GEOCHEMICAL CONSTRAINTS FOR DEPOSITIONAL ENVIRONMENT OF THE HYPERSALINE LACUSTRINE QIANJIANG SHALES , JIANGHAN BASIN, CHINA
There are 193 salt rhythms in QJF. Within each rhythm, both wireline logs and elemental geochemical data indicate that the inter-salt shales occur either in monocyclic or multicyclic clastic-carbonate-evaporatic laminates.
Data of Well QYP-2 indicates the TOC of inter-salt shale (19%) is much higher than that of intra-salt shales (2%).High salinity and sulphate enrichment are not conducive for the accumulation of organic rich shales. The TOC contents of the inter-salt shales appear to increase with silica content. Thin-section analyses reveal clear evidence for TOC concentration in algal/bacterial lamination, often associated with volcanic ash. SEM-EDS analysis reveals that silica occurs mainly in shapes of cyanobacteria, believed to have formed by penecontemporaneous metasomatism of silica solution in cyanobacterial sheaths. Plot of elemental Al-Fe-Mn data against those in previous works supports a biogenic silica origin in the inter-salt shales. Combined trace element composition with chlorine content, it is possible to identify the position within each sedimentary cycle that corresponds to the period of anoxia, and thus to predict where maximum TOC may be expected.
Results of Rock-Eval analysis indicate that most of the inter-salt shales contain type I and II organic matter. Based on the presence of n-alkanes with clear even-over-odd carbon predominance, extremely high phytane contents, low pristane/phytane ratios, and hopanoid and steroid biomarker distributions, relative abundance of gammacerane, arylisoprenoid alkanes and a number of aromatic compounds, it is possible to reconstruct the depositional settings and paleoclimatic conditions.
Evaporates in the QJF possess all necessary elements for self-contained petroleum systems. The use of chemostratigraphy in the definition of a stratigraphic framework and calibration of wireline log data appears promising for regional mapping of organic rich shales.