To investigate variation characteristics of interlayer water bound up with organic matter in smectite, organo-clay complexes extracted from grinded source rock samples were determined using thermo-XRD, DTA and PY-GC. The d001 diffraction peak of organo-clay complexes is postponed from 250 to 550°C before reaching 1.00 nm and accompanied by exothermal peaks on DTA and organic matter with abundant C20-C30 carbon detected by PY-GC, which is different from single smectite and indicates the existence of organic matter in the interlayer of smectite. Water desorption characteristics of organo-clay complexes are in consistent with smectite at 100 and 600°C, but different from smectite at 550°C with an additional dehydration peak and a remaining d001 diffraction peak, suggesting the water removed at 550°C is interlayer adsorption water rather than constituent water of clay minerals. Comparing the dehydration order and water loss, we conclude that part of interlayer water of smectite may act as the "bridge" that binds organic matter and smectite, which results in water-expelled lag beyond 250°C and may provide a good medium for hydrocarbon migration and oil pool formation.
Highly-mature carbonate source rock is essential to the exploration of oil and gas in southern China. In this study, the carbonate strata in the relatively well-developed Lower Permian Chihsia Formation (located in Chaohu, Lower Yangtze) were targeted, and the formation and influencing factors of source rock were discussed based on paleoenvironment reconstruction using comprehensive sedimentology, palynofacies, and organic geochemistry data. The results demonstrate that the Chihsia Formation is oxygen-deficient biogenic carbonate sediments, with marked variations in the organic and inorganic components, formed during a period of Permian transgression. The formation of source rock is the combined result of high bioproductivity and oxygen-deficient environment generated by transgression and oxygenation events which frequently occurred during transgression periods. Source rock was affected by self-dilution effects and diagenesis, causing its heterogeneous distribution in many intervals. Source rock is over 40 m thick, and can be identified based on its different organic, biological, and mineral composition characteristics. The carbonate rock appears to require no specific clay content in order to become a source rock. The combination of sedimentology, palynofacies, and organic geochemistry has provided an effective means for evaluating and predicting high-maturity carbonate source rock in the region.
