SEDIMENTOLOGICAL SIGNIFICANCE OF NATURAL GAMMA RAY LOGGING DATA OF MARINE CARBONATE: A CASE OF THE WELL L1 OF QINGXUDONG FORMATION, LOWER CAMBRIAN IN SOUTHEAST SICHUAN BASIN

Sequence interfaces and the maximum flooding surface are obviously observed in the U, Th, K curves. Therefore, the Th/U and Th/K curves can be applied for detailed subdivision of sequences. Changes in Th/U ratio are well associated with water depth and oxidation-reduction conditions. The low Th/U ratio indicates deepening of water depth and increasing in reducibility. The value of Th/K ratio is also an indicator to the exposure time of sediments in a weathering environment. Using the clay mineral chart, we may qualitatively determine the types of clay minerals and the changes in content. The changes in clay mineral assemblage are usually regarded as the responses to climatic changes. The natural gamma logging curve and the clay mineral chart of the well L1 are applied in this paper to recover the evolution of the sedimentary environment in the Qingxudong period, from sq1 to sq4. It suggests that the water became shallower gradually, the oxidation was increasing, drought remained stable, and the sedimentary facies gradually evolved from a low-energy subtidal zone to supratidal evaporates in this period.