Natural and Laboratory Oxidation of Low-Organic-Carbon-Content Sediments:  Comparison of Chemical Changes in Hydrocarbons

Abstract

Callovo-Oxfordian argillaceous rocks collected in HTM 102 borehole recently drilled in the northeast of the Paris Basin (France) have been studied. Several parameters suggest a low maturity level of the organic matter. The chloroform soluble material from low-total-organic-carbon-content (ranging between 0.4 and 1%) samples is characterized by the presence of unsaturated biomarkers. A Callovian sample has been oxidized in a ventilated oven at 60 °C and 130 °C. This sample has also been pre-extracted and then oxidized at 130 °C for 64 h. To validate these experiments, the samples have been stored for 3 years. Additional studies on two cuttings, collected from older boreholes drilled in the same geological formation in the Paris Basin, have also been carried out. The chemical changes in the bitumen phase of the Callovo-Oxfordian sediments upon natural alteration and laboratory oxidation are similar in nature and in magnitude. At 60 °C−256 h or for 3 years of storage, the relative abundance of the biomarkers decreases and the n-alkane distribution is shifted toward low-molecular-weight compounds. Data clearly show that the effects of oxidation are more pronounced at 130 °C−1024 h and for the Paris Basin cuttings originating from older boreholes. The gas chromatography−mass spectrometry analyses performed on the aliphatic fraction reveal mature distributions for hopanes and steranes. The results indicate that the effects of natural oxidation can be simulated in a ventilated oven. The artificial oxidation of a pre-extracted sample suggests that the kerogen alteration induces the generation of stable biomarkers. These results demonstrate that core samples undergo a rapid air oxidation, which significantly modifies the geochemical signature of the organic matter. Then, subsequent interpretations concerning the depositional environment, oil-to-source rock correlations, or maturity assessment may be affected by the natural oxidation of organic matter during long-term storage of sediments.