Abstract:

The study of shale facies characteristics and sedimentary model can help to reveal the characteristics and distribution of fine-grained source rocks.The lithofacies assemblage and sedimentary model of the Lower Cambrian shale in the northeastern margin of the Tarim Basin are analyzed by means of outcrop description, thin-section petrographic observation and organic matter content measurement.The sea-level of the Lower Cambrian in northeastern Tarim Basin was characterized by rapid transgression, continuous deepening and rapid shallowing.The sea-level change may have directly affected the shale development.Six lithofacies and three lithofacies assemblages are developed from bottom to top in the Lower Cambrian in the study area: (1)the semi deep-water shelf lithofacies assemblage includes siliceous shale facies, siliceous clay-shale mixed facies and phosphorous shale facies; (2)the deep-water shelf lithofacies assemblage includes siliceous shale facies and clayey shale facies; (3)the shallow water shelf lithofacies assemblage includes calcareous shale facies and siliceous calcareous shale facies.In the Early Cambrian, the northeastern Tarim region was likely in a semi-deep-water shelf environment, with strong extension and frequent hydrothermal activity.The hydrothermal activity has a dual role in the enrichment of organic matter, which is not conducive to organic matter enrichment in the siliceous shale in the semi-deep-water shelf environment, and to the organic matter enrichment in the phosphorous shale in the semi-deep-water shelf environment.With the continuous sea-level rise, tectonic activity and hydrothermal activity decreased, and the deep-water shelf lithofacies assemblages began to develop.During this period, the organic matter was well preserved.The sedimentation rate was slow, and the paleoproductivity was high.The total organic carbon (TOC) content in the shale was relatively high.During the sedimentation of the Xidashan Formation, the environment was generally of shallow water shelf.During this period, the content of silicic minerals in the sediments decreased, while that of calcareous minerals increased.The preservation conditions of organic matter were poor, the sedimentation rate was high, and the TOC content in the shale was relatively low.We considered that Lower Cambrian phosphorous shale facies and the deep-water shelf lithofacies assemblage in the northeastern Tarim basin represent favorable intervals.

Key words: Tarim Basin, Lower Cambrian, shale lithofacies, sedimentary model