Genetic mechanism of multi-scale sedimentary cycles and their impacts on shale-oil distribution in Permian Lucaogou Formation, Jimusar Sag, Junggar Basin

Abstract

Complex lithofacies of Lucaogou Formation in Jimusar Sag promote strong heterogeneity of oil distribution in fine grained sedimentary rocks. It is of great significance to define the formation mechanism of fine-grained sedimentary rocks for favorable reservoir prediction and exploration target selection in Jimusar Sag. Based on detailed petrographic characterization, in-situ geochemical parameter testing, and high-resolution cycle analysis, sedimentary cycles on the micron to meter scales were successfully identified in Lucaogou Formation from Jimusar Sag. Precession-forced paleo-environmental evolution mainly induces the deposition of meter-scale sedimentary cycles. In the period of low precession, the paleo-environment is cold and dry, the lake level falls. Silt-grained particles advance toward the center of the lake basin carried by gravity current, thus siliciclastic sediments are mainly deposited. In the period of high precession, the climate is warm and humid, the lake level rises. The inputs of siliciclastic sediments are limited and the temperature increases, which are conductive to the carbonate deposition. On this basis, high-frequency paleo-environmental evolution caused by solar activity (70–110yr cycle) further induces the formation of sedimentary cycles on micron-centimeter scale. When the precession is low, the rise and fall of lake level controlled by solar activity is contribute to the deposition of tuff-rich lamina and silt-grained felsic lamina, respectively. The period of high precession is under the background of overall high lake-level, the rise and fall of lake level, along with fall and rise of temperature, controlled by solar activity finally induce the deposition of tuff-rich lamina and carbonate lamina, respectively. The development of multi-scale sedimentary cycles controlled by Milankovitch cycle and solar activity cycle have important implications for shale oil enrichment. The fine-grained sediments deposited during the period of low precession and intense solar activity dominate feldspar dissolution pores and intergranular pores, which are favorable for shale oil enrichment.