Geochemical Analysis of Petroleum-- Reducing Exploration Risk

S ummary
Geochemical analyses of petroleum hydrocarbon (crude oil and gas) are conducted to reduce various risks associated with the exploration,development and production stages of oil/gas fields. Using geochemical data, such as the molecular and isotopic composition data of gases and thebiomarker compositions of oils, we can evaluate paleoenvironments, the ages and maturity of hydrocarbons, reservoir continuity, migration andaccumulation processes, etc. to enhance petroleum discovery rates and improve the efficiency of reservoir development plans.

Composition and Isotope Analysis of Gas

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Hydrocarbon gases are either derived from the metabolism of microorganisms (microbial/biogenic origin) or generated thermally from thesource rock (thermogenic origin). These origins are distinguishable
through analysis of chemical compositions and isotopic ratios.

Biomarker Interpretation for Exploration

To separate and identify various compounds, such as biomarkers
in crude oils, gas chromatography-mass spectrometry (GC-MS)
is used. Various compounds are separated by GC based on
differences in their boiling points and then identified according
to the mass spectra specific to individual compounds using MS.

Reservoir Geochemistry (Fingerprint Analysis)
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Changes in the compositions ofbiomarker compounds in oils andsource rock bitumens reflect
their paleoenvironments, maturity,and ages. This information can bederived
throughbiomarker
analysis.

Migration and Accumulation of Oil and Gas

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Chemical compositions of crude oils differ slightly between eachreservoir, even if the source of two reservoirs is the same.
These slight differences can be evaluated through fingerprint
analysis using GC chromatograms. Fingerprint analyses areapplied in reservoir continuity and connectivity studies as well as
production allocation studies of different reservoirs.

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Migration andaccumulation processes
can be estimated using
isotope analysis.
The left figure showsthat the gases generated
in a source rock have
moved east and upward
along a fault because of
their buoyancy, and were
supplied from east towest in the reservoirformation.
Such studies can reduce
migration risks in furtherexploration anddevelopment stages andenhance discovery rates.

Geochemical analyses of petroleum through GC, GC-MS, and isotopic
ratio-MS are used in various evaluation studies as described below:(1) Exploration geochemistry — Clarification of the petroleum
system of a target basin through oil–source rock correlation andevaluation of migration and accumulation using biomarker andisotope ratio analyses.
(2) Reservoir geochemistry — Evaluation of reservoir continuity
and production allocation through correlation of different
reservoirs using biomarker, fingerprint, and isotope ratio
analyses.
These analyses contribute to enhancing the success rates ofexploration and improve the efficiency of reservoir development
planning. Geochemical analysis therefore contributes to risk
reduction at all stages of petroleum exploration and production.

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