Oil-source mass spectrometry analysis is a core geochemical experiment that uses GC-MS, GC-MS/MS, and isotope-ratio mass spectrometry to characterize the molecular fingerprints of crude oils and source rock extracts. This technical guide explains the sample types, testing items, analytical methods, quality controls, and practical applications of oil-source correlation studies in petroleum exploration and reservoir evaluation.
Technical Overview
The oil-source mass spectrometry analysis experiment is a systematic analytical workflow used in petroleum geochemistry to determine the genetic relationship between crude oils and the source rocks that generated them. The technique combines solvent extraction, liquid chromatography fractionation, and high-sensitivity mass spectrometric detection to identify and quantify biomarkers—molecular fossils such as steranes, terpanes, isoprenoids, and aromatic compounds—that retain diagnostic information about biological origin, thermal maturity, depositional environment, and post-generation alteration.
Because biomarker distributions are inherited from the original organic matter and are relatively resistant to secondary processes, mass spectrometry provides a reliable "molecular fingerprint" for oil-oil and oil-source rock correlation. Modern accredited third-party laboratories and petroleum research institutes routinely perform this experiment following national and industry standards such as GB/T 18606 and SY/T 5119, supported by ISO/IEC 17025 quality management systems.
Testing Samples
The experiment accepts a wide range of geological and produced fluid samples. Proper sampling, labeling, and storage are essential to prevent contamination and volatile loss.
Crude Oil and Condensate Samples
Surface production oils, wellhead samples, DST/RFT fluids, and condensates can be analyzed directly after dehydration and removal of particulates. A typical submission of 5–20 mL of oil is sufficient for a full biomarker and isotopic workup.
Source Rock Cuttings and Core Samples
Drilling cuttings and conventional or sidewall cores are cleaned to remove drilling mud, ground to fine powder, and extracted with organic solvents to obtain the soluble organic matter (chloroform bitumen "A" or total extract). A minimum of 50–100 g of rock is generally recommended, depending on total organic carbon content.
Reservoir Rock and Oil Sand Extracts
Reservoir cores, oil-stained sands, and shows are extracted to recover movable hydrocarbons for comparison with produced oils, supporting reservoir continuity and fill-history studies.
Kerogen and Isolated Organic Matter
After extraction and carbonate/silicate demineralization following procedures such as GB/T 19144, isolated kerogen can be characterized by pyrolysis-GC-MS to assess source potential and generate kerogen-specific fingerprints for direct oil-source comparison.
Oil Seeps, Bitumens, and Contamination Screening Samples
Natural oil seeps, solid bitumens, and samples potentially contaminated by oil-based drilling fluids can be submitted to distinguish indigenous formation hydrocarbons from synthetic or diesel-based mud additives.
Testing Items
The analytical program is organized into the following testing items, each targeting a specific geochemical question:
- Saturated hydrocarbon biomarkers: n-alkane distribution, acyclic isoprenoids (pristane/phytane ratio), C27–C29 regular steranes, diasteranes, C27–C35 hopanes and moretanes, gammacerane, β-carotane, oleanane, and Ts/Tm ratios.
- Aromatic hydrocarbon biomarkers: phenanthrene and methylphenanthrene series (MPI-1, MPI-3 maturity indices), dibenzothiophene and methyldibenzothiophenes, monoaromatic and triaromatic steranes, and alkylnaphthalenes.
- Light hydrocarbons and diamondoids: adamantane and diamantane concentrations for assessing thermal cracking and high-maturity condensates.
- Molecular ratios and calculated parameters: carbon preference index (CPI), odd-even predominance (OEP), sterane C29 20S/(20S+20R), C29 ββ/(αα+ββ), homohopane distribution C35/C34, and moretane/hopane ratios.
- Compound-specific stable carbon isotopes: δ13C values of individual n-alkanes and isoprenoids by GC-IRMS, plus bulk δ13C of saturated and aromatic fractions, to distinguish marine, lacustrine, and terrigenous organic input.
- Quantitative biomarker concentrations: absolute concentrations (µg/g oil or µg/g TOC) of key biomarkers using internal standards.
Testing Methods
The experiment follows a multi-stage workflow that combines sample preparation, chromatographic separation, and mass spectrometric detection.
Sample Preparation and Fractionation
Rock samples are cleaned, dried, and pulverized, then extracted by Soxhlet apparatus or accelerated solvent extraction using dichloromethane and methanol. Crude oils are deasphalted with n-hexane. The total extract or oil is separated by silica gel–alumina column chromatography into saturate, aromatic, and polar (resin) fractions in accordance with methods such as SY/T 5119. Asphaltenes are precipitated and weighed separately, completing the SARA characterization.
GC-MS Analysis
Saturate and aromatic fractions are analyzed on a gas chromatograph coupled to a quadrupole or magnetic-sector mass spectrometer, typically with a 30–60 m DB-5MS type capillary column, helium carrier gas, and 70 eV electron ionization. Data are acquired in both full-scan mode (for compound identification) and selected ion monitoring mode (for sensitive quantification of characteristic ions such as m/z 191 for terpanes and m/z 217/218 for steranes). Procedures follow the methodology described in GB/T 18606.
GC-MS/MS Analysis
For complex or biodegraded samples where co-elution limits conventional GC-MS, tandem mass spectrometry in metastable reaction monitoring mode is applied. Parent-to-daughter transitions, for example m/z 372→217 for C27 steranes or m/z 370→191 for C29 pentacyclic triterpanes, provide significantly enhanced selectivity and lower detection limits.
GC-IRMS and Pyrolysis-GC-MS
Compound-specific isotope analysis is performed by gas chromatography–isotope ratio mass spectrometry to measure δ13C of individual compounds with precision typically better than ±0.5‰. Kerogen and source rock samples may additionally be characterized by pyrolysis-GC-MS to release bound biomarkers for direct comparison with produced oils.
Quality Assurance and Quality Control
Each analytical batch includes procedural blanks, duplicate samples, internal standard recovery checks, and calibration verification. Reference oils of known biomarker composition are analyzed routinely to confirm instrument stability. Reputable testing is conducted in ISO/IEC 17025 accredited third-party laboratories or qualified petroleum geochemistry research facilities, ensuring traceable and defensible results.
Applications
Oil-source mass spectrometry analysis supports a broad range of exploration and development decisions:
- Oil-source rock correlation: establishing genetic links between discovered petroleum accumulations and potential source intervals to validate petroleum system models.
- Thermal maturity assessment: constraining generation stage and comparing oil maturity with source rock maturity using sterane and aromatic maturity parameters.
- Depositional environment interpretation: distinguishing marine, lacustrine, swamp, and evaporitic source settings from gammacerane, Pr/Ph, and homohopane signatures.
- Biodegradation evaluation: grading the extent of microbial alteration and reconstructing the original oil composition using resistant biomarkers.
- Reservoir connectivity and charging history: comparing oil families across fault blocks and reservoir compartments to identify multiple charges or barrier-controlled compartmentalization.
- Unconventional resource evaluation: characterizing shale oil and tight oil mobility, maturity, and oil window placement in self-sourced reservoirs.
- Contamination identification: differentiating genuine reservoir hydrocarbons from oil-based mud and synthetic drilling fluid contamination in exploration wells.
FAQ
How much sample is required for the analysis?
For crude oils, 5–20 mL is typically sufficient. For rock samples, 50–100 g of cuttings or core is recommended, although samples with high total organic carbon may require less. Highly depleted samples can often be accommodated using GC-MS/MS with its lower detection limits.
What is the difference between GC-MS and GC-MS/MS in this experiment?
GC-MS monitors a single stage of mass analysis and is suitable for most oils and source extracts. GC-MS/MS performs an additional precursor-to-product ion transition, which removes matrix interference and co-eluting peaks, making it the preferred method for trace-level, heavily biodegraded, or high-maturity samples.
Can biodegraded oils still be analyzed?
Yes. Although n-alkanes and light hydrocarbons are consumed during biodegradation, polycyclic steranes and terpanes are highly resistant and remain diagnostic. GC-MS/MS analysis of these compounds allows correlation and alteration grading even for severely degraded heavy oils.
How long does a complete oil-source correlation study take?
A routine single-sample biomarker analysis typically requires 5–10 working days, while a full oil-source correlation project involving multiple oils, source extracts, kerogens, and isotopic analyses may require two to four weeks depending on sample count and complexity.
Which standards govern this experiment?
Commonly applied standards include GB/T 18606 for biomarker analysis of source rocks and crude oils by GC-MS, SY/T 5119 for family component (SARA) separation, and GB/T 19144 for kerogen isolation. Testing quality is managed under ISO/IEC 17025 laboratory accreditation requirements.
How are the results interpreted for oil-source correlation?
Interpretation involves multivariate comparison of biomarker ratios (for example, C27/C28/C29 sterane distributions, Ts/Tm, and gammacerane index) and isotope profiles between oils and candidate source rocks. A strong match across multiple independent parameters indicates a genetic relationship, which is then integrated with burial history and migration pathway modeling to build a coherent petroleum system narrative.
Can contaminated or weathered samples be analyzed?
In most cases, yes. Laboratory cleaning protocols remove water-based mud, and diagnostic compound screening distinguishes oil-based mud additives from indigenous hydrocarbons. Moderately weathered seeps and bitumens retain sterane and terpane fingerprints suitable for correlation, although data interpretation must account for the alteration effects.
