Abstract:

Carboniferous Keluke Formation of the Norhtern Qaidam Basin is a shaledominated succession, which shows great potential in shale gas exploration. Finegrained sedimentary rocks (shale or mudstone) of Keluke Formation were deposited in the epeiric sea environment, which are quite different from siliceous marine shale and carbonaterich lacustrine shale, and have not been wellstudied. Therefore the research on lithofacies of Keluke shale is of great significance in sedimentology and petroleum geology. With easy to identify as the main principle, 3 kinds of rock properties, including mineral composition, sedimentary structure, fossils and other foreign matters are chosen to define the shale lithofacies in Northern Qaidam Basin. According to outcrop and core observation as well as thin section study and XRD analysis, 10 lithofacies of Carboniferous shale in Northern Qaidam Basin have been recognized, which are very helpful in interpreting the sedimentary environments. Among all these lithofacies, coal, horizontal bedding carbonaceous claystone, lenticular bedding silty claystone, wavy bedding argillaceous siltstone and flaser bedding siltstone were formed in tidal flat environment, While siderite rich clay stone was formed in lagoon, and massive marlstone and bioclastic limestone were formed in carbonate platform. In a special case, shell rich silty mudstone and shellrich mudstone were formed in the transition zone between restricted platform and lagoon, as the result of massive death of shell forming organisms mixed with the terrigenous detrital influx. Because of the gentle slope of the epeiric sea floor, the sedimentary systems are affected significantly by short term sea level fluctuations. With rapid shoreline migration, shale lithofacies of different microenvironments overlapping with each other frequently in the vertical direction. According to outcrop and core observation, 3 types of lithofacies assemblages have been recognized, which are formed in restricted carbonate platformlagoontidal flat facies assemblage, restricted carbonate platformlagoon facies assemblage, and restricted carbonate platform skeletal shoal facies assemblage. Finally, a generalized model for the epeiric sea shale of Keluke Formation is established.

Key words: Northern Qaidam Basin, Carboniferous, Keluke Formation, mudstone shale, sedimentary environment