Microseepage of hydrocarbons from accumulations to the sediment surface, mediated by pressure and buoyancy forces, is a well-known phenomenon. Physical and chemical properties of the overburden influence the flux rates of volatile hydrocarbons, and interpretation of such data can be challenging. Classical geochemical surface sampling mostly relies on higher hydrocarbons, because the gaseous compounds may not be detected representatively due to core decompression during the sampling procedure.

Offshore Resource Group (ORG) developed a Sub-Sea Unit (SSU) in the last few years; the system consists of a sediment penetration and pumping system equipped with environmental sensors and video coverage and most relevant two kinds of chemical detectors: an UV-VIS probe targeting poly-aromatic hydrocarbons (PAHs), and a gas-stripper-Selected Ion Flow Tube (SIFT) Mass Spectrometry targeting organic compounds up to C12. Both analytical systems are sensitive enough to determine environmental concentrations associated with microseepage. Performed surveys were assisted by the MPOG® technique (Microbial Prospecting for Oil and Gas), both on unconsolidated sediment material received via the lander system, and by gravity coring.

This article represents a number of case studies in which the results provide more understanding about the technique, acknowledge uncertainties, and evaluate its potential as a risking tool for hydrocarbon exploration.


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