Hydrocarbon migration

Introduction

Primary migration refers to the expulsion of hydrocarbons from the source rock into suitable carrier beds. Secondary migration describes the movement of hydrocarbons along these migration conduits to a suitable trap. Good conduits for migration may include porous and permeable sandstones, limestones and open fault systems.

APT can help you understand the likely migration pathways to your prospects using an integrated 3D model.

Migration rates and distances

Theoretical (England et al. 1987, 1991; England 1994), experimental (Dembicki and Anderson 1989) and modelling (Sylta et al. 1997) studies indicate that secondary migration is fast on a geological time-scale and, in most settings, it is faster than the rate of overburden deposition required to initiate and maintain hydrocarbon generation and expulsion (England 1994), so the timing of primary migration is a reasonable guide to that of secondary migration (Dembicki and Anderson 1989).

Most commercial accumulations are explained by relatively short lateral migration distances (<30 km), constrained by the dimensions of the drainage areas surrounding individual traps. Although there are well-known examples of relatively long-distance secondary migration – for example Siri Fairway, Danish North Sea (Ohm et al. 2006) or Williston Basin, Western Canada (Osadetz et al., 1992, 1994; Burrus et al., 1996) – they are uncommon (Demaison and Huizinga 1994).

Using a global database Sluijk and Nederlof, 1984 found an approximately log-normal distribution of migration distances, with mode around 10 km and a long tail up to values well in excess of 100 km. An overall migration factor was defined, dependent upon lateral and vertical migration distances and a number of subfactors. Major lateral migration subfactors were fracturing and faulting parallel to the migration pathway, which were considered to enhance migration, and cross-faulting or lithological/structural barriers, which likely impede migration. Important vertical migration subfactors were the average lithology of the stratigraphic section between source and reservoir, and also the presence of fracturing and faulting. In a multivariate discriminant analysis it was found that the occurrence of hydrocarbon charge was related to the migration subfactors but not to the migration distances. This suggests that migration distance per se does not prevent charging, although such a viewpoint is probably overly simplistic and the nature of the migration pathway (comprising the various subfactors) is more important. From migration modelling exercises, Sylta et al., 1997 suggest that in efficient carrier systems migration velocities in excess of 100 km/a and migration distances in excess of 1000 km are possible.

Relative secondary migration distances of hydrocarbons from a particular kitchen may be obtained from GC-MS analysis of some pairs of compounds.

Migration modelling

Flowpath modelling

The flowpath or ray-tracing migration model is buoyancy driven. Migration pathways follow the path of maximum gradient until barriers are met. Petroleum migrates vertically until it encounters a sealing horizon after which it moves in the direction of the most structurally elevated position below this sealing surface until it reaches the edge of the seal. Flow path modelling is computationally simple, the software operates on high-resolution grids and also efficiently defines drainage areas, solves for fill and spill histories and mixing in reservoirs.

Darcy flow modelling

Darcy flow represents a physically correct model for transport of multiphase fluids that considers all of the relevant physical processes because it involves not only buoyancy, capillary forces, and pressure gradient, but also transient physics, thanks to the viscous terms. The Darcy model is well suited to follow transient flow in low-permeability areas. Darcy flow is computationally intense and solution times for fine grids may be long.

Invasion percolation modelling

Invasion percolation is based upon the assumption that on the time scales for secondary migration, flow is controlled mainly by buoyancy and capillary pressures and that viscous forces are negligible and may be discounted. Invasion percolation is relatively quick and especially useful to simulate secondary migration on high resolution grids.

References

Burrus, J., Wolf, S., Osadetz, K., Visser, K., 1996. Physical and numerical modelling constraints on oil expulsion and accumulation in the Bakken and Lodgepole petroleum systems of the Williston Basin (Canada-USA). Bulletin of Canadian Petroleum Geology 44, 429–445.

Demaison, G., Huizinga, B.J., 1994. Genetic Classification of Petroleum Systems Using Three Factors: Charge, Migration, and Entrapment, in: Magoon, L.B., Dow, W.G. (Eds.), The Petroleum System - from Source to Trap. pp. 73–89.

Dembicki Jr., H., Anderson, M.J., 1989. Secondary migration of oil: experiments supporting efficient movement of separate, buoyant oil phase along limited conduits. AAPG Bulletin 73, 1018–1021.

England, W.A., 1994. Secondary Migration and Accumulation of Hydrocarbons, in: Magoon, L.B., Dow, W.G. (Eds.), The Petroleum System - from Source to Trap. pp. 211–217.

England, W.A., Mackenzie, A.S., Mann, D.M., Quigley, T.M., 1987. The movement and entrapment of petroleum fluids in the subsurface. Journal of the Geological Society 144, 327–347.

England, W.A., Mann, A.L., Mann, D.M., 1991. Migration from Source to Trap, in: Merrill, R.K. (Ed.), Source and Migration Processes and Evaluation Techniques. pp. 23–46.

Ohm, S.E., Karlsen, D.A., Roberts, A., Johannessen, E., Hoiland, O., 2006. The Paleocene sandy Siri Fairway: An efficient “Pipeline” draining the prolific Central Graben? Journal of Petroleum Geology 29, 53–82.

Osadetz, K.G., Brooks, P.W., Snowdon, L.R., 1992. Oil families and their sources in Canadian Williston Basin, (southeastern Saskatchewan and southwestern Manitoba). Bulletin of Canadian Petroleum Geology 40, 254–273.

Osadetz, K.G., Snowdon, L.R., Brooks, P.W., 1994. Oil families in Canadian Williston Basin (southwestern Saskatchewan). Bulletin of Canadian Petroleum Geology 42, 155–177.

Sluijk D., Nederlof M.H., 1984. Worldwide geological experience as a systematic basis for prospect appraisal, in Demaison, G., Murris, R.J. (Eds), AAPG Memoir 35, pp. 15–26.

Sylta, Ø., Pedersen, J.I., Hamborg, M., 1997. On the vertical and lateral distribution of hydrocarbon migration velocities during secondary migration. Geofluids II 1997 extended abstracts, 55–58.