Migration and Trap Filling Models

This post compiles some of the images from my recent posts on LinkedIn, to show the important role capillary pressure plays in petroleum migration and trap filling. These models assume that migration rate is extremely slow, limited by supply rates from the source rock, and thus migration is always in equilibrium with the capillary pressure field of the geological system - the dynamic effects of viscosity (thus Darcy flow rates) can be safely ignored. In fact we have never observed any distribution patterns of petroleum pools that can not be explained by capillary pressure alone. All accumulations are constrained by a capillary system, except occasionally hydrodynamics and gravity (tar sands) play a role in part of the accumulation. The variation of pore throat sizes laterally due to facies change, and vertically due to different lithologies is the greatest force that controls the migration process and the distribution of petroleum pools - at all scales microscopic to 100 km+ scales, and tight reservoirs to the traditional traps.    

These models use very simple geological models to demonstrate the useful physical principles, and real world geology is much more heterogenous and variable - we need to have in mind what we see in cores, on logs, and in outcrops when we make models. Models are useful because they help us understand the physics, and interpret observations, and make predictions, with the difference between nature and model in mind.  

Figure 1. Capillary displacement pressure, and Sw-Pc (Sw-Height) curves are fundamentally what control the trap filling process, and the resulting saturation profiles, and the variable oil water contacts. 

Figure 2. The saturation distribution in the model above. Saturation is a function of both height above FWL (ie. capillary pressure (Po-Pw), and the rock type. The low saturation occurs in tight rocks thus volume is reduced by both porosity and saturation.  

Figure 3. An idealized model to show OWC can be tilted if a systematic change in pore size exist across the entire field. Observation of a tilted, or variable OWC is not always due to hydrodynamics. When studying tilted OWCs, we should investigate not only the pressure gradient, but also capillary data, which can be inferred from porosity/permeability data. The Tin Fouye Tabankort (TFT) field in Algeria may be such an example.

Figure 4. A model demonstrating the mechanism of stratigraphic traps after Tim Schowalter 1979. Note that capillary seals are relative - a trap is formed when a tighter rock is above or up dip of a less tight rock. So reservoir rock of one accumulation can be the seal for another accumulation. In nature these changes are more subtle and hard to draw the boundaries. The main observation of these mechanisms are the correlation between saturation and rock quality.  

Figure 5. The purpose of this model is to demonstrate the effect of storage along migration pathway on the distance of migration for a given volume generated by the source rock. The poor reservoir (silty, or shalely) stores less along the carrier, and the result is that same volume of supply will travel further in the same time that volume is generated compared to a better quality carrier bed - everything else being equal. Effective carrier beds do not have to be very good quality.  

Now a couple of real examples:

Figure 6. The Parshall field on the eastern side of the Williston basin. The middle Bakken reservoir gradually thins to the east with lower porosity and permeability. The field does not reach the actual pinch out of the middle Bakken. This model shows that the gradual change of the middle Bakken facies is responsible for the trap, rather than the pinch out. This is probably true with many of the subtle accumulations elsewhere in the Middle Bakken, and in other unconventional plays. The traps are subtle!

Figure 7. The Kraka field in the Danish North Sea has a tilted FWL. This is a quick model to show how it works based on data from this paper.

Please feel free to use these images in your research or teaching. You may reference the petroleum system blog, by Zhiyong He, founder of  ZetaWare Inc. 

Key references:

T. T. Schowalter, 1979, Mechanics of Secondary Hydrocarbon Migration and Entrapment; AAPG Bulletin vol. 63 (5): 723–760.

T. T. Schowalter and P. Hess, 1982, Interpretation of Subsurface Hydrocarbon Shows;

AAPG Bulletin, V66, No.9, pp. 1302-1327

P. Frykman et al., The history of hydrocarbon filling of Danish chalk fields, Geological Society London Petroleum Geology Conference series · January 2004