Pyrolysis, Porosity and Productivity in Unconventional Mudstone Reservoirs: Free and Adsorbed Oil

Abstract

Rock-Eval pyrolysis analysis of unconventional mudstone reservoirs use the S1 peak (kg free liquid per tonne of rock) to determine the volume of oil that can be potentially produced by hydraulic fracking. The data presented in this paper were obtained from a study of the unconventional oil potential of the marine late Jurassic Kimmeridge Clay Formation of the southern Viking Graben area of the UK and Norwegian sectors of the North Sea. Typical marine silty mudstones with Ͼ2%TOC contains Ͻ1kg/t (0.1wt. %) of free oil when immature at depths Ͻ2.5km and Tmax Ͻ420°C. The peak saturation of free oil rises to values of 12kg/t, with a typical peak average of ϳ6kg free oil/tonne of rock (0.6wt.%). Such average values are reached when mature at depths of 3.4km and Tmax values of 435°C in the North Sea. Converting Rock-Eval S1 yields to volume percent, the average mature mudstone contains about 1.8vol% of free oil. How does this compare with the accessible porosity of the mudstone?Conventional log analysis for porosity measures everything that is not mineral grains as pore space, but fluid injection methods (mercury, helium) measure the effective porosity, a more realistic measure of open pores that are available to petroleum. In the case of log-derived porosities, closed porosity and structured (fixed) water are counted, but appear not to be open to Rock-Eval free oil (S1) and hence for 'saturation'. Comparing typical peak S1 volumes (1.8 vol. %) against mudstones porosities (1.62 vol. %), suggests, within error, that the open porosity is fully saturated with oil at peak maturity. In addition the distributions of porosity and S1 yields are similar.However, if the S1 yield at peak maturity is totally a function of porosity; the S1 yield should be independent of the amount or type of kerogen. This is not so. Plotting S1 yield against TOC shows a strong positive correlation, with a gradient of 0.7kg/t per 1%TOC and an intercept of about 1%TOC. This gradient indicates that the S1 'oil' is partitioned between the kerogen and the porosity. Given the size of the molecules relative to the micro-porosity of the largely amorphous kerogen, the oil is likely to be absorbed onto the kerogen surface. This explains the asymmetry (or shoulders) seen on the pyrograms of S1 peaks, the main peak representing easily liberated liquids in open porosity and the shoulder the more strongly adsorbed liquids. The additional adsorbed liquid is more efficiently recovered and can be quantified by solvent extraction of the mudstones, where saturates ϩ aromatics are shown to be over twice the Rock-Eval S1 yield. Thus, like Unconventional gas, this evidence suggests that Unconventional liquids too may derive from 'free' and 'adsorbed' oil, and that both are mainly stored within organic (kerogen) porosity.