Noble gases solubility models of hydrocarbon charge mechanism in the Sleipner Vest gas field

Abstract

Noble gases are chemically inert and variably soluble in crustal fluids. They are primarily introduced into hydrocarbon reservoirs through exchange with formation waters, and can be used to assess migration pathways and mechanisms, as well as reservoir storage conditions. Of particular interest is the role groundwater plays in hydrocarbon transport, which is reflected in hydrocarbon–water volume ratios. Here, we present compositional, stable isotope and noble gas isotope and abundance data from the Sleipner Vest field, in the Norwegian North Sea. Sleipner Vest gases are generated from primary cracking of kerogen and the thermal cracking of oil. Gas was emplaced into the Sleipner Vest from the south and subsequently migrated to the east, filling and spilling into the Sleipner Ost fields. Gases principally consist of hydrocarbons (83–93%), CO₂ (5.4–15.3%) and N₂ (0.6–0.9%), as well as trace concentrations of noble gases. Helium isotopes (³He/⁴He) are predominantly radiogenic and range from 0.065 to 0.116 R_A; reported relative to air (R_A = 1.4 × 10⁻⁶; Clarke et al., 1976; Sano et al., 1988), showing predominantly (>98%) crustal contributions, consistent with Ne (²⁰Ne/²²Ne from 9.70 to 9.91; ²¹Ne/²²Ne from 0.0290 to 0.0344) and Ar isotopes (⁴⁰Ar/³⁶Ar from 315 to 489). Air-derived noble gas isotopes (²⁰Ne, ³⁶Ar, ⁸⁴Kr, ¹³²Xe) are introduced into the hydrocarbon system by direct exchange with air-saturated water (ASW). The distribution of air-derived noble gas species are controlled by phase partitioning processes; in that they preferentially partition into the gas (i.e., methane) phase, due to their low solubilities in fluids. Therefore, the extent of exchange between hydrocarbon phases and formation waters – that have previously equilibrated with the atmosphere – can be determined by investigating air-derived noble gas species. We utilize both elemental ratios to address process (i.e., open vs. closed system) and concentrations to quantify the extent of hydrocarbon–water exchange (i.e., volumetric gas–water ratios). These data are discussed within the framework of several conceptual models: (i) total gas-stripping model, which assumes all noble gases have been stripped from the water phase, thus defining the minimum volume of water to have interacted with the hydrocarbon phase; (ii) equilibrium model, which assumes equilibration between groundwater and hydrocarbon phase at reservoir P, T and salinity; and (iii) open and closed system gas-stripping models, using concentrations and elemental ratios. By applying these models to Ne–Ar data from Sleipner, we estimate volumetric gas–water ratios $\left(\frac{{\text{V}}_{\text{g}}}{{\text{V}}_{\text{w}}}\right)$ between 0.02 and 0.07, which are lower than standard geologic gas–water estimates of ∼0.24, estimated by combining gas-in-place estimates with groundwater porosity estimates. Sleipner Vest data can be best approximated by an open system model, which predicts more than an order of magnitude more groundwater interaction during migration than geologic estimates, indicating a dynamic aquifer system and/or a hydrous migration pathway. In an open system, the extent of gas loss can be estimated to be between 8 and 10 reservoir volumes, which have passed through the system and been lost (i.e., filled and spilled).

Introduction

The principles behind petroleum systems exploration are straightforward but the applications are not; approaches typically require temporal and spatial determination of hydrocarbon source, secondary migration pathways and the occurrence of suitable trapping structures. In practice, each of these parameters within basin systems is multivariate, which together means that most exploration targets are highly complex systems. Basin evolution models provide a temporal framework in which the thermal maturity and timing of hydrocarbon expulsion from source rocks can be predicted. These are dependent on non-linear estimations of subsidence, filling, compaction of sediments and fluids with a range of chemical, physical and mechanical properties over time-scales that can include multiple tectonic processes. Geochemical observations, such as biomarker and stable isotopic information, from surface seeps and subsurface hydrocarbon discoveries are often used to identify source rock and/or maturity information and provide further iteration and confidence in predictive models. There are few techniques that can be used to identify and quantify the controls on secondary migration processes and pathways; noble gases provide such a tool.

The unique and well-defined noble gas isotopic and abundance compositions of different fluid sources (i.e., air-derived, radiogenic and mantle) in sedimentary basins, combined with their chemical inertness, can be used to identify and quantify the physical mechanisms that control interactions between water, oil and gas – thus providing essential information in developing our understanding of petroleum systems (Ballentine et al., 2002, Prinzhofer, 2013). Noble gases dissolved in water that has equilibrated with the atmosphere – air-saturated water (ASW) – provides the main vector for bringing air-derived noble gases into the subsurface. The unique noble gas isotopic composition of air is preserved in the water but the elemental composition is modified by the differential solubility of the noble gases at recharge or burial (e.g., Kipfer et al., 2002). Atmosphere-derived heavy noble gases (Kr and Xe), can be adsorbed and fixed within carbonaceous sediments, which when released into the fluid system, provides an additional source of these noble gases with an atmospheric isotopic composition (Torgersen and Kennedy, 1999, Zhou et al., 2005).

Contact between groundwater and subsurface oil or gas phases results in the redistribution, or partitioning, of the air-derived noble gases between the different phases. The resulting noble gas composition of each phase is a function of the thermodynamic conditions of the system and the extent and type of contact. In principle, this preserves a record of that interaction in each phase, which has been modeled using different approaches in a number of previous studies (Zartman et al., 1961, Bosch and Mazor, 1988, Zaikowski and Spangler, 1990, Ballentine et al., 1991, Ballentine et al., 1996, Hiyagon and Kennedy, 1992, Pinti and Marty, 1995, Torgersen and Kennedy, 1999, Schwarzenbach et al., 2003, Zhou et al., 2005, Zhou et al., 2012, Gilfillan et al., 2008, Gilfillan et al., 2009, Hunt et al., 2012, Aeschbach-Hertig and Solomon, 2013, Darrah et al., 2015). Further information is provided from readily identifiable noble gas nuclides derived from radioactive decay (e.g., ⁴He, ⁴⁰Ar), which can provide temporal constraints. Within open fluid systems the flux of radiogenic isotopes into and out of the fluid system controls their concentration within the fluids (Torgersen and Clarke, 1985, Torgersen and Ivey, 1985, Castro et al., 1998a, Castro et al., 1998b, Zhou and Ballentine, 2006, Torgersen, 2010). Within closed fluid systems the rate of accumulation in the groundwater (Solomon et al., 1996, Cook et al., 1996, Tolstikhin et al., 1996, Ballentine et al., 2002, Holland et al., 2013) is determined by the solid phase parent radionuclide concentrations and efficiency of release (Tolstikhin et al., 2010, Hunt et al., 2012, Darrah et al., 2014, Lowenstern et al., 2014, Barry et al., 2015) from the mineral into the fluid phase.

Earlier studies of noble gases in hydrocarbon bearing systems have mostly investigated simple models that provide lower limits for groundwater involvement in hydrocarbon migration by assuming total gas stripping of the water phase (e.g., Ballentine et al., 1991), or scenarios in which water and hydrocarbon phases are at simple equilibrium (Ballentine et al., 1996). More recent studies have shown that open system Rayleigh fractionation processes operating on the ASW noble gas signatures can be identified and modeled in coal gas systems to identify both the volume and age of water in contact within this unconventional hydrocarbon system (Zhou et al., 2005). The potential remains to take similar models to conventional hydrocarbon systems to help identify the role of groundwater and style of secondary hydrocarbon migration within conventional oil and gas systems (Hunt et al., 2012, Darrah et al., 2014, Darrah et al., 2015).

In this study, we present noble gas isotope and abundance data as well as major gas compositional and isotopic data from the Sleipner Vest natural gas fields of the Norwegian North Sea (Fig. 1). We have chosen this field because the trapping structure has filled completely and continued gas charging is thought to have caused gas to spill from this system and charge other nearby accumulations further along the secondary migration path. This is a classic open system ‘fill and spill’ scenario. Here we develop a series of increasingly more complex models to account for the observed noble gas data. We compare estimates of groundwater involvement assuming: (i) total gas stripping of groundwater to provide a minimum water estimate (zero order model); (ii) equilibration of hydrocarbon and water phases (first order model); (iii) accumulation of the gas in the reservoir due to groundwater gas stripping, along with simultaneous gas loss via spill (second order, open system model) and (iv) accumulation of gas in the reservoir due to gas stripping, without gas spill (second order, closed system model). Both closed- and open-system models describe how noble gas concentrations and elemental ratios evolve when the interaction of the hydrocarbons with ASW occurs during migration to the reservoir. Using air-derived noble gas isotopes in our models allows us to understand the extent of exchange between hydrocarbon phases and formation waters. Thus, for the first time, we are able to assess the utility of noble gases in determining the origin and subsequent migration history of gases within this type of hydrocarbon system.