The Ediacaran–Cambrian boundary in the Tarim Basin, NW China: Geological data anomalies and reservoir implication

Abstract

Abstract In this study, unconformities (T90) formed during the succession from the Ediacaran to the Cambrian were identified based on five types of data, namely, field outcrops, C and O isotopes, major and trace elements, well logs and seismic data. The outcrop data show notable lithological changes above and below the T90 interface. The C and O isotope data show significant negative δ13C and δ18O excursions at the T90 interface. The negative δ13C excursion at the T90 interface occurred concurrently with the negative δ13C excursions that occurred globally during the same period. The average values of Ca, Mg, Nd, Pr, Ce and La were higher below the T90 interface than that above the T90 interface and that the average mass fractions of Zr, Sr, Zn, Ba, Mn, S, Si, Ni, Fe, Al, Rb, K, Cl and P were lower below that interface than that above it. The well log data show that at the T90 interface, the concentrations of U and Th and the mass fraction of K increased suddenly (from 97.66 to 104.308 ppm, from 210.005 to 232.086 ppm and from 11.609% to 12.99%, respectively) and the gamma ray value increased significantly, reflecting a brief land exposure of the interface. Onlap and truncation patterns were visible on the seismic sections above and below the T90 interface. Various types of unconformities have developed at the T90 interface in various areas of the Tarim Basin. Paraconformities are mainly distributed in the Kuluketage and Kalpin outcrop areas as well as the depressions in the northern Tarim Basin and a large area north of the Tarim Basin. Angular unconformities are distributed near the north wing of the central uplift belt. The paleogeomorphology of the T90 unconformity interfaces limited the distribution of sedimentary facies in the overlying strata, thereby affecting the distribution of the hydrocarbon source rocks of the Yurtus Formation, and controlled the development of karst reservoirs in the underlying strata.