Geological and geochemical implications of the complicated carbonate diagenetic process in the Lower Ordovician buried hills of the eastern Tazhong Low Rise, NW China, using Well M1 as an example

Abstract

There are many structural buried hills in the east of Tazhong Low Rise that have significantly valuable for oil/gas exploration and research. In this work, geochemical and geological methods were used to research the complicated diagenetic process of Yingshan carbonate. The Yingshan carbonate underwent four diagenetic stages with three types of calcite, which diagenetic environment was complex and variable. During the Early Ordovician of syndiagenetic stage, the high-magnesium CI calcite develops along the micro-fracture and has no or very weakly cathodoluminescence color. In the following early diagenetic, the Yingshan carbonate subsided and developed the CIII calcite along the corroded fissure and pressolved sutures. After the M1 buried hill formed during the Late Silurian, the Yingshan carbonate experience the supergene stage, and suffered the weathering and denudation for a long period. During the Carboniferous karst, two cave horizons developed in the M1 buried hill, corresponding with two sets of Carboniferous carbonate rocks by the stratigraphic correlation. The cement CII calcite contains less Sr, Na, Mg, and more Mn and Fe than sea water, showing bright red in the cathodoluminescence. During the following middle and late diagenetic stage, M1 buried hill underwent a process of persistent burial and slight uplifting with CIII calcite cementing from the formation water. During the Permian, the hydrothermal fluid intensely influenced the buried hill, leaving the hydrothermal minerals.