BG Exploration and Production India Ltd, Midas Building, Sahar Plaza, M.V. Road, ... architecture, structural restoration modelling uses geometrical models of ...
GEOMOD2004
9-11 JUNE 2004 – EMMETTEN - LAKE LUCERNE
FOUR-DIMENSIONAL MODELLING OF SEDIMENTARY BASIN ARCHITECTURE AND HYDROCARBON MIGRATION
S.M. Clarke*, S.D. Burley* & **, G.D. Williams*, A.J. Richards*, D.J. Merredith* (*) Basin Dynamics Research Group, School of Earth Sciences & Geography, Keele University, Staffordshire, ST55BG, England (**) BG Exploration and Production India Ltd, Midas Building, Sahar Plaza, M.V. Road, East Andheri, Mumbai, 400 049, India
Summary Structural restoration, fault-seal analysis and hydrocarbon migration are largely treated as separate, predictive approaches to investigating basin geohistory and petroleum systems. Basin and fault-seal modelling techniques use geometrical and geo-mechanical solutions to static problems of basin architecture, structural restoration modelling uses geometrical models of basin geohistory, whilst hydrocarbon migration modelling approaches employ fluid dynamic solutions to flow through geometrically static basins. Each of these separate modelling approaches in their own fields are advanced and sophisticated but not integrated with each other. Lack of integration results in a static solution to migration modelling and renders a risk-driven, multiple realisation approach to petroleum systems analysis impractical. In this paper, we describe an integrated, 4D basin modelling workflow in which structural restoration and fault-seal analysis, together with their evolving geohistories and geometries, can be incorporated into an invasion percolation (IP) hydrocarbon migration modelling technique. Four-Dimensional Basin Analysis Basin modelling, or ‘petroleum systems analysis’, is a powerful approach for predicting the timing of petroleum generation and the distribution of hydrocarbon phase in the sub-surface to reduce exploration risk (Burley et al., 2000). Basin architecture and its evolution are two of the essential inputs to building a basin model. Basin geohistory influences the sediment infill, whilst evolving fluid conduit geometries define potential hydrocarbon migration pathways and thus hydrocarbon accumulations. Commercial and research basin modelling tools already enable many petroleum system processes to be predicted with considerable usefulness, including subsidence, compaction, pressure, maturation, secondary migration and trapped hydrocarbon phase and volumes (Mann et al., 1997; Wendebourg, 2000). These processes are dynamic, evolving through time and may occur in different areas of a basin at the same time. Fluid flow conduits, and potential migration along them, are also 3D networks throughout the basin. Accurate prediction of hydrocarbon accumulations therefore requires modelling in four dimensions: 3D space and time. Without the time component, basin models are essentially static, retaining Present Day geometries. However, incorporation of basin architecture and its evolution into 4D basin models is still simplistic in its approach. Commercial, 3D, secondary migration simulators do not directly incorporate structural restoration beyond vertical decompaction and backstripping, whilst commercial structural restoration tools do not simulate hydrocarbon generation and migration processes. The importance of structural restoration in determining basin geohistory, for incorporation into predictive petroleum system simulators, is considered in this paper. Initial results of a new, integrated, modelling methodology linking 4D fault restoration software with a hydrocarbon migration simulator based on Invasion Percolation (IP) techniques are presented. Accurate restoration of a basin geohistory is demonstrated to be critical to the modelling of hydrocarbon migration, and thus for petroleum system analysis. Sequential Restoration and Basin Modelling in Four Dimensions Accurate estimation of the evolution of subsurface geometry through time requires reconstruction of the regional to local scale tectonics and the simulation of porosity reduction through compactional processes. The key to incorporating structural restoration in basin simulators is to represent basin geometry and fault restoration as a geometrical modelling environment. Until now, two different 194
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techniques have been used to tackle the problem of subsurface geometry, porosity and pressure prediction. Traditionally, the gross geometry has been estimated using inverse models that range from large-scale tectonics to structural reconstruction with mechanical decompaction, and the generated time steps used as input into hydrocarbon migration modelling (e.g. Huggins et al., 2004). However a more detailed estimate of porosity and pressure can be obtained through forward simulation integrated with a coupled pressure-temperature model, resulting in a further reduction of porosity through diagenesis. Structural restoration through time involves a retro-deformational process in which sequential ‘snapshots’ of the basin evolution are portrayed. Automated retrodeformation can be achieved using a ‘piercing point’ technique (fig. 1), which analyses and automatically computes the required displacement vector field needed to restore the displaced geometry. This modelling approach uniquely defines matching points on the fault hanging-wall and footwall cut-offs and computes the required heave vectors for each restoration step. Restoration is performed using a well-known geometrical technique – the vertical/inclined shear algorithm – which results in a volume balanced solution, key to the successful geometrical modelling of basin stratigraphy (Williams et al., 1997). Geometrically perfect restorations are achieved without stratigraphical surface gaps or overlaps in the restored model. Four-Dimensional Fault Seal Analysis Faults, their geometries, and their movement histories through time are important controlling elements of hydrocarbon migration and entrapment within sedimentary basins. When faults undergo displacement, their fluid transmissibility properties change as a result of juxtaposed strata and the development of a fault zone of material between faulted blocks. These effects have to be combined with structural restoration and basin modelling and incorporated into models of hydrocarbon migration for accurate migration pathways to be predicted through time. By adopting a 4D modelling approach to the prediction of fault transmissibilities resulting from the processes of cross-fault juxtaposition (Allan, 1989) and argillaceous fault rock development (Fristad et al., 1997) these processes can be incorporated into basin modelling and hydrocarbon migration simulation. Cross-fault juxtaposition can be modelled by constructing a fault-surface section (Allan, 1989) to investigate seal and leak points resulting from juxtaposed combinations of reservoir and non-reservoir lithologies. This technique produces a static, 2D model that demonstrates juxtaposition relationships for a given temporal stage (usually Present Day) in the deformational history of the fault. Accurate modelling relies on the correct interpretation of faulted structural geometry and stratigraphy. The geometry of a fault that is curved along strike will not be represented correctly in the fault-surface section and, if a curved fault is resolved onto a flat plane, stratigraphical cut-off areas will not necessarily be preserved. Furthermore, the representation of a fault as a fault-surface section does not allow the interaction of multiple faults on sealing capacity to be investigated, or the juxtapositions of lithologies at other temporal stages in the structural evolution of the fault to be analysed. Using a 4D solution these problems can be addressed. By mapping properties of the faulted blocks to the footwall and hanging-wall of the fault surface directly in 3D, inaccuracies inherent in the 2D solution are reduced (fig. 2). Furthermore, the interaction of juxtaposition seal between multiple faults can be 195
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explored and, by working in 4D, the juxtaposition relationships at other temporal stages in the evolution of the fault(s) can be determined. There are many examples of faulted, hydrocarbonbearing, sedimentary basins in which faults can seal hydrocarbons even in the presence of juxtaposed reservoir units (e.g. Fristad et al. 1997). In these cases, fault transmissibility is dependent on the properties of the fault zone. Argillaceous smearing is an example of a fault-zone process that can affect transmissibility. This process has attracted much attention due to its particular relevance to mixed arenaceous and argillaceous sequences and calibration with field examples (Foxford et al., 1996). Many workers have produced 1D and 2D models to quantify argillaceous smearing and, of these models, the Shale Gouge Ratio or SGR (Fristad et al., 1997) is commonly employed for the analysis of fault transmissibility. By developing a 4D solution to the SGR it is possible to incorporate the true 3D variations in fault surface and block geometry through time into the prediction. Using this approach (fig. 3) an increased correlation between predicted gouge and outcrop examples can be demonstrated (Clarke et al., 2004). Moreover, by integrating argillaceous gouge modelling into the basin workflow it is possible to explore the effects of gouge from multiple linked faults on the sealing capacity of a reservoir, determine gouge at other temporal stages in the evolution of the fault (fig. 4) and incorporate these effects into models of hydrocarbon flow. Hydrocarbon Migration Hydrocarbon migration simulators rarely consider the effects of structure on migration and entrapment (see Huggins et al., 2004). Previous attempts at integrating the effects of faults into migration simulators have employed the results of fault analysis methods to provide a transmissibility multiplier applied to the cellular grid of a finite element (FE) migration model (e.g. Manzocchi et al., 1999), but these tools are designed for static reservoir models. They go some way to introducing the effects of faults into migration simulators but the geometry and evolution of the fault is not carried forward and the migration model becomes structurally static. Furthermore, the lengthy process times 196
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of FE-driven migration simulation renders a risk-driven approach to basin analysis and migration impractical. The effects of poorly constrained input parameters on potential outcomes cannot be adequately assessed. By adopting a 4D approach to migration modelling based on Invasion Percolation (IP) techniques (Meakin et al., 2000; Cathuruthers, 2003) it is possible to integrate the 4D effects of basin architecture and geohistory with hydrocarbon migration simulation. The technique models flow pathways (not properties) based on the statistical determination of the easiest path through the basin model. The effects of 3D fault-block geometry, transmissibility, and their temporal variation, can be incorporated (fig. 5). Using this technique, migration pathways can be simulated in minutes (rather than days or weeks) allowing multiple realisations of the same model to analyse risk associated with possible trap fill. This approach does not replace full FE-driven simulation since these techniques determine properties of the flow (such as quantities and phases) essential to the evaluation of the economic viability of the modelled accumulations. However, IP-driven flow modelling provides multiple realisations that enable the sensitivity of models to input geological parameters to be investigated and high-risk prospects to be eliminated. Conclusions The integrated modelling of basin architecture, geohistory, fault-seal analysis and hydrocarbon migration produces a valuable tool for a risk-driven, basin analysis workflow. Using this workflow, it is possible to explore the structural evolution of hydrocarbon-bearing basins in 4D and integrate this with fault-seal analysis and migration pathway modelling to produce a model in which the effects of structure, rock properties and basin geohistory on hydrocarbon migration can be explored. Using IPdriven hydrocarbon migration results in model process times of the order of minutes (rather than days) and the effects of poorly constrained input parameters, varying structural geometries, deformation stages, approximated rock properties and changes in fault seal capacity on trap fill can be determined to minimise risk in hydrocarbon exploration. REFERENCES • Allan, U.S. 1989 Model for hydrocarbon migration and entrapment within faulted structures. AAPG Bul..,73 803-81 • Burley, S.D., Clarke, S., Dodds, A., Frielingsdorf, J. Huggins, P., Richards, A., Warburton, I.C. and Williams, G.D. 2000 New
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insights on petroleum migration from the application of 4D basin modeling in oil and gas exploration. Jl Geochem. Exploration, 70, 465-470. Carruthers, D.J. 2003 Modelling of secondary petroleum systems migration using invasion percolation techniques. Ch 3 in: Duppenbecker, S and Marzi, R. (eds) Multi-dimensional Basin Modelling. AAPG Discovery Series, No. 7. Clarke, S.M., Burley, S.D. & Williams, G.D. 2004 (In Press) Dynamic fault seal analysis and flow pathway modelling in threedimensional basin models. Proceedings of the 6th Petroleum Geology Conference. London Foxford, K.A., Garden, I.R., Guscott, S.C., Burley, S.D., Lewis, J.J.M., Walsh, J.J. & Watterson, J. 1996 The field geology of the Moab Fault. In Huffman, A.C., Lund, W.R. & Godwin, L.H. (eds.) Geology & Resources of the Paradox Basin Special Symposium. Utah Geological Association & Four Corners Geological Society Guidebook 25 Fristad, T., Goth, A., Yeilding, G. & Freeman, B. 1997 Quantitative fault seal prediction: a case study from Oseberg Sud. In Moller, P. & Koestler, A.G. (eds.) Hydrocarbon Seals: Importance for Exploration & Production. NPF Special Pub. 7 107-124 Huggins, P., Burley, S.D., Sylta, O., Tommeras, A., Bland.,S, Kape, S., and Kusznir, N. 2004 (In press) Structural Restoration Techniques in 3D Basin Modelling: Implications for hydrocarbon migration and accumulation. Mar Pet.Geol. Mann, U., Hantschel, T. Schaefer, R.G., Kroos, B., Leythaeuser, D., Littke, R. and Sachsenhofer, R.F. 1997. Petroleum migration: mechanisms, pathways, efficiencies and numerical simulations., p403-509. In: Welte, D.H., Horsfield, B. and Baker, D.R. (eds) Petroleum and Basin Evolution. Springer. 535pp. Manzocchi, T., Walsh, J.J., Nell, P. & Yeilding, G. 1999 Fault transmissibility multipliers for flow simulation models. Petroleum Geoscience 5 53-63 Meakin, P., Wagner, G., Vedvik, A., Amundsen, H., Feder, J. & Jossang, T. 2000 Invasion percolation and secondary migration: experiments and simulations. Marine & Pet. Geol. 17 777-795 Wendebourg. J. 2000 Modelling multicomponent petroleum migration in sedimentary basins. Jl Geochem. Exploration, 70, 651656. Williams, G., S. Kane, Buddin, T.S. and Richards, A. 1997, Restoration and balance of complex folded and faulted rock volumes: flexural flattening, jigsaw fitting and decompaction in three dimensions. Tectonophysics, 273, 203-218
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