Depositional controlling factors on pore distribution and structure in the lower Silurian Longmaxi shales: Insight from geochemistry and petrology

Abstract

The present study investigates the lower Silurian (Rhuddanian-Aeronian) Longmaxi Formation of the Sangzhi block South China's Upper Yangtze Platform to establish the role of depositional environment as determinant of pore structure evolution. Research methods include clay grain microfabric analysis using a scanning electron microscope with cathodoluminescence, analysis of total organic carbon and major and trace element concentrations, X-ray diffraction analysis, and porosity measurements. Our results suggest that Rhuddanian shale accumulated in a suboxic environment that experienced relatively high paleoproductivity thereby enriching the sediment in organic matter (OM) and biogenic quartz. These combined to yield sediment characterized by relatively high porosity. OM appears to have been the main contributor to porosity with OM pores comprising the dominant pore type. Aeronian time was marked by an increased supply of terrigenous sediment, especially clay, and establishment of oxygenated bottom conditions. The increased supply of terrigenous sediment and decreased paleoproductivity resulted in sediment deficient in OM resulting in low porosity. Pore spaces in clay minerals are subordinate to interparticle pores, the dominant porosity of Aeronian shale. The transition of organic-rich Rhuddanian shale containing biogenic silica upward into clay-rich Aeronian shale deficient in OM is accompanied by a change in porosity and dominant pore type that reflects the changing depositional environments of these deposits. The up-section evolution of sedimentary environment reflects the effects of varying water depth and degree of water exchange with the open ocean related to sea level fluctuation. Documented changes in porosity related to depositional environment have a direct influence on shale gas reservoir capacity. Specifically, a suitable sedimentary environment creates the conditions for high-quality shale formation.