A hydrous pyrolysis study to ascertain how gas yields and the extent of maturation for a partially matured source rock and bitumen in isolation compared to their whole source rock

Abstract

In order to ascertain whether kerogen and the heavy oil or bitumen generated during the initial stages of source rock maturation then go on to produce similar gas yields compared to the natural situation when they are present in source rocks, a study has been conducted on an immature type II Kimmeridge Clay Formation (KCF) source rock (Dorset, United Kingdom), with an initial vitrinite reflectance (VR) of 0.31% Ro and total organic carbon (TOC) content of 14.4%. Water (hydrous) pyrolysis experiments were conducted on the whole immature source rock, the isolated partially matured bitumen-extracted source rock and the isolated bitumen (bitumen generated and extracted from the initial source rock) and mixtures of the isolated partially matured source rock and bitumen using a 22ml Hastalloy vessel at 320–420°C for 7–48h over a pressure range of 115–500bar. Hydrocarbon gas yields and the increase in VR were greater for the whole rock experiments compared to the experiments on the isolated partially matured source rock and isolated bitumen (no mineral matter present), as well as the experiments on mixtures of the isolated partially matured isolated source rock and bitumen combined. Hydrocarbon gas yields and VR values were found to decrease at 500bar compared to 180bar at 350°C where the effects of pressure retardation were found to be much more significant for the partially matured source rock maturation and bitumen in isolation than for the whole source rock. At 420°C, gas generation was not affected by pressure, but VR decreased going from 310bar to 450bar. The results obtained demonstrate that the interplay of inherent mineral matter, reactant phase, source rock/kerogen, bitumen and pressure are key factors in determining the extent of hydrocarbon generation and source rock maturation in geological basins.