|Abstract: ||Abstract: The experimentally determined kinetics for hydrocarbon generation from source rocks have been routinely evaluated for predicting oil and gas occurrences in petroleum systems. Most conventional, open-system pyrolysis techniques provide kinetic parameters for predicting only bulk petroleum (bitumen + oil + gas). The non-synchronous generation of bitumen, oil and gas, however, hinders the application of the bulk kinetics to the accurate prediction of the timing and yield ratio of gas/oil. The present study attempts to measure the kinetic parameters of liquid petroleum (bitumen + oil only) generation from source rocks in a closed-system by improving the innovative technique to monitor in situ the fluorescence spectra of pyrolysates during pyrolysis (Huang et al,1998).
The starting samples include eleven oil-prone kerogens demineralized from marine source rocks (Type II), sulfur-rich marine source rocks (type IIS), lacustrine source rocks (type I), a torbanite (boghead coal), and a separated liptinite maceral. The pyrolysis experiments were conducted at five heating rates (1, 3, 8, 25, 50 deg./min) at temperatures up to 600°C using a hydrothermal diamond anvil cell mounted on a microscope equipped with reflected blue light from a high pressure mercury lamp HBO 100. The fluorescence intensity increase during oil generation was monitored in situ to estimate the kerogen-to-(bitumen+oil) transformation, which, in turn, was used to calculate the kinetic parameters for liquid petroleum generation.
The preliminary results show that fluorescence responses of highly oil-prone samples (e.g. lacustrine) occur in the temperature range closely corresponding to those of bulk petroleum (mostly bitumen + oil) generation, confirming that fluorescence intensity is approximately synchronized with the extent of the liquid petroleum generation. The calculated activation energies (Ea) and its distribution using KINETICS program (Braun et al, 1987), by assuming a similar frequency factor (7 to 9 x 1013 1/sec), is very close to that (54 kcal/mol) determined by the Rock-Eval method. Application to other oil-prone source rocks reveals that most studied kerogens tend to possess narrow Ea distribution (< 2 kcal/mol), close to a single overall Ea, implying a narrow liquid petroleum generation window. The activation energies for the studied samples, except Type IIS and torbanite, are similar (53 to 55 kcal/mol) if the similar frequency factor (Ao) was assigned during the KINETICS modeling. The extrapolation to geological conditions using 10°C /My heating rate shows that the peaks of liquid generation from these kerogens occur in a relatively small temperature range (from 143°C to 158 °C with interval < 20°C)( Figure 1.). The Ea for the sulfur-rich kerogens are slightly lower (53 kcal/mol or less) whereas the Ea for torbanite is slightly higher (55 kcal/mol). At the geological heating rate, one of the sulfur-rich kerogens generates peak liquid petroleum at lowest temperature (143°C) while torbanite generates liquid at the highest temperature (167°C). The abnormal kinetic behaviors of these two types of source rock are consistent with the previous observations (Lewan et al, 2002; Dessot et al, 1997).