Journal of the Geological Society, London, Vol. 144, 1987, pp. 327-347, 21 figs, 8 tables. Printed in Northern Ireland The movement and entrapment of petroleum fluids in the subsurface W. A.ENGLAND, A. S. MACKENZIE',D. M. MANN & T. M. QUIGLEY Geochemistry Branch, BP Research Centre, Sunbury+n-Thames, Middlesex TW16 7LN, UK Abstract: This paper discusses the migration of petroleum from its formation in a source rock to its subsequent possible entrapment in a reservoir. The chemical and physical properties of petroleum gases and liquids are stressed, particularly their phase behaviour undersubsurface conditions which is shown to be a very important factor in determining migration behaviour. Engineering correlations are presented for estimating the propertiesof petroleum fluids under geologically realistic conditions. The directionsand magnitudes of theforces acting on migrating petroleumare deduced from the combined effects of buoyancy and water flow in compacting sediments. These forces are combined, using a fluid potential description. This procedureallows the direction of migration to be defined. The rate of migration is thenestimated from the properties of thesediments involved, allowing a distinction to be made between 'lateral' and'vertical' carrier beds. This simplified approach is suitable for rapid predictive calculationsin petroleum exploration. It is compared with the more complex 3-D computer modelling approaches which are currently becoming available. Migration losses are related to the cumulative pore volume employed by the petroleum in establishing a migration pathway. The petroleum migration mechanism is shown to be predominantly by bulk flow, with a small diffusive contribution for light hydrocarbons over distances less than c. 100m. The loss factors involved in secondary migration are estimated fromfield evidence. Themechanism of reservoir filling is presented as a logical extensionto those described formigration. This, together with the inefficiency of in-reservoir mixing by diffusion or convection, is shown to tend to cause significant lateral composi- tional gradients in reservoirs over and above the gravitationally induced vertical gradients described by other workers. Symbols and units used Verticalforce acting onunit volume of A Scaling constant in k, =A@*,m' petroleum, N m-3 or Pa m-' Acceleration due to gravity, 9.81 mS-' BG formationGas volumea factor, for single- phase gas reservoir: Surface gas:oilratio of petroleum fluids expelled from a source rock, kg kg-' volume of gas + dissolved condensate Surface gas:oilratio of a subsurface under subsurface conditions petroleum liquid, kg kg-' volume of gas measured at STP height of sedimentary column, m Carrier bed thickness, m B0 Oil formation volume factor: Maximum height of petroleum column that volume of oil + dissolved gas in subsurface a seal can support, m volume of oil at STP Characteristic segregation length for a given compound, m C Number of components Intrinsic permeability, m* c, number,Capillary pq/y Boltzmann constant, 1.38X 10-23JK-' cc Compaction coefficient Molecular weight of ith component, kg (kg C" Coefficient of consolidation mol) - CGR Condensate:gaspetro- a relatingratio, to Subsurface mass of petroleum liquid or gas, leum gas, kg kg-' kg d m diameter, Grain Surface mass of petroleum liquid (oil or D Diffusion coefficient, m' s-l condensate), kg F Number of degrees of freedom; or force on Surface mass of petroleum gas, kg unit volume of fluid, N m-3 or Pa m-l number of moles FK Rate of enthalpychangecaused by unit Reynolds number, pqtlp volume of kerogen breakdown, J m-3 sK1 Pressure, Pa; or number of phases (for use FP Force acting onvolume unit of petroleum with phase rule) fluid, Nm-3 or Pa m-' Darcy flux perunit cross-sectional area of Fw Forceacting on unitvolume of water, N m-3 rock, m3 m-' s-l or Pa m-' Vertical component of Darcy flux, m3m-' S-1 'Presentaddress BP PetroleumDevelopment (Norway) plc., Lateral Darcy flux, m3 s-l Forusbeen 35, P.O. Box 197, 4033 Forus, Norway. Vertical Darcy flux, m3S-' 327 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/2/327/4888812/gsjgs.144.2.0327.pdf by guest on 01 October 2021 328 W. A. ENGLAND ET AL. r, f Pore throat radius, mean pore throat radius, Table 1. Compositions of subsurface petroleum liquid from the m Bruce field, UK continental shev, reservoired at 40 MPa and 105 "C; Hydrodynamic radius of the ith component, and surface gases and oils produced at STP (COR = 0.3 kg kg-') m R Gas constant, 8.314 X Id JK-' (kg mo1)-' Subsurface petroleum R, Rayleigh number liquid Surface gas Surface oil S Petroleum saturation of pore space, m3 m-3 Com- ponents M(%) mol(%) M(%) mol(%) M(%) mol(%) t Time, S T Temperature, K 1.1 1.92.6 4.3 V Volume, m3 0.1 0.3 0.4 0.4 vi Molarvolume of ithcomponent, m3 (kg 10.9 49.9 68.311.8 moI)-' 3.3 0.1 0.028.1 11.0 12.6 W Length scale for estimating Peclet number 3.4 0.1 5.8 7.8 13.1 0.5 W Horizontal length of camer bed, m 3.01.4 0.4 3.84.7 10.4 2.52.6 0.9 2.52.5 6.9 X Horizontal dimension, m Mole fraction of ith component 2.8 2.3 4.3 5.81.8 1.6 Xi 4.810.8 5.3 3.5 0.8 3.4 X Composition 6.314.9 8.2 4.0 0.2 1.0 Y Horizontal width of carrier bed, m 5.1 10.5 6.72.9 0.08 0.4 z Vertical dimension, m 3.7 1.9 0.1 0.027.0 4.9 Z Compressibilityfactor to correct ideal gas 3.0 1.4 4.1 5.3 law for non-ideal behaviour: PV = ZnRT 2.9 1.2 3.9 4.5 (Y Dip of beds, degrees or radians 2.9 1.1 3.9 4.3 3.0 1.1 4.0 4.1 (YW Coefficient of expansion for water, K-' B Contact angle of petroleum-water interface 2.7 0.9 3.6 3.4 on pore wall, measured through petroleum, 2.6 0.8 3.5 3.1 2.3 0.7 3.1 2.5 degrees, or radians 1.9 0.5 2.5 1.4 Interfacial tension, or energy, Nm-' or Jm-* 1.4 0.4 1.9 0.9 Proportions of subsurfacepetroleum fluids, 0.9 0.2 1.2 0.9 liquids orgases that are gas at the surface, kg 0.9 0.2 1.2 0.6 kg-' 0.7 0.2 0.9 0.3 Tortuosity factor, m m-' 0.4 0.09 0.5 0.3 Dynamic viscosity, Pas or kg m-' sC1 0.3 0.06 0.4 0.1 Density, kg m-3 0.2 0.04 0.3 0.06 Subsurface petroleum gas density, kg m-3 0.1 0.02 0.1 0.06 0.1 0.02 0.1 0.06 Surface petroleum gas density, kg m-3 0.07 0.01 0.1 0.06 Subsurface petroleum liquid density, kg m-3 0.04 0.007 0.05 0.03 Surface petroleum liquid (oil or condensate) 0.02 0.003 0.03 0.01 density, kg m-3 26.57 (MW. 450) 4.2 16.0 36.2 Subsurface petroleum fluid density, kg m-3 100 100100 100 loo 100 Subsurface rock density, kg m-3 Subsurface water density, kg m-3 C, refers to a molecule with n carbon atoms. Total stress, effective stress, Pa Porosity, average porosity, surface porosity, m3 m-3 grouped by carbon number. Thus, the C6 fraction includes Fluid potential, Pa or J m-3 normal and branched hexanes (C,H,,) as wellas the Electrostaticpotential, volts, mechanical unsaturated compound benzene (C6&). The compounds potential, J with carbon numbers of five or less are mainly found in the Petroleumpotential, water potential, Pa or gaseous phase under surface conditions, while the heavier J m-3 hydrocarbons are mostly found in the liquid state. Vectordifferential of verticaland lateral Petroleum is formed in the subsurface in finegrained water potential, N m-3 or Pa m-' source rocks and its generation is well understood (e.g. Tissot & Welte 1984). Some fraction of the organic remains Understanding the movement of petroleum fluids through of dead organisms deposited with the rocksmay be the pores of sedimentary rocks is of enormous commercial preserved to form a solid, insoluble constituent known as importance. Much has been written onthe extraction of kerogen (e.g. Durand 1980). Kerogen ischemically stable petroleum fluids from the pores of underground reservoirs until c. lOO"C, at which point some of the bonds within (e.g. Dake 1978); but the understanding of how these fluids kerogen are broken and mobile petroleum fluids are moved towards and accumulated in the reservoirs is produced; if their volume within the pores is adequate to somewhat superficial. An improved appreciation of this form an inter-connected phase, expulsion may occur (Cooles process will help to plan extraction programmes, and et al. 1985). To create accumulations from which petroleum increase the precision of petroleum exploration. Petroleum may be extracted economically, the petroleum must migrate is defined as both crude oil and natural gas; 'oil' and 'gas' into the pores of coarser, more permeable 'reservoir' rocks. are descriptions applied to petroleum fluids under surface It is not uncommon for petroleum to migrate more than conditions of pressure and temperature. 2 km vertically and 100 kmlaterally from its origin to a A typical petroleum composition isshown in Table 1. reservoir. The thousands of naturally occurring compounds have been Thereare several procedures for quantifying the Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/2/327/4888812/gsjgs.144.2.0327.pdf by guest on 01 October 2021 MOVEMENT OF PETROLEUM FLUIDS 329 generation andexpulsion of petroleum fromsource rocks.