Comparison of palaeo oil charges with currently reservoired hydrocarbons using the geochemistry of oil-bearing fluid inclusions

S.C. George, M. Lisk, P.J. Eadington, R.A. Quezada, F.W. Krieger, P.F. Greenwood, CSIRO Division of Petroleum Resources and Australian Petroleum Cooperative Research Centre, and M.A. Wilson, Department of Chemistry, University of Technology Sydney Abstract This paper describes the use of oil-bearing fluid inclusions as time-specific markers of different oil migration events. Fluid inclusions are small samples of pore fluid trapped in framework minerals due to brittle deformation and fracturing during burial, or in diagenetic minerals during crystallisation. Recent advances in analytical techniques and instrumentation mean that it is now feasible to obtain detailed geochemical data on the trapped oils, which can be directly compared with either reservoired oils or with putative source rocks. Two techniques are currently being used, an off-line method which is suitable for comparing high molecular weight biomarker distributions and an on-line method which provides compositional data for the complete fluid which is trapped, including low molecular weight hydrocarbons and gases. In this paper the results from two case studies involving the Jabiru and South Pepper oilfields are discussed. At Jabiru the oil trapped in fluid inclusions is genetically-related to, but is less mature than, the currently reservoired charge. This suggests continued expulsion of progressively more mature oil from the same or similar source rock facies. At South Pepper the fluid inclusion oil is both more mature and derived from a different, more calcareous source rock than the currently reservoired oil. The early oil is most likely derived from Triassic source rocks and was heavily biodegraded before re-charge of the South Pepper structure with Jurassic-sourced oil and gas. Molecular composition of inclusion (MCI) studies such as these are powerful tools for elucidating the petroleum charge history to reservoirs. They provide molecular geochemical data on early oil charges which, when compared to presently reservoired oil, can be used to describe the changing nature of reservoir fluids through time. They are successfully being applied to other reservoirs on the North-west Shelf of Australia, either in dry holes or where there is currently gas and where fluid inclusion oil represents the only sample of oil charge. Introduction Generated oil migrating to a reservoir rarely displays a uniform composition over time. Source rock horizons become more mature during burial, so will generate oil with progressively changing geochemistry. Deeper source rocks start generating oil, leading to variations in geochemical source parameters in the later oil charges. Earliest oil charge is likely to fill the largest pores and continued migration will fill progressively smaller pores and may mix with earlier charges. Reconstructing the oil charge history from the currently reservoired petroleum is complex, with considerable uncertainty as to the timing, source and phases of oil charge. One difficulty is the absence of end-member signatures due to the homogenisation process in the reservoir. One approach is the use of oil-bearing fluid inclusions as time-specific markers of different oil migration events. Oil inclusions are small samples (usually <10 m in diameter) of pore fluid encapsulated in framework minerals such as quartz, feldspar and carbonate as they crystallise. Oil inclusions form during the crystallisation of diagenetic minerals and through the brittle deformation and fracturing of framework minerals (detrital or diagenetic) during burial. They can be readily detected by fluorescence microscopy because of their distinctive fluorescence emission colours. The presence of high abundances of oil inclusions in the gas or water zones of reservoirs have previously been used to detect palaeo oil columns or residual zones. Recent advances in analytical techniques and instrumentation mean that it is now feasible to obtain detailed geochemical data on the trapped oils, which can be directly compared with either reservoired oils or with putative source rocks. Several previous studies have examined the molecular composition of oil inclusion. One important application of the analysis of these samples of palaeo-oil is to correlate and compare them with currently reservoired oils and deduce hydrocarbon charge histories. P. 159