Crude oil group composition analysis, commonly known as SARA analysis, quantifies the saturate, aromatic, resin, and asphaltene fractions that govern how a crude oil behaves during production, transportation, blending, and refining. Accredited third-party laboratories perform this testing in accordance with internationally recognized ASTM and IP standards to support reservoir characterization, flow assurance studies, compatibility assessment, and crude valuation. This article explains the analytical principle, applicable sample types, measured parameters, standard test methods, and typical industrial applications of hydrocarbon group-type analysis.
Technical Overview
Crude oil group composition analysis (also referred to as hydrocarbon family or group-type analysis) separates the soluble organic matter of a crude oil into chemically distinct fractions based on polarity, aromaticity, and solubility behavior. The most widely used framework is the SARA scheme, which divides the oil into four fractions: saturates (normal and iso-paraffins together with naphthenes), aromatics (mono-, di-, and poly-nuclear aromatic hydrocarbons), resins (polar compounds containing heteroatoms such as nitrogen, sulfur, and oxygen), and asphaltenes (high-molecular-weight, highly polar material that is insoluble in normal alkanes but soluble in aromatic solvents such as toluene).
The relative proportions of these four fractions exert a controlling influence on nearly every physical and chemical property of petroleum fluids. SARA distribution correlates with viscosity, density, interfacial behavior, emulsion-forming tendency, wax and asphaltene precipitation risk, thermal conversion yields, coke formation propensity, and catalyst deactivation potential in downstream processing. Because the resin and asphaltene fractions stabilize the colloidal structure of crude oil, group composition data are also used to calculate stability indices, such as the colloidal instability index (CII), defined as the ratio of (saturates + asphaltenes) to (aromatics + resins).
Group composition analysis is typically complemented by simulated distillation, elemental analysis, density and viscosity measurement, and PVT studies. Together, these data form the chemical foundation for reservoir fluid characterization, refining strategy development, and commercial crude quality assessment.
Testing Samples
Stabilized (Dead) Crude Oil
The most common sample type is stabilized crude collected from wellheads, separators, export meters, storage tanks, or shipping terminals. Samples should be homogeneous, free of water and suspended solids, and collected in sealed containers to prevent loss of light components and oxidation of the polar fractions. A minimum of 10 to 50 grams is generally sufficient for full SARA determination by column chromatography.
Live Oil and Recombined Reservoir Fluids
Pressurized bottomhole samples or recombined fluids from PVT studies are first flashed to atmospheric conditions, and the resulting stock-tank oil is subsequently analyzed for group composition. This allows SARA results to be integrated with phase-behavior and asphaltene-onset-pressure studies.
Condensates and Light Crude Oils
Gas condensates and light crude oils contain a high proportion of volatile saturates, so careful handling, including chilled transport and analysis in sealed vessels, is required to prevent evaporative loss of light ends from biasing the saturates fraction.
Heavy Oil, Extra-Heavy Oil, and Natural Bitumen
Heavy and ultra-heavy feedstocks are characterized by elevated resin and asphaltene contents. Pre-heating and thorough homogenization are typically necessary before sub-sampling to ensure representative aliquots of these viscous materials.
Refinery Streams and Residual Fractions
Atmospheric and vacuum residua, vacuum gas oils, deasphalted oils, slurry oils, and visbreaker or coker products can all be characterized by the same group-type framework, providing essential input for conversion-unit feed evaluation and fouling studies.
Source Rock Extracts and Core Samples
In petroleum geochemistry, organic extracts obtained from core or cutting samples by Soxhlet or accelerated solvent extraction are analyzed for group composition to support oil-source correlation, maturity assessment, and biodegradation studies.
Testing Items
A standard crude oil group composition report typically includes the following measured and derived parameters, all expressed on a sample mass basis:
- Saturates content (wt%)
- Aromatics content (wt%)
- Resins (polar fraction) content (wt%)
- Asphaltenes content (wt%), determined as n-heptane insolubles and, on request, n-pentane insolubles for comparison
- Total mass recovery and mass-balance closure for the complete separation
- Derived indices, including the saturates/aromatics ratio, the resins/asphaltenes ratio, and the colloidal instability index (CII)
- Optional extended determinations, such as maltene sub-fractionation and proximate n-alkane solubility profiling
Testing Methods
Asphaltene Precipitation
The analysis normally begins with precipitation of the asphaltene fraction using a large excess of normal heptane, following ASTM D6560 and IP 143, or ASTM D3279 for n-heptane insolubles. The precipitated asphaltenes are washed, dried, and weighed gravimetrically, while the filtrate (maltenes) proceeds to chromatographic separation.
Adsorption Column Chromatography
The maltene fraction is separated into saturates, aromatics, and resins by elution through packed columns of activated silica gel and alumina using solvents of increasing polarity. ASTM D4124 describes the classical four-fraction separation scheme, while ASTM D2007 (clay-gel adsorption chromatography) provides an alternative approach widely applied to petroleum fractions. Each eluted fraction is solvent-stripped and quantified gravimetrically.
Thin-Layer Chromatography with Flame Ionization Detection (TLC-FID)
For high-throughput screening and small-sample applications, automated TLC-FID instrumentation separates the four SARA fractions on quartz rods coated with sintered silica, which are then scanned by a flame ionization detector. ASTM D7996 and IP 469 standardize this rapid micro-scale technique, which requires only milligram quantities of sample.
High-Performance Liquid Chromatography (HPLC)
Instrumental HPLC methods with mass-sensitive detectors offer automated group-type separation with good reproducibility, and are increasingly used for routine monitoring of large sample sets in refinery and research laboratories.
Quality Assurance
Accredited third-party laboratories operating under ISO/IEC 17025 quality systems validate group composition methods through duplicate analysis, spike recovery, and mass-balance verification, with acceptance criteria typically requiring total recovery between 97% and 103%. Reference crude oils and internal quality control samples are analyzed alongside client samples to demonstrate method control.
Applications
Group composition data are used across the entire petroleum value chain, from the reservoir to the refinery and into environmental and commercial domains:
- Refinery feedstock evaluation: predicting distillate yields, conversion-unit performance, coke formation, and catalyst deactivation potential.
- Flow assurance: assessing asphaltene precipitation and deposition risk in wellbores, flowlines, and processing facilities, often combined with onset-pressure measurements.
- Blending compatibility: evaluating whether crude oils or refinery streams will remain stable when mixed, using CII and resin-to-asphaltene ratios to anticipate sludge formation and fouling.
- Geochemistry and exploration: supporting oil-oil and oil-source correlation, thermal maturity evaluation, and biodegradation grading through saturate and aromatic distributions.
- Enhanced oil recovery: characterizing fluid behavior during gas injection, solvent flooding, and chemical EOR processes.
- Environmental forensics: fingerprinting spilled oil and tracking weathering-induced changes in the polar and asphaltene fractions.
- Crude marketing and valuation: providing chemical evidence for quality disputes, price differentials, and pipeline specification compliance.
FAQ
What is the difference between n-heptane and n-pentane asphaltenes?
The precipitation solvent defines the asphaltene fraction. n-Pentane dissolves part of the heavy polar material that n-heptane precipitates, so n-pentane insoluble contents are consistently higher. Modern standards, including ASTM D6560 and IP 143, specify n-heptane as the reference solvent, and results should always be reported with the solvent identified.
How much sample is required for SARA analysis?
Classical column chromatography generally requires 1 to 10 grams of oil, with 10 grams or more recommended for heavy feedstocks. TLC-FID methods can produce a full four-fraction profile from less than 10 milligrams, making the technique suitable for archived or extremely limited samples.
Which method should be selected for a given project?
Column chromatography (ASTM D4124 / ASTM D2007) offers the highest accuracy and is preferred for reference characterization and contractual disputes. TLC-FID (ASTM D7996 / IP 469) is preferred when rapid turnaround, high sample throughput, or minimal sample consumption is the priority. Many projects combine both, using TLC-FID for screening and column chromatography for confirmation.
How is SARA data used to judge crude stability?
As a widely applied rule of thumb, a colloidal instability index below approximately 0.7 indicates a stable system, while values above 0.9 suggest a significant risk of asphaltene flocculation or sludge formation. These thresholds are empirical and should always be interpreted alongside onset measurements and operational experience.
Can biodegraded or heavily weathered crudes be analyzed?
Yes. Biodegraded oils typically show depleted saturates and enriched resins and asphaltenes, and the method is in fact a key tool for grading biodegradation. Sample preparation protocols are adjusted to handle the elevated viscosity and high polar content of such fluids.
What turnaround time is typical?
Depending on the method and laboratory workload, SARA analysis by column chromatography is commonly completed within five to ten working days, while TLC-FID screening can often be returned within two to five working days. Urgent programs can usually be accelerated by prior arrangement with accredited third-party laboratories.
