Natural and laboratory oxidation of low-organic-carbon-content sediments: Comparison of chemical changes in hydrocarbons

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 wlow-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 degrees C and 130 degrees C. This sample has also been pre-extracted and then oxidized at 130 degrees 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 degrees 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 degrees 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.