Vertical heterogeneity and the main controlling factors of the Upper Ordovician-Lower Silurian Wufeng-Longmaxi shales in the Middle Yangtze region

Abstract

The Upper Ordovician-Lower Silurian Wufeng-Longmaxi shales in the Middle Yangtze area have strong vertical heterogeneity, affecting the gas-bearing capacity of shale reservoirs and the fracturing ability.This study integrates wire-line logs, drilling cores, thin section observations, total organic carbon content, X-ray diffraction mineralogy measurement, and major, trace elements measurement.Under the stratigraphic sequence framework in the study area, the vertical variation of TOC content, mineralogical composition and shale lithofacies were studied.Combined with geochemical proxies for paleoclimate and paleoenvironmental conditions, the main factors controlling vertical heterogeneity were discussed.The results show that the Wufeng-Longmaxi shales have two 3rd-order sequences.The Wufeng Formation is divided into the transgressive systems tract(TST1) and high stand systems tract(HST), and the Longmaxi Formation is divided into the transgressive systems tract(TST2) and the early high stand systems tract(EHST).During the transgression period, the shale has high silica content, low clay content, and high organic carbon content, and mainly developed siliceous shale facies and clay-bearing siliceous shale facies.In the early high-stand systems tract period, shale developed silty bands and lenticular bedding, with high clay content, and low organic carbon content, and mainly developed siliceous-clay mixed shale lithofacies.The vertical changes of paleoclimate indicators(CIA, Rb/Sr), paleo-productivity indicators(Cu/Al, P/Al), redox environmental indicators(MoEF, UEF), and terrestrial input(Ti, Zr) indicators indicate a high productivity, anoxic sedimentary environment, low terrestrial input and relatively dry and cold climate conditions for the transgression period, while during the high-stand systems tract deposition, the study area has low productivity, oxic-dysoxic environment, high terrestrial input and relatively warm and humid climateconditions.The paleo climate, paleo productivity, redox conditions, volcanic activity, terrestrial input and bottom currents are the main factors controlling the stratigraphic heterogeneity of the Wufeng-Longmaxi shales in the study area.Finally, by comparing with some parameters of the shale in the Upper Yangtze region, it is found that the study area shows the differences in deeper burial depth, thinner organic shale, lower gas content and more brittle mineral content.

Keywords

• wufeng-longmaxi shales
• heterogeneity
• shale lithofacies
• sedimentary paleoenvironment
• organic matter enrichment