Geochemistry in petroleum exploration

INTRODUCTION

1.1 PHILOSOPHY

We must understand how oil & gas are formed and use this knowledge to locate new HC reservoirs
1.2 FORMATION OF OIL & GAS

o Example of natural gas characteristics according to their maturity:

Evolution

C1/C2+

IC4/nC4

Early diagenesis

>0.97

>6

Catagenesis

<0.98

<1

Metagenesis

>0.97

o Plants and algae’s are buried in fine grained sediments and absence of O2.

o The organic matter is transformed in kerogen by low temperature chemical & biological reactions.

o The large molecules of kerogen are the precursors of oil & gas.
1.2.1 DIAGENESIS

o Methanogen microorganisms transform debris in biogenic CH4
1.2.2 CATAGENESIS

o Temperature rises and the bacterial action stops (temp >80°C)

o Thermal reactions break
the kerogen in smaller molecules called bitumen in an early stage, then,
with increasing maturity, to oil, condensate and finally gas.
1.2.3 METAGENESIS

o The thermal process continues and creates smaller molecules: thermal CH4

1.2.4 MIGRATION

o The HC are expelled from the source rock and migrate to a trap where it will accumulate.
1.3 APPLICATION

3 types of geochemical models : organic facies, thermal maturity and volumetric.
2 ORGANIC FACIES

2.1 THE CARBON CYCLE

o Most of organic matter is
returned to the atmosphere through the carbon cycle (photosynthesis),
only 1% of the photosynthetic production is preserved in sediments.

o The oxidation in the sediments (down to 300m) will bring the Total Organic Carbon (TOC) value down to 0.1%.
2.2 FACTORS INFLUENCING ORGANIC RICHNESS

The 3 factors influencing the amount of organic matter in a sediment are: productivity, preservation and dilution.
2.2.1 PRODUCTIVITY

o Shallow water is the most productive environment (Plants + light: 20 000t/km2)

o In high water zones, water upwelling can increase productivity
2.2.2 PRESERVATION

o Preservation is the most important factor for organic richness. It is linked to:

􀂾 Anoxia : linked to stagnancy, the sediments are very dark.

􀂾 Oxygen Minimum Layer
(OML): the O2 request is higher than the production, due to decay of
organic matter falling from the upper photic zone.

2.2.3 DILUTION

􀂾 Rapid burial

If the burial becomes extremely rapid, the dilution may become higher than the preservation (Specially in shales)
3 ORGANIC CHEMISTRY AND ISOTOPES

3.1 NAMES & STRUCTURES

3.1.1 HYDROCARBONS

o Only C & H

􀂾 Alkanes or paraffins or saturated:

• straight line = n-alkanes (CnH2n+2) (normal…)

• Non linear = iso, meta, etc or use the methyl (CH3) radical

• Cyclic (CnH2n) every C is linked to 2 C and 2 H

• Isopropenoids: straight chain of C with a methyl on every 4 C

Unsaturated: 2differentheptanes

• Able to combine with additional H

• Alkenes or olefins: double bond between C’s : react easily and do not persist in geological environment

• Aromatics are cyclic
alkenes but they are stable (BTX): they alternate single and double
bonds (delocalised electron) between the C’s
3.1.2 HETEROCOMPOUNDS (NSO)

o Contains also N, O or S

o Very often converted to HC during dia & catagenesis

o Porphyrins (from chlorophyll) are often present in oil

o Asphaltenes: big , highly aromatic molecules
3.2 STEREOCHEMISTRY & STRUCTURES

o Stereochemistry: 3D representation of the molecules

o Isomer: Same chemical formula but different atomic arrangement.(ie; nC4, iC4)
3.3 REACTIONS

o Oxidation: loss of e- which goes to the oxidation agent, or loss of H

o Reduction: gain of e- which comes from the reduction agent or gain of H

o Isomerisation: an isomer is converted to another
3.4 ISOTOPES

o Variation of the number of neutrons

4 KEROGEN

4.1 INTRODUCTION

o Kerogen = large organic matter molecules, insoluble in organic solvents.

o Bitumen = soluble portion.
4.2 FORMATION

JCD 09/2003 5/7 GEOCHEMISTRY
4.3 COMPOSITION

Maceral equivalent of a mineral for organic matter
4.4 MATURATION

o Maturation is due to high temperatures during a long time

o A mature kerogen becomes
more aromatic , as these molecules can stack neatly, the reflectance
increases. The vitrinite reflectance assesses the kerogen maturity.

5 RESERVOIR TRANSFORMATIONS

Two types of transformations:

o Non thermal process

􀂾 Water washing

􀂾 Biodegradation

􀂾 Leakage, transmigration

o Thermal process :

􀂾 Cracking

􀂾 Deasphalting

5.1 WATER WASHING AND BIODEGRADATION

o Unable to differentiate the 2 phenomenon as they occur frequently together and biodegradation hides the water washing effect.
5.1.1 WATER WASHING

o affects high soluble components: benzene, toluene, light alkanes.
5.1.2 BIODEGRADATION

o affects n alkanes.

o GOR, light HC content increase and API gravity decrease.

o Viscosity, sulfur content increase.

o Temperature above 80°C will kill bacteria and stop biodegradation. (around 2000m deep)
5.2 OIL SEGREGATION BY GRAVITY

5.3 LEAKAGE OF CAP ROCK, DYSMIGRATION THROUGH FAULT

o Can be reversed ! (top with a lot of gas but less pressure than the bottom or different pore sizes)

May result in 2 reservoirs,
the upper one with high API gravity (light oil or gas), the lower one
with residual heavy HC and low API gravity.
5.4 INFILLING RESERVOIR WITH GASES, NATURAL DEASPHALTING

o Deasphalting may be due to gas injection (i.e. heptanes) or thermal cracking, the result is lighter oil and solid residues.

o When the oil becomes lighter, the asphaltenes become less soluble and precipitate.

o Deasphalting brings an API gravity increase and a sulfur content decrease.
5.5 THERMAL CRACKING, MATURATION

o Function of time and temperature (linked to depth).

o Variation of depth, due to tectonic movements can stop or restart cracking process.

o With maturity, API gravity increases , Pour point and viscosity decrease.

o Break the molecules in lighter ones, the high maturity produces dry gas.

o The iC4/nC4 ratio will be
high (i.e. > 8) in an immature HC (diagenesis) and will decrease (ie
<1) in the maturity zone (Catagenesis), as maturation creates
predominantly n alkanes.

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