CPG Geochemistry & Source Rock Analysis Flashcards

What does the term 'expulsion efficiency' mean in petroleum geochemistry?

Answer: The fraction of generated hydrocarbons that actually migrates out of the source rock versus the amount retained

Expulsion efficiency describes what proportion of the total hydrocarbons generated by a source rock is expelled into the migration system rather than remaining as residual bitumen.

What geochemical parameter is used to assess the depositional environment (oxic vs. anoxic) of a source rock?

Answer: Pristane/phytane (Pr/Ph) ratio derived from isoprenoid biomarkers

The pristane/phytane ratio reflects redox conditions during organic matter deposition: Pr/Ph 3 suggests oxic, marine or terrestrial environments.

In US Geological Survey (USGS) resource assessments, what probabilistic method is commonly used to estimate undiscovered hydrocarbon volumes in a petroleum province?

Answer: Play analysis with Monte Carlo simulation to aggregate lognormal distributions of accumulation sizes

USGS national assessments use play-level probabilistic analysis with Monte Carlo simulation to generate F95–F50–F5 resource distributions for undiscovered conventional and continuous accumulations.

Which analytical technique is used to directly measure the carbon isotopic composition (δ¹³C) of individual biomarker compounds in a crude oil?

Answer: Compound-specific isotope analysis (CSIA) using GC-IRMS

Compound-specific isotope analysis (CSIA) couples gas chromatography with isotope ratio mass spectrometry (GC-IRMS) to measure δ¹³C of individual compounds separated from complex mixtures.

What is a 'geochemical anomaly' in surface exploration, and how is it used in petroleum prospecting?

Answer: An elevated concentration of hydrocarbons or their alteration products in surface soils/sediments/water, indicating possible subsurface seepage from a petroleum accumulation

Surface geochemical anomalies (elevated methane, ethane, propane, or microbial communities sustained by hydrocarbon seepage) can indicate a leaking subsurface petroleum system and help rank prospects.

In petroleum geochemistry, what does 'mixing' of oils from different source rocks or maturity levels produce in a GC-MS biomarker analysis?

Answer: Intermediate biomarker ratios that do not match any single source rock end-member, complicating oil-source correlation

Mixed oils display biomarker ratios intermediate between their source end-members, which can falsely suggest a non-existent mixed source rock or obscure true correlations.