Jones, D. M., G. Macleod, S. R. Larter, D. L. Hall*, A. C. Aplin, M. Chen
Newcastle Research Group In Fossil Fuels and Environmental Geochemistry, Postgraduate Institute, Newcastle upon Tyne, Drummond Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK.
*Amoco Exploration and Production Technology Group, P. O. Box 3385, Tulsa, OK, 74102, USA.
Much recent development in the field of molecular characterisation of included petroleum is documented in the literature (Jensenius and Burruss, 1990, Macleod, 1993, Karlsen et. al., 1993, Macleod 1994, George et. al., 1995); however, obtaining accurate molecular characterisation of the minuscule volumes of petroleum contained in petroleum-bearing fluid inclusions hosted in diagenetic cements remains difficult and non-routine. Thorough cleansing of the surface of the cements hosting the petroleum inclusions is required to remove non-included petroleum contamination. The cleaned mineral grains hosting the petroleum-bearing inclusions of interest, are then crushed below solvent (crush-leach), and the organic molecular marker compounds or "biomarkers" concentrated and analysed. A vital step in the process is detailed, pre-analytical petrographic analysis to allow generations or populations of petroleum fluid inclusions to be identified and enable geochemists to accurately interpret what is being analysed.
Within the laboratories of the Newcastle Research Group, we have suspected that the petroleum being analysed during our "crush-leach analysis" of petroleum fluid inclusions was at times not just included petroleum, but also petroleum firmly adhered to the surface of the cements being analysed. As we are analysing tiny volumes of petroleum any surface contaminant, however small, will become significant and could invalidate the GC-MS data and any interpretations based on it. To address this potential problem, we have developed an analytical methodology in which surfaces of the cements being crushed are spiked with standard compounds, and successive cleaning and GC analysis of the solvent washings, indicates when all surface-adhered petroleum compounds and standards have been removed. Then, and only then, is the host cement crushed below solvent, and the liberated petroleum analysed via GC-MS.
With these new capabilities we have more confidence that truly included petroleum is being exclusively analysed. Two studies serve to illustrate the technique. Carbonate-hosted petroleum inclusions were identified in several dry holes from frontier portions of the Veracruz Basin where there is limited well control and limited petroleum production. Identification of liquid petroleum bearing fluid inclusions in these rocks, including fairly shallow and young lithologies, suggests that some petroleum migration has occurred in the recent geological past. Microthermometric data suggests three general hydrocarbon zones as a function of depth: 1) shallow, heterogeneous oil and gas; 2) intermediate, near-bubble/dew point oil/condensate; and 3) deep, undersaturated condensate. Biomarker analysis of oil inclusions ties migrated petroleum to marine, carbonate-sourced oils of variable maturity with similarities to produced oils.
We will also present data from a North Sea reservoir, where "biomarker" and compositional analysis of included petroleum has been combined with micro-thermometry to construct a filling history for the reservoir. The compositional and micro-thermometry data have also been used to identify and time compartmentalisation of the reservoir. In the past, the reservoir units contained a homogenous black oil, and were in fluid and pressure communication. Now, however, the reservoir is pressure-compartmentalised and contains condensate and black oil. Such information has a direct impact on production strategy and near field exploration plans.