Abstract
This is the second of a series of papers to investigate the role of organic compounds on wettability, porosity and permeability of carbonate rocks. These are important characteristics that influence the performance of petroleum reservoirs. In this paper, we report on analyses of natural sediments and rocks to determine the abundance of carboxylic acids and its influence on rock wettability. Two recent and four ancient carbonate samples were extracted by a methanolic potassium hydroxide procedure. Alkane and carboxylic acid fractions were analyzed by gas chromatography. Alkanes were a much larger fraction of the total organic carbon (TOC) in ancient rocks than in recent sediments. Acid concentrations averaged 77 μg-acid/g-dry-sediment in the recent sediments and 14 μg-acid/g-dry/sediment in the ancient rocks. Calculations indicate that the extracted acids are insufficient to coat the carbonate surfaces (and thereby influence wettability and carbonate reactivity). However, many of the acids are bound to the kerogen, and they probably attach the kerogen to the mineral. Imbibition tests showed that carbonates can be oil-wet with as little as 0.1% TOC or 6μg-acid/g-dry-sediment.
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