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Types of Crude Oil: Sweet vs. Sour Crude

How H2S and sulphur content separate sweet from sour crude, and what that means for corrosion, processing and safe carriage.

6 min read
Beginner
Ship Construction & Naval Architecture
Key Principles at a Glance 5 points
  • Crude is oil in its natural or raw state, which has not yet been processed or refined.
  • Sweet crude has hydrogen sulphide (H2S) less than 25 ppm and sulphur between 0.42% and 0.50%; it contains only small amounts of H2S and CO2 and is commonly processed into gasoline, kerosene and high-quality diesel.
  • Sour crude has hydrogen sulphide (H2S) more than 25 ppm and sulphur more than 0.50%; impurities are greater and more processing is required, so it is commonly processed into heavy fuel oil.
  • The H2S content is the key safety concern — it is a toxic and flammable gas, so sour crude demands stricter atmosphere control and gas monitoring.
  • Higher sulphur and impurities increase corrosion and refining cost, which is why sour crude commands a lower price and is used for heavier products.

1. What is Crude Oil?

Crude oil is oil in its natural or raw state which has not yet been processed or refined. It is a complex mixture of hydrocarbons with varying amounts of sulphur compounds, dissolved gases, and impurities.

Why Classification Matters:

  • Refining cost and products: The impurities present determine how much processing is needed and what products the crude can yield.
  • Corrosion: Sulphur compounds attack steel and tanks, especially when combined with water.
  • Safety: Dissolved hydrogen sulphide (H2S) is highly toxic and flammable, creating a serious hazard during carriage and handling.

Crude is broadly classified into sweet and sour crude, based on its H2S content and sulphur content.

PETROLEUM CHEMISTRY | CRUDE OIL CLASSIFICATION RAW CRUDE OIL Unrefined Liquid Hydrocarbons SWEET CRUDE • H2S Content: < 25 ppm • Sulphur Content: 0.42% – 0.50% • Low acidity • Low refinery corrosion Yield: Gasoline • Kerosene • Diesel SOUR CRUDE • H2S Content: > 25 ppm • Sulphur Content: > 0.50% • Severe steel corrosion • Toxic gas risk Yield: Heavy Fuel Oil (HFO) • Bitumen 1. DEFINITION OF CRUDE • Raw, unrefined petroleum • Complex hydrocarbon mixture • Quality set by oil field origin Natural State 2. SWEET CRUDE CRITERIA • H2S under 25 ppm • Sulphur between 0.42%–0.50% • High value gasoline & diesel Premium Feedstock 3. SOUR CRUDE CRITERIA • H2S over 25 ppm • Sulphur exceeds 0.50% • High refining cost (Heavy Fuel) Heavy Contaminants
Figure 1: Crude Oil Classification. Crude in its raw, unrefined state is classified as sweet or sour based on its hydrogen sulphide (H2S) and sulphur content, which in turn determines its refining products and handling hazards.

2. Sweet Crude

Sweet crude is the cleaner, more valuable crude oil:

  • Contains hydrogen sulphide (H2S) of less than 25 ppm.
  • Contains sulphur of between 0.42% and 0.50%.
  • Contains only a small amount of H2S and CO2.
  • It is commonly processed into gasoline, kerosene and high-quality diesel.
Why It Is Called "Sweet"

The low sulphur and H2S content makes sweet crude less corrosive and less hazardous, requires less refining, and yields higher-value light products. This is why it is prized by refiners.

SWEET CRUDE | COMPOSITION & DISTILLATION MATRIX < 25 ppm H2S CONCENTRATION Hydrogen Sulphide Safe Limit 0.42%–0.50% SULPHUR CONTENT Low Total Sulphur Range MINIMAL ACID GASES • Negligible CO2 & H2S presence • Non-aggressive to cargo tanks & pumps HIGH-VALUE REFINED PRODUCTS MOTOR GASOLINE High Octane Rating Boiling Range: 35°C – 200°C KEROSENE / JET A-1 Commercial Aviation Fuel Boiling Range: 150°C – 275°C PREMIUM DIESEL Ultra-Low Sulphur Automotive Boiling Range: 250°C – 350°C 1. LOW SULPHUR • Strictly 0.42% to 0.50% S • Gentle on tank coatings • Minimal SOx gas emissions 0.42%–0.50% S 2. H2S SAFETY • H2S under 25 ppm limit • Safe ullage headspace • Lower crew toxicity risk < 25 ppm H2S 3. REFINING MARGIN • Yields high-value distillates • Low processing complexity • Commands market price premium High Value Crude
Figure 2: Sweet Crude. With H2S below 25 ppm and sulphur between 0.42% and 0.50%, sweet crude is cleaner and is commonly refined into gasoline, kerosene and high-quality diesel.

3. Sour Crude

Sour crude is the heavier, more contaminated crude oil:

  • Contains hydrogen sulphide (H2S) of more than 25 ppm.
  • Contains sulphur of more than 0.50%.
  • Contains small amounts of H2S and CO2, but the impurities are more; removing those impurities requires more processing and higher charges.
  • It is commonly processed into heavy fuel oil.
Carriage Hazard — H2S

Hydrogen sulphide is a toxic, flammable gas that is heavier than air and can accumulate in tanks, void spaces and pump rooms. Sour crude cargoes demand stringent gas monitoring, closed gauging, and strict enclosed-space precautions before any tank entry.

SOUR CRUDE | TANK ULLAGE HAZARDS & REFINERY CHARGE > 25 ppm H2S HAZARD LEVEL High Ullage Gas Release > 0.50% TOTAL SULPHUR Forms Corrosive Acid CARGO TANK CROSS SECTION TOXIC H2S VAPOR ULLAGE Heavier than air • Collects in lower pockets SOUR CRUDE CARGO LAYER High Viscosity • Refined to Heavy Fuel Oil (HFO) REFINING & COST PENALTY • Heavy Fuel Oil (HFO) Fraction: Mainly yields marine bunker residual fuels • Desulfurization Charges: Demands high-pressure hydrogen treating Lower market crude price reflects higher cost 1. HIGH H2S LEVEL • H2S exceeds 25 ppm • Highly toxic & flammable • Paralyzes sense of smell > 25 ppm Toxic 2. HIGH SULPHUR • Sulphur exceeds 0.50% • Accelerates pipeline rust • Requires inert gas blanket > 0.50% S Corrosive 3. HEAVY END PRODUCT • Processed to Heavy Fuel Oil • High refinery hydrotreating • Traded at discount price Heavy Fuel Oil Yield
Figure 3: Sour Crude. H2S above 25 ppm and sulphur above 0.50% make sour crude more contaminated and costly to refine, so it is commonly processed into heavy fuel oil. The high H2S content also makes it a significant toxic-gas hazard.

4. Sweet vs. Sour: Comparison & Operational Implications

Property Sweet Crude Sour Crude
Hydrogen sulphide (H2S)Less than 25 ppmMore than 25 ppm
Sulphur content0.42% – 0.50%More than 0.50%
ImpuritiesLowHigh (more processing needed)
Typical productsGasoline, kerosene, high-quality dieselHeavy fuel oil
CorrosivityLowerHigher
H2S safety hazardLowerHigher — strict gas control

Operational Implications on Board:

  • Corrosion control: High-sulphur sour crude increases corrosion of tank plating and pipelines; inert gas and coating protection become more important.
  • H2S monitoring: Sour crude requires personal H2S detectors, fixed gas detection, and strict control before pump-room or tank entry.
  • Cargo documentation: The Safety Data Sheet and cargo info must state H2S and sulphur levels so the correct precautions are taken.
  • Refining economics: The extra treatment for sour crude explains its lower market value compared to sweet crude.
Surveyor Asks

Define crude first, then give the two thresholds: H2S 25 ppm and sulphur 0.50%. State what each type is refined into (sweet → gasoline/kerosene/diesel, sour → heavy fuel oil) and mention the H2S hazard on tankers.

SWEET VS SOUR | THE TWO OPERATIONAL THRESHOLDS CRITICAL BOUNDARY VALUES 1. H2S Threshold: 25 ppm SWEET < 25 ppm SOUR > 25 ppm 2. Sulphur Threshold: 0.50% SWEET: 0.42%–0.50% SOUR: > 0.50% EXAM MEMORY FORMULA Sweet = Clean • Sour = Corrosive / Heavy TANKER CARRIAGE SAFEGUARDS A. Closed Gauging (UTI Tapes) Hermetically sealed vapor lock prevents H2S escape B. Personal Multi-Gas Monitors Mandatory alarm set at 5 ppm H2S on crew chest C. Inert Gas System (O2 < 5%) Inert atmosphere prevents flammable ignition 1. CORROSION DEFENSE • Sour sulphur attacks steel • Requires epoxy tank coating • Anodes protect against pits Coating & Inert Gas 2. H2S TOXICITY • 100 ppm destroys smell nerve • 500 ppm fatal in minutes • Strict enclosed space entry Personal Gas Meter 3. REFINING VALUE • Sweet yields premium petrol • Sour yields heavy bunker fuel • Dictates crude pricing index Commercial Premium
Figure 4: Comparison & Implications. The 25 ppm H2S and 0.50% sulphur thresholds separate the two crudes. Sour crude drives higher corrosion, demands strict H2S monitoring, must be documented correctly, and is cheaper because of its higher refining cost.