Residual carboxylic acids (nonpyrolytic) in mudstones and their implications and constraints on sandstone diagenesis, Bozhong Depression, offshore Bohai Bay Basin, East China

Abstract

Nonpyrolytic residual carboxylic acids were measured in Paleogene mudstones (0.60% < Ro < 0.88%) from the Bozhong Depression, offshore Bohai Bay Basin (East China). These residual compounds were composed of C3-C10 fatty acids and abundant naphthenic and aromatic acids. The residual water-soluble species, which were composed of formic (methanoic) acid, propionic (propanoic) acid and butyric (butanoic) acid with a predominance of propionic acid (average: 74.4%), exhibited two decreases with increasing depths: the first decrease at a depth of ∼3150 m by ∼35% of the maximum (Ro: 0.7%–0.75%; temperature: 110–115 °C) and the second at a depth of ∼3550 m by ∼12.7% of the maximum (Ro: 0.81%–0.82%; temperature: 130–135 °C). The first decrease was identified as an export-driven process, whereas the second decrease was a thermal-breakdown-driven process.The implications for sandstone diagenesis are also analysed. The predominant window of secondary porosity generation should generally be coeval with the hydrocarbon charge. Carboxylic acids transported from underlying mudstones and naphthenic acids/CO2 generated by hydrocarbon biodegradation are the main source of acids at shallow depths. Shallow depths have an injection time of carboxylic acids (including naphthenic acids/CO2) earlier than expected (∼0.30–0.35%Ro/11 Ma at shallow depths, younger than ∼0.7%Ro/38 Ma at deep depths). These factors cause rocks at shallow and deep locations to have different diagenetic evolution paths. Therefore, it is inappropriate to use diagenetic models of shallow depths to predict the evolution of deep reservoirs.