Paleoenvironment reconstruction of the Middle Ordovician thick carbonate from western Ordos Basin, China

Abstract

Reconstructing paleoenvironments has long been considered a vital component for understanding the development and evolution of carbonate reservoirs. The Middle Ordovician Period is considered the archetypical greenhouse interval, and also a critical period in biological evolution. The Middle Darriwilian isotope carbon excursion has been observed in many areas of the world and may be related to the biological explosions caused by decreases in the temperature. The thick carbonate rocks in the fifth member of the Middle Ordovician Majiagou Formation in the Dingbei area of the Ordos Basin were chosen as an example, based on the concentration of major, trace and rare earth elements as well as C, O and Sr isotopic analyses, the paleoenvironment was reconstructed. And its impact on natural gas exploration was analyzed. The results show that the seawater paleotemperature was 29 °C, suboxic-anoxic paleoredox conditions were observed, and the seawater paleosalinity was high. A large number of plankton in the biological explosion caused a rapid increase in the total organic carbon in carbonate rocks, which provided natural gas as supplemental source rocks. Affected by early meteoric water, the dissolution of gypsum laid the foundation for high-quality reservoirs, and the residual gypsum also further preserved natural gas. This study provides new data for the paleoenvironment and a theoretical basis for further natural gas exploration.