Jurassic paleoclimate reconstruction and its influence on organic matter enrichment in Kuqa Depression, Tarim Basin

The Tarim Basin is the most important oil and gas basin in western China. The paleoclimate of the Jurassic Period in the Tarim Basin remains poorly understood, and research on the enrichment of organic matter in coal-measure source rocks is relatively scarce. Therefore, to reconstruct the Jurassic paleoclimate and its influence on the enrichment of organic matter in coal-measure source rocks, we studied 13 samples from nine wells in the eastern Kuqa Depression of the Tarim Basin. We found that the corrected chemical index of alteration (CIA*), the chemical index of weathering, the plagioclase index of alteration, ln(Al2O3/Na2O), and Na/K can characterize the degree of chemical weathering in the study area. The degree of chemical weathering during the Jurassic period was high and declined gradually, indicating that the paleoclimate was relatively warm and humid, progressively changing to hot and dry. Paleoclimatic parameters composed of hydrophilic and hydrophobic elements indicate climate fluctuation. The parameters Ca/Mg, Fe/Mn, and Rb/Sr indicate the paleoclimatic changes in warmth and humidity in the study area. The Yangxia Formation in the Early Jurassic was found to be relatively hot and dry, whereas, in the Late Jurassic, the Qigu Formation experienced a severe hot and dry event. The cross plot of CIA* and total organic carbon (TOC) showed that samples with higher TOC content exhibit a higher degree of chemical weathering, indicating that the overall warmth and humidity played an important role in controlling the accumulation of organic matter in coal-measure source rocks. It was found that when the overall climate was warm and humid, a relatively hot and dry climate was conducive to the enrichment of organic matter in coal-measure source rocks.