Coupling mechanism between sea level changes and pore heterogeneity of marine shale reservoirs driven by astronomical orbital cycles: Lower Silurian Longmaxi shale in the Upper Yangtze area, South China

Abstract

Marine cycles driven by astronomical orbital periods, may reflect sea-level fluctuations, which considerably influence organic matter (OM) accumulation, redox conditions, and terrigenous input, and subsequently impact the heterogeneity of shale reservoirs. This study uses multivariate analysis to examine astronomical orbital cycles and sea-level fluctuations, and investigate coupling mechanism between seal level changes and pore heterogeneity in Lower Silurian Longmaxi marine shale reservoirs. We conducted a high-resolution cyclic stratigraphic study of the GR series and performed associated geochemical analyses. Milankovitch cycles were identified, including ∼1.2 Myr obliquity, ∼405 kyr long eccentricity, and ∼95 kyr short eccentricity cycles. Climate change driven by ∼1.2 Myr long obliquity cycles resulted in two climate fluctuations (hot-humid and dry-cold), causing eustatic changes with two transgression-regression cycles (2 transgressive system tracts (TST) and 2 regressive system tracts (RST) identified. The changes in sea level during these periods considerably influenced the heterogeneity of shale pores, leading to variations in pore types and characteristics in response to sea-level fluctuations. For instance, during TST1, deglaciation occurred, causing a rapid rise in sea level and the deposition of organic-rich shales in a deep-water anoxic environment with limited terrigenous influx. This environment, combined with abundant biogenic silica, contributed to the accumulation of OM and promoted porosity, making organic pores dominant during the TST1 to RST1 period. Subsequently, the Upper Yangtze area shifted to a semi-restricted dysoxic - oxic deep-water shelf with the fall in sea level. This change resulted in decreased paleoproductivity and increased terrigenous influx, directly leading to reduced pore development, with inorganic pores becoming the main pore type during the TST2 to RST2 period.