Environmental geochemistry

Samples

There are similar aspects to checking for leaks from production facilities and surface prospecting, as they both involve identifying petrogenic hydrocarbons at or near the sediment surface. Samples can be gaseous, liquid or solid. The last usually comprises unconsolidated sediment, which may contain gaseous and/or liquid hydrocarbons. So basically the same types of analyses apply as for conventional exploration studies, but the sampling methods necessarily differ, particularly for subsea sites, and are covered in surface sampling.

APT can perform geochemical analyses on gas, oil and sediment samples to search for petrogenic hydrocarbons and possibly correlate them with known hydrocarbons, but does not undertake field work.

Gas

Gas samples in cylinders, bags or IsoTubes are analysed in the same normal way, and headspace gas from sediment, in IsoJars or tins, can also be sampled as for cuttings. Isotopic and molecular compositional analyses are recommended in order to provide the maximum information, particularly for subsea samples, because of the expense and difficulty of resampling.

Headspace gas can identify any areas of high gas yields in sediments, typically associated

with active seeps and/or high biological activity. The sources are differentiated by methane abundance and isotopic composition of the various hydrocarbon components.

Oil

The liquid hydrocarbon content of surface sediments can be very low and overwhelmed by polar natural products from an active biological community, so great care is required to avoid potential contamination.

Rather than routine Soxhlet type extraction, the use of ultrasound is recommended. Otherwise, extraction and fractionation procedures to obtain hydrocarbon fractions, and their subsequent analysis, are as for reservoir/source rock extracts.

Potential problems

  • Contamination is always a concern with low levels of hydrocarbons are expected, so the use of suitable sampling equipment is vital
  • Low level petrogenic contributions may be masked by indigenous non-gaseous hydrocarbons
  • Headspace gas samples from an oxic sediment column may well have experienced biodegradation