Carboniferous Organic Reef Development Characteristics and Geological Significance in Northeast Margin of Qaidam Basin

Key words: 
• northeast margin of Qaidam Basin /
• Carboniferous /
• organic reef /
• coral /
• reef-building organism

Abstract: The organic reef in the Carboniferous System along the northeast margin of Qaidam Basin was reported in this paper, and the organic reef development characteristics and regional distribution were descripted. Based on field research and indoor microscopic analysis, combining core observation and description, using geochemistry and petrology method, summarizing the biological, stratum and petrology feature of organic reef in Carboniferous, northeast margin of Qaidam Basin. The article analyzed the main output layer and scale of the organic reef, discussed the palaeoenvironment and petroleum geological significance in northeast margin of Qaidam Basin. The results showed that research area's Reef-building organism is mainly composed by Tetracoralla and also include some Tabulata, the morphology of coral reef is very abundant, such as pyramidal-shape or plate-shape solitary coral, tubular, fasciculate and cellular colonial coral. Reef-inserted organisms were mainly Brachiopod and crinoid. Coral reef is punctiform or discontinuous banded in stratums, three kinds of types are identified by coral reef developing laws and distributional characteristics:clusters of vertical tubular coral, clusters of horizontal extension tubular coral and clusters of the top downward sloping tubular coral. According to biological content in rock, organic reef can be classified to coral skeletons limestone, coral grain limestone, biological granular limestone, containing biological granular limestone and micrite. In Early Carboniferous epoch in northeast margin of Qaidam Basin, the sedimentary environment is mainly carbonate platform while Late Carboniferous epoch it became tidal flat environments which was a transitional zone between marine and continent. Organic reef in the study area is mainly located in the third or fourth section in Huaitoutala Formation of Lower Carboniferous. It developed good source rock and thick layer of oil sands, showing strong ability of hydrocarbon generation and oil migration process.