A Geological Model for the Origin of Fluid Compositional Gradients in a Large Saudi Arabian Oilfield: An Investigation by Two-Dimensional Gas Chromatography (GC × GC) and Asphaltene Chemistry

Abstract

The heavy oil rim of a large Saudi Arabian oilfield has been shown to be in vertical and lateral equilibrium, matching predictions of the gravity term from the Flory-Huggins-Zuo equation of state for asphaltenes in the form of 5.2 nm clusters of the Yen-Mullins model. The large (10x) vertical gradient of asphaltene concentration over a very large perimeter (>> 10 km) of the oilfield provided a stringent test of this equation of state fit. Two-dimensional gas chromatography (GC×GC) and stable isotope analysis δD and δ 13 C were used to determine consistency of the liquid phase components with equilibration and the effects of biodegradation or thermal maturity on the observed asphaltene gradient. These analyses confirm homogeneity of equilibrated liquid phase components of similar chemical character and equilibrated asphaltene isotopes. Biodegradation is minimal and there is no maturity variation among the samples. Thus, the large asphaltene gradient did not result from these secondary processes and is not remnant from how the reservoir charged with crude oil. The results are consistent with original findings that the oil column is equilibrated. Thermodynamic equilibration over such large distances (>10 km) requires convective currents and provides constraints on fluid dynamic processes in reservoirs. A simple 1-D three-component single-phase model is introduced to account for asphaltene accumulation by way of convective currents established from a diffusive gas front at the top of the oil column.

Evaluation of the Performance of Cryogen-Free Thermal Modulation-Based Comprehensive Two-Dimensional Gas Chromatography-Time-of-Flight Mass Spectrometry (GC×GC-TOFMS) for the Qualitative Analysis of a Complex Bitumen Sample

“…As seen in Figure 5A, C 29 αββ-20, eluting at a 1 D retention time of about 71 min was initially identified when evaluating the m/z 217 for steranes in the group analysis. The compound identity was confirmed based on an analogous mass spectrum found in the literature [6]. EICs assisted in extracting the individual compounds since cyclic terpanes and diasteranes were found in low abundances and overlapping within the biomarker region.…”

“…Additionally, the triaromatic species are said to be more sensitive to thermal maturity, since they originate from the aromatization of monoaromatic species, which causes them to lose a methyl group [19]. The final group, methyl hopanes, was identified by m/z 205 (red) [6]. These compounds were found in high abundance and easily identified.…”

“…GC×GC-TOFMS proved again to be an appropriate tool by resolving typical 1D GC coelutions and providing spectral deconvolution. Biomarker identification by GC×GC-TOFMS was also shown as a powerful tool in studying the chemical attributes of asphaltene [6]. By studying the biomarkers and biomarker ratios, the thermal maturity was determined.…”

Asphaltenes: Separations, structural analysis and applications

“…GC is a primary method for identifying low-MW and thermally stable hydrocarbons in asphaltenes. One-dimensional GC (1D-GC) is largely employed for analyzing heteroatomic (like N and S) compounds in asphaltene with selective detectors, such as nitrogen chemiluminescence detector (NCD), nitrogen phosphorus detector (NPD), sulphur chemiluminescence detector, pulsed flame http://engine.scichina.com/doi/10.1016/j.jechem.2018.10.004 photometric detector and atomic emission detector [126][127][128][129] . The limits of detection were usually about 0.4 pg N/s and 0.5 pg S/s for nitrogen and sulfur, respectively [126,129,130] .…”

“…GC ×GC/TOF MS could also identify chemical structures of O-and S-compounds in asphaltenes [132,133] , mainly containing diols, phenolic compounds, benzofurans, naphthols, and indanols. Additionally, GC × GC assisted by stable isotope determination ( δ D and δ 13 C) were applied to affirm homogeneity of equilibrated liquid-phase components of similar chemical property and equilibrated asphaltene isotopes in Saudi Arabian oilfield [128] .…”