Gulf of Nuna: Astrochronologic correlation of a Mesoproterozoic oceanic euxinic event

The ca. 1.4 Ga Velkerri and Xiamaling Formations, in Australia and the north China craton, respectively, are both carbonaceous shale deposits that record a prominent euxinic interval and were intruded by ca. 1.3 Ga dolerite sills. These similarities raise the possibility that these two units correlate, which would suggest the occurrence of widespread euxinia, organic carbon burial, and source rock deposition. Paleomagnetic data are consistent with Australia and the north China craton being neighbors in the supercontinent Nuna and thus permit deposition in a single large basin, and the putative stratigraphic correlation. However, lack of geochronological data has precluded definitive testing. The Xiamaling Formation has been shown to exhibit depositional control by orbital cycles. Here, we tested the putative correlation with the Velkerri Formation by cyclostratigraphic analysis. The Velkerri Formation exhibits sedimentological cycles that can be interpreted to represent the entire hierarchy of orbital cycles, according to a sedimentation rate that is consistent with Re-Os ages. Comparison of the inferred durations of the euxinic intervals preserved in both the Xiamaling and Velkerri Formations reveals a nearly identical ∼10-m.y.-long oceanic euxinic event. This permits the interpretation that the two hydrocarbon-rich units were deposited and matured in the same basin of Nuna, similar to the Gulf of Mexico during the breakup of Pangea.

Analysis on the difference of material composition and reservoir space of Mesoproterozoic dark marine shale in the Yanshan area

The dark marine shales which are thick in the Mesoproterozoic Hongshuizhuang formation and Xiamaling Formation in the Yanshan area are the subject of this study. The material compositions and pore characteristics were analyzed, and the reasons for the differences in the adsorption capacity of shale were emphatically discussed through geochemical analysis, X-ray diffraction, scanning electron microscope, low-temperature nitrogen adsorption, mercury injection experiment, isothermal adsorption, and other analysis methods. The results indicate that the samples are rich in high organic carbon content, and vitrinite reflectance suggests that the degree of thermal evolution is in the late-mature to high-mature stage; the mineral compositions mainly consist of quartz and clay minerals. The micropores are relatively developed, which mainly include intergranular pores, intragranular pores, microcracks, and many other pores. Results from the nitrogen adsorption analysis suggest that the pore structures can be divided into three types. Porosity has obvious fractal characteristics. The fractal dimensions of the adsorption pore are larger than those of the seepage hole. The difference of shale adsorption between the Hongshuizhuang Formation and Xiamaling Formation is mainly determined by the volume of micropores and transition pores and the different mineral compositions.