Study of Structure and Biomarkers of Organic-Clay Complexes in Immature Oil Source Rocks, Shengli Oil Field, China

LU XIANCAI, HU WENXUAN, Department of Earth Sciences , Nanjing University, 210093, P.R. China; ZHOU GUANGJIA, MIAO DEYU, CHEN ZHILIN, Shengli Petroleum Research Institute, Dongying, 275152, Shandong, P.R.China

This paper studies the organic-clay complexes in immature oil source rocks of Shengli Oil Field, using supercritical CO₂ fluid extraction (SFE) and chloroform extraction(CE) to treat source rock samples, X-ray diffraction (XRD)and Fourier transform infrared spectrometer (FT-IR) to determine the structures of the extracted clay fractions and gas chromatography (GC) and gas chromatography /mass spectrometry (GC-MS) to analyze the extracts and its biomarkers.

The result of SFE and CE shows that the SFE yield of low maturity rocks, which contain abundant swelling clays, is higher than that obtained by CE, whereas those of high maturity rocks, containing fewer swelling clays, obtained by SFE and CE are similar to each other. In addition, the SFE extracts are rich in low-molecular-weight hydrocarbons (<C₂₅) relatively to the CE extracts.

SFE with CO₂ flow under different pressures and temperatures were performed and different yields of extracts were got. Afterward, the intact clay fractions and those treated by various methods including CE, SFE under different conditions were investigated by XRD and FT-IR, and it is found that the basal spacing (d001) of swelling clays changes regularly. Both the intact clays and the treated clays by CE have three d001 values: 1.292, 1.120 and 0.998nm. But it is not the same for the clays treated by SFE under different conditions. As the amount of extracts increase, namely, as the organic component in the complexes decrease, the d001 values get fewer and smaller from 1.292, 1.120 and 0.998nm to 1.292, 0.998nm and, then, to 0.998nm. It is proposed that the solvable organic matter, especially low-molecular-weight hydrocarbons, which can be extracted by SFE, could be adsorbed into interlamellar spaces of swelling clay layers to form organic-clay complex, which lead to the change of the basal spacing of the clays.
The hydrocarbon fractions of SFE and CE extracts were analyzed by GC-MS. The result suggests that SFE could roughly replace the CE technique, which is generally used to analyze biomarkers. But a little discrepancy still exists in maturity and environmental parameters. As the amount of extracts obtained by SFE under different conditions varies the biomarker parameters change slightly. For example, the 20S/(20S+20R) of C₂₉ steroids of an immature sample fluctuates from 0.03 to 0.10. The difference observed may be resulted from the organic component isolated from organic-clay complex by SFE, which contains much information of early diagenesis and have been protected by clay minerals during post-adsorbed stage.

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