Abstract
Abstract Compound-specific isotope analysis (CSIA) of individual organic compounds is a powerful but underutilized tool in petroleum exploration. When integrated with other organic geochemical methodologies it can provide evidence of fluid histories including source, maturity, charge history and reservoir processes that can support field development planning and exploration efforts. The purpose of this chapter is to provide a review of the methodologies used for generating carbon and hydrogen isotope data for mid- and high-molecular-weight n -alkanes. We discuss the factors that control stable carbon and hydrogen isotope compositions of n -alkanes and related compounds in sedimentary and petroleum systems and review current and future applications of this methodology for petroleum exploration. We discuss basin-specific case studies that demonstrate the usefulness of CSIA either when addressing particular aspects of petroleum exploration (e.g. charge evaluation, source rock–oil correlation, and investigation of maturity and in-reservoir processes) or when this technique is used to corroborate interpretations from integrated petroleum systems analysis, providing unique insights which may not be revealed when using other methods. CSIA of n -alkanes and related n -alkyl structures can provide independent data to strengthen petroleum systems concepts from generation and expulsion of fluids from source rock, to charge history, connectivity, and in-reservoir processes.
Highlights
Petroleum geoscientists use organic geochemistry as an essential tool in oil and gas exploration and field development planning
Low-cost, high-throughput bulk data are commonly used to screen for source rock quality (e.g. per cent total organic carbon (%TOC), hydrogen and oxygen indices) and thermal maturity (Tmax, vitrinite reflectance equivalent)
We focus on compound-specific analysis of higher-molecular-weight n-alkanes and related compounds because: (a) they are the most abundant hydrocarbon groups present both in the source rock extracts and reservoir oils; (b) they are easy to extract, separate and analyse; (c) they can be analysed for both stable C and H isotope compositions using the same sample and using the same gas chromatograph isotope ratio mass spectrometer (GCIRMS) instrument; (d) they provide a reasonable scope for in-depth review; and (e) they demonstrate the potential for growth of these underutilized techniques
Summary
The initial step of sample clean-up and fraction separation depends on the matrix, i.e. whether it is a source or reservoir rock or a liquid. The whole oil or the saturate fraction with n-alkyl compounds usually must first be analysed using a gas chromatograph flame ionization detector (GC-FID) or gas chromatograph mass spectrometer (GC-MS) to quantify the amount of sample needed to achieve reproducible results on the IRMS. To achieve the most precise δ13C and δ2H measurements, n-alkane peaks should have baseline resolution (if the sample contains other compounds in addition to n-alkanes) and a sufficient signal-tobackground ratio (which is system specific); compound-specific measurements are made using the GC-IRMS coupled with combustion (δ13C) or high-temperature conversion (δ2H) reactors, respectively. We recommend that the end users carefully evaluate the IRMS chromatograms to ensure the separation of compounds is adequate, the baseline is clean, and the integration of individual peaks is consistent throughout the run and from sample to sample
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