an atomic S/C >0.04. Sulfur content can be an important bulk parameter for interpretation, not only for correlating crude oils to their to parent source rocks, but also to evaluate the viability of a discovery/field in economic terms.
Formation of sulfur-rich source rocks
Most primary sulfur in petroleum systems originates from early diagenetic reactions between deposited organic matter and aqueous sulfide species (Peters et al., 2005). In environments where anoxic (low oxygen) conditions prevail, specific species of anaerobic microorganisms (sulfate-reducing bacteria) use the sulfate () within the water body to oxidise organic molecules, producing hydrogen sulfide (H2S). Within siliclastic depositional environments, where dissolved iron species are usually in high abundance, the metals can remove the H2S by the formation of iron monosulfides (FeS, and ultimately iron pyrite (FeS2), resulting in low-sulfur kerogens and hydrocarbons. However, in less clastic facies such as carbonate muds for example, the low concentration of dissolved iron inhibits the removal of H2S as iron sulfides, and the excess sulfide becomes incorporated into the kerogen. Lacustrine environments may contain sulfur-rich Type I and IIS kerogens, but these are generally not widespread, as such environments usually do not contain enough sulfate for the bacteria to utilise. The Miocene Monterey Formation in California is an extreme example of a euxinic marine source rock, with kerogen containing between 8-14 wt.% chemically bound sulfur (Baskin & Peters, 1992). It is this source interval that led Orr (1986) to propose a new ‘Type II-S’ kerogen.
Figure 1: Model of sulfate reduction by sulfate-reducing bacteria and conversion of hydrogen sulfide into iron pyrite (SRB = sulfate-reducing bacteria). Image taken from ig.NET (IGI, 2019)
Low maturity petroleum generation and character
Sulfur-Sulfur (S-S) and Carbon-Sulfur (C-S) bonds are abundant in high sulfur kerogens and are more prone to cleavage at lower temperatures than Carbon-Carbon (C-C) bonds. This results in sulfur-rich source rocks generating petroleum at lower maturities than other kerogens (Martin, 1993). This corresponds to a shift in the average activation energy distribution (Figure 2) from a higher range (taking more energy to begin the chemical reaction) in a typical Type II kerogen (predominantly 52-55 kcal/mole) to a lower range of 48-51 kcal/mole for a Type II-S kerogen. It is also hypothesized by Lewan (1998), that the formation of active sulfur radicals generated during the initial stages of thermal maturation is the main controlling factor on increased petroleum formation rates, rather than the relative weakness of C-S bonds.
Figure 2: Distributions of activation energies for Type II and Type II-S kerogens (based upon Rock-Eval pyrolysis data (Tissot et al., 1987)
The initial oil expelled will have the maximum sulfur content, with sulfur content decreasing with increasing source rock maturity as a result of dilution by further generation of non-sulfur compounds (Orr & White, 1990). Most sulfur in crude oils is organically bound (bound to carbon) and such organo-sulfur compounds are usually the most abundant NSO compounds in petroleum. In hydrocarbons expelled from Type IIS kerogens, the oil may contain in excess of 40% NSO compounds (Peters et al., 2005). Organically bound sulfur compounds in petroleum can be subdivided into several chemical groups:
Figure 3: Chemical groups of organic sulfur compounds in oil (taken from Peters et al., 2005)
High sulfur-oils are relatively undesirable for the industry as they can typically be very heavy and viscous (due to the early-mature nature of the expelled fluids), and thus, difficult and costly to refine. Sulfur species are also poisonous for all catalytic processes. These can be temporary, although the full effects can be permanent depending on process conditions (Grove, 2003). In addition, the reduction in sulfur oxide emissions from the energy production and distribution sector is a key target for environmental agencies. However, due to the large resources of heavy, sulfur-rich oils around the world, and as conventional, light-oil accumulations gradually become depleted, there may be a requirement to gain further insight into the most productive and environmentally friendly way to exploit these crudes.
References
Baskin, D. K. and K. E. Peters (1992) "Early generation characteristics of a sulfur-rich Monterey kerogen." AAPG Bulletin76(1)
Grove D.E (2003) “Sulfur as a Catalyst Poisin.” Platinum Metals Review47 (1)
Lewan, M. D. (1998). "Sulphur-radical control on petroleum formation rates." Nature391(6663)
Martin, G., (1993) “Pyrolysis of organosulphur compounds. In: Patai, S., Rappoport, Z. (Eds.), The Chemistry of Sulphur Containing Functional Groups.” Wiley, pp. 395–437.
Orr, W. L. (1986) "Kerogen/asphaltene/sulfur relationships in sulfur-rich Monterey oils." Organic Geochemistry10(1-3)
Orr, W. L. and C. M. White (1990) Geochemistry of Sulfur in Fossil Fuels. Washington, DC, American Chemical Society.
Peters et al., (2005) Biomarkers and Isotopes in the Environment and Human History. Cambridge Cambridge University Press.
Tissot, B. P., et al. (1987) "Thermal history of sedimentary basins, maturation indices and kinetics of oil and gas generation." AAPG Bulletin71(12)
Waldo et al. (1990) Sulfur speciation in heavy petroleums: Information from X-ray absorption near-edge structure, Geochimica et Cosmochimica Acta, 55, pp. 801-814