Middle to Upper Ordovician stable carbon isotope stratigraphy and sedimentary facies in the Shunbei and Tahe areas, northern-central Tarim, China

Abstract

The Shunbei area, located in northern-central Tarim (Northwest China), represents a new, important area for petroleum and gas exploration. This study presents high-resolution, conodont biostratigraphically controlled stable carbon isotope data from 540 samples of four deeply buried (∼7000–8000 m) wells in the Shunbei area and its nearby Tahe area spanning the Middle to Upper Ordovician for detailed chemostratigraphic correlations. Additionally, thin sections are obtained from the two studied areas for sedimentary facies analysis. The δ13C data from Shunbei area correlate well with the results from Tahe area and other outcrops in northwestern Tarim. The major global δ13C excursions of the mid-Darriwilian Isotopic Carbon Excursion (MDICE, ∼+1‰) and the Guttenberg Isotopic Carbon Excursion (GICE, ∼+1‰) are recognized from the upper Yijianfang and the Lianglitag formations respectively. Two minor (∼+0.5‰) δ13C positive peaks from the Yingshan Formation of the Shunbei area are comparable to lower Darriwilian strata of platform facies in Tarim, providing a potential tool for correlating lower Darriwilian hydrocarbon reservoirs, such as the Yingshan and Yijianfang formations in the Tarim Basin. A major sedimentary gap is detected in the lower Qiaerbak Formation of the Shunbei area and the upper Yijianfang Formation of the Tahe area, indicating an erosional event occurred in the early Sandbian which could be correlated with the globally tectonic-active interval during the Darriwilian–Sandbian transition. The completely preserved MDICE and GICE in the Shunbei area, as well as its relatively deeper-water sedimentary environment in comparison with the Tahe area, collectively suggest that the Shunbei area is of significance for further geochemical studies on the Middle and Late Ordovician environments and their resources.