Sequential extraction—a useful tool for reservoir geochemistry?

Little is known about the microscale distribution and potential molecular fractionation of bitumen residing in reservoir rocks. Few data exist on the comparative organic geochemistry of hydrocarbons in authigenic inclusions formed in reservoir rocks and the bitumen in the pore space. A three step sequential extraction technique has been applied to clastic petroleum reservoir samples, using a suite of solvents with increasing polarity, in order to address these questions. In the analyzed samples the following main trends were observed: (i) the first extract represents > 95% of the total extract; (ii) the relative polar compound content increases going from the first to the last extract; (iii) the last extract has generally a slightly lower apparent maturity as assessed using biomarker parameters. Additionally a reduction in the degree of biodegradation from the first to the last extracts was observed, implying tentatively that biodegradation takes place dominantly in the center of the pores. Fluid inclusion extracts show distinct source characteristics, however the last extract appears to approach the fluid inclusion extract signature in some aspects. The use of sequential extraction as a routine tool in the reservoir geochemical context is judged to be limited: however it may help to solve specific problems.