3.
Geochemical Surveys and Analyses
Awang H. Satyana (SKK Migas) INDONESIAN PETROLEUM ASSOCIATION (IPA)
SHORT COURSE, YOGYAKARTA, 6-10 APRIL 2015
Geochemical Surveys
• Surface geochemistry (surface geochemical prospecting)
• Well geochemistry (hydrocarbon mud logging,
wireline logs, geochemical logs)
Surface Geochemistry
• Investigate geochemical anomaly at surface related to oil and/or gas accumulations at subsurface.
• Basic principle
– Any sediments are unlikely to be totally impervious to hydrocarbons. If generated, both light and heavy hydrocarbon molecules can therefore migrate upwards from deep sedimentary sequences via permeable carrier beds and structures such as faults in the overlying sediments. This will cause detectable accumulations in near-surface sediments, a sufficiently detailed examination of these using sensitive analytical techniques should be capable of detecting and characterizing the seeping hydrocarbons.
– These thermogenic hydrocarbons will be introduced into an environment
where there are also varying amounts of biogenic hydrocarbons, generated by the action of bacteria on organic matter in the sediments at shallow levels, also distinguishable by the analyses.
Surface Geochemical Prospecting
• The search for near-surface or surface anomalies of hydrocarbons, which could indicate the occurrence of petroleum accumulations in the
subsurface.
• The methodology covers a range of techniques, from observation of
visible oil seepage at the surface (macro-seep) to detection of micro-seeps in near surface sediments using sensitive analytical technique conducted in onshore and offshore areas.
• A surface geochemistry project can be divided into five different phases : planning, sampling, preservation of samples, analyses and
interpretation/integration of the data with the geology.
Micro-seeps Survey
•
Map the light hydrocarbons expressed through micro-seeps from active petroleum and natural gas reservoirs at depth. The maps, developed from the survey, serve as a guide to identifying prospective areas in both frontier and mature basins. The light hydrocarbons, when surveyed, analyzed and mapped, produce a graphic, geochemical picture of the source rock below. This picture shows whether or not a given area contains petroleum source rocks; whether the area is more oil or gas prone; and finally, whether or not the gas is over cooked and non-productive, or the oil too heavy to produce by drilling alone.•
Such studies can greatly reduce the area to be searched; helping to focus more expensive exploration techniques, such as 3D geophysical surveys, on those portions of the basin most likely to contain petroleum accumulations and potential reservoirs.•
The relationships, of the C1 - C4 gases, tell the story of the source rock below. The relationships (ratios) of C1-C2, C1-C3, and C1-C4 indicate whether an area is more oil or gas prone. These ratios also indicate thermal maturity. Magnitudes are controlled by the permeability of migration pathways, and subsurface pressure.Obviously, major fault tectonics can affect migration pathways, and strongly influence seepage magnitude.
• It uses airborne geochemical technology to map hydrocarbon micro-seeps associated with oil and gas reservoirs.
• Data channels measure total hydrocarbons, propane plus and pentane plus.
Combined mapping often results in the ability to distinguish between oil and gas- prone targets.
Airborne Geochemical Survey
ALF : ultra-violet light is emitted by a pulsed laser from the
aircraft, and will induce fluorescence from
any oil slick on the sea surface.
Thompson et al. (1991)
Geochemical Surveys at Offshore
Geochemical Surveys at Offshore
gravity corer vibro corer piston corer heat flow probe
Geochemical Surveys at Offshore :
Sampling Probes (examples)
Geochemical Surveys at Offshore
Geochemical Surveys: Results
Bjorøy and Ferriday (2003)
Deep marine geochemical survey in Indonesia, “Sea-floor Seep Analysis” using piston coring program
Noble et al. (2009)
Orange et al. (2009)
Orange et al. (2009)
multibeam backscatter
“Sea-floor Seep Analysis” using multibeam & piston coring program
Orange et al. (2009)
Biomarker examples (m/z 191) of sea-floor oil seeps
Noble et al. (2009)Well Geochemistry
• Oil and gas accumulations occur in sedimentary sequences that contain hydrocarbons in drilled well sections. Prospecting should be directed toward those areas and formations where the shale gas readings on the mud log are high.
• Mapping gas yields from mud logs can help identify prospective areas for oil and gas.
• Wireline logs may be used to evaluate oil source rock potential if they have been suitably calibrated with geochemical data and if the TOCs are > 1.5 %. Good oil source rocks within the oil window show high electrical resistivities, high gamma ray response, low densities, high neutron porosity, and low transit time on the sonic log.
• Geochemical log allow quick recognition of important changes in geochemical parameters with depth. They also provide a basis for identifying the best source intervals, along with any anomalous data.
Wireline logs and oil source rock potential
Meyer and Nederlof (1984) Creaney and Allan (1990)
head space gas analyses
Hunt (1996)
Hunt (1996)
Hunt (1996)
Peters (1986)
Geochemistry: Analysis-Based Sciences
• Petroleum geochemistry have always been analysis-based sciences. Proper application of modern analytical technology has been critical to our ability to describe the chemical composition of fossil fuels and kerogens, to predict source potential of sedimentary rocks, to correlate samples with each other, and to understand alteration processes like biodegradation.
• Without these sophisticated analytical capabilities, petroleum
geochemistry would still be in a very primitive state.
Geochemistry: Analysis-Based Science
•no sample
•no analysis
•no answer
modified Waples (1985)
Be careful : most of the well samples are not sources
top of oil window
kitchen kitchen
trap trap trap
Analytical Techniques (examples)
• TOC (total organic carbon) determination
• Rock-eval pyrolysis
• Kerogen isolation
• Microscopic kerogen evaluation
• Spore coloration
• Vitrinite reflectance
• Elemental (CHNSO) analysis of kerogen
• Bitumen extraction
• Column chromatography
• Gas chromatography of saturated hydrocarbons
• Gas chromatography of aromatic and asphaltene hydrocarbons
• Gas chromatography-mass spectrometry
• Physical properties of oil (API, pour point, viscosity)
• Trace element analysis of oil (sulphur, nickel, vanadium content)
• Chemical analysis of oil (asphaltene, volatile, wax content)
• Compositional analysis of gas
• Carbon isotopes
• Hydrogen isotopes
• Sulphur isotopes
• Headspace gas analysis
Robertson Research
geochemical analyses protocol
exampleGeochemical Analyses Project
• Sampling and sample preparation
• Screening analyses
• Follow-up analyses
• Computer modeling & predictions
• Interpretation & reporting
Timing : 2-3 weeks for screening analyses
6-8 weeks on complete analyses for one well
Waples (1985)
Waples (1985)
Waples (1985)
column chromatographic separation
rock-eval pyrolyzer
gas chromatographer (GC)
gas chromatographer-mass spectrometer (GC-MS)
