Print this pageSealing Properties of Source Rocks: The Example of the North Sea’s Kimmeridge Clay Formation
Olufemi A. Jokanola1, Andrew C. Aplin2, Steve R. Larter1, and Kuncho D. Kurtev2
1 University of Calgary, Calgary, AB
2 University of Newcastle, Newcastle upon Tyne, United Kingdom
The Kimmeridge Clay Formation (KCF) is the North Sea's primary source rock, efficiently expelling petroleum on restricted geological timescales. However, the KCF also acts as a cap rock in some parts of the North Sea, retaining substantial column heights. Here, we consider this apparent conundrum by combining a petrophysical study of KCF mudstones with a charge – leak model which calculates steady state column heights in dynamic petroleum systems.
Petrographic studies define three general KCF lithofacies: silt-rich, clay-rich and laminated. Independent of both lithology and effective stress, porosity decreases linearly from 26 to 3% at burial depths between 1.5 and 4.5km. Based on pore size distribution data, all samples are good to excellent capillary seals, with calculated oil column heights ranging from 200m for the more porous, silty samples to 4000m for the less porous, laminated samples. However, we question whether the capillary seal model is appropriate for source rocks, which we suggest are likely to have oil-wet pore systems. In this case, the dynamic column height will be defined by the relative rates of charge and leakage. Leakage rates are a function mainly of seal permeability and thickness. The permeability of these KCF samples ranges from 0.01 - 10nD, with thicknesses between 50 and 200m. Dynamic charge – leak modeling shows that as long as petroleum is being supplied to the reservoir, substantial column heights could be held by the KCF. If the source rock is switched off, 50% of the oil column would dissipate in 2 – 26 Ma, for caprock thicknesses of 15 – 150m. In most geological circumstances, the KCF source thus acts as an effective seal.