Sedimentary geochemical records and their depositional environmental significance during Cambrian-Ordovician transition in well YD 2, eastern Tarim Basin, NW China

Abstract

In the eastern part of Tarim Basin, thick carbonate rocks were developed in the Upper Cambrian, while thick grey-black mudstone in the Lower Ordovician. Due to the complex tectonic and geological conditions in the basin, there are still many uncertainties in the study of the sedimentary environment, post-diagenesis and hydrocarbon accumulation conditions during the Cambrian-Ordovician transition. In this paper, 24 drilling core samples of the key strata of the Upper Cambrian-Lower Ordovician in the well YD2 were selected as the main research objects. Their mineral composition, major and trace elements (including REE) were analyzed. The results showed that the proportion of carbonate-siliciclastic sources in the rocks was controlled by the sedimentary environment, especially seawater depth. The oxic-suboxic sedimentary environment is indicated by the redox-sensitive elements and other trace elements (especially REE). This is consistent with the mineralogical features of the samples and the relatively low TOC. The elements Mn, Cr, V, Ni, on the whole, show a trend of first increasing, then decreasing, and repeated occurrence, indicating the sea level fluctuation cycle. In addition, according to the Ba/Sr and Mn/Sr ratios, the upper Cambrian carbonate rocks were normal marine sediments with weak post-diagenesis. Quartz, dolomite and calcite veins filling the fissures of the samples may have formed in the stage of intense uplift denudation, which was unfavourable to the formation of large oil and gas reservoirs.