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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Best Practices for the Collection, Analysis, and
Interpretation of Seabed Geochemical Samples to
Evaluate Subsurface Hydrocarbon Generation
and Entrapment
Apache Corporation
Houston Texas
The detection and measurement of migrated hydrocarbons in near-surface marine sediments has become a relatively routine exploration method in the petroleum industry to better understand hydrocarbon charge. The presence of near-surface migrated thermogenic hydrocarbons provides strong evidence an active petroleum system is present as well as critical information on source, maturity and migration pathways. There are multiple methods currently applied by industry contractors to collect, prepare, extract, and analyze near-surface migrated hydrocarbons contained within marine sediments.
To improve the detection of seabed migrated thermogenic
hydrocarbon seepage, core samples should be collected
along major migration pathways (cross stratal leakage features)
identified by conventional deep seismic
and high-resolution
seafloor imaging technology. Not all targeted cores will hit
the designated feature and thus collecting replicates along
key migration features is critical. Collecting sediment samples
below the Zone of Maximum Disturbance to avoid possible
transition zone alteration effects and ROM (recent organic
matter) masking problems is critical. Choosing the coring device
best suited for local seabed conditions will maximize both
penetration and sample recovery. Real time imaging provides
greater detail to confirm feature and provides a specific feature
to target.
Multiple sections per core should be collected at variable depths providing a depth profile. Geochemical analysis should include a full range of hydrocarbon types; light hydrocarbon gases (C1 to C5), middle boiling point gasoline plus range (C5 to C10), and high molecular weight hydrocarbons (C15+). Two types of geochemistry samples should be collected; one to capture the volatile light hydrocarbons (C1 to C10+) and non-hydrocarbon gases; and a second for the higher molecular weight hydrocarbons (C12+). The light hydrocarbons require special handling and containers to limit volatile loss and prevent post sampling microbial alteration. Bulk sediment measurements such as quantity of organic matter and sand percent can be very helpful.
The identification of the background versus anomalous
populations is critical when evaluating anomalous seabed hydrocarbons.
Note that the sediment hydrocarbons are normally
highly altered and may not resemble conventional reservoir gas
or oil. Mapping thermogenic hydrocarbon seeps (oil and gas)
relative to key cross-stratal migration pathways via fluid flow
modeling and seismic
attribute
analysis provides an effective
petroleum systems evaluation tool to better understand the
seepage relative to subsurface hydrocarbon generation and
entrapment.