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Abstract: Thucholites have mostly been observed in the intergranular pores of sandstones and conglomerates. In this study, investigation of thucholites in Mesoproterozoic shales has been undertaken to explain the origin and maturation of the organic matter. An optical microscope and a scanning electron microscope equipped with an Oxford X-ray energy-dispersive spectrometer (SEM-XDS) were used to study the thucholite in the Hongshuizhuang Formation (1.4Ga) from the Yanshan Basin of northern north China. The rare and scattered thucholites are present in the shales and do not occur in the dolomites/argillaceous dolomites. The thucholite contains an apparent radioactive mineral particle as the core. The inner rim around the radioactive mineral core generally shows higher reflectance and non-fluorescence, whereas the outer rim displays low reflectance and strong fluorescence. These contrasting observations indicate higher polymerization in the inner rim relative to the outer rim. The mineral grains in the thucholites are mostly monazite and less commonly, zircon. The radioactive mineral grains of granodiorite provenance fell into algal mats at the sediment–water interface through seawater transportation, followed by polymerization of organic matter by ionizing radiation from the radioactive mineral matter through geological time. This process of formation of thucholites in shales is different from that in sandstones and conglomerates. The outer rim of the thucholites shows strong fluorescence, indicating that the thermal maturity of these 1.4Ga old sediments is still in the oil window, consistent with the low equivalent vitrinite reflectance values (0.82–0.88%) calculated from protobitumen reflectance. The reflectance of the outer rim is close to the average of protobitumen reflectance of the whole sample, suggesting that radioactive heat has limited effect on the thermal maturity of the organic matter. [Copyright &y& Elsevier]

The overmature Precambrian to Lower Paleozoic marine shales from China contain relatively simple organic matter (OM) composition, which is dominated by in-source solid bitumen (i.e., solid bitumen in the hydrocarbon source rocks). The thermal evolution behavior of the original OM and the determination of thermal maturity in the Precambrian to Cambrian marine shales have been challenging for decades. The vitrinite-like maceral (VLM) is widely present in these marine shales, and its origin is still unknown. To address these issues, the immature Proterozoic Xiamaling shales from China and the immature Cambrian Alum shales from Sweden, and a Chinese immature Carboniferous coal were heat-treated at temperature range of 300 °C to 550 °C. The carbonized residue of the artificially matured samples was examined for their morphological and reflectance variation, and the results were compared with data on the other overmature natural shales from China and Sweden. OM components are similar in the Xiamaling and Alum immature oil shales, consisting of filamentous algae, matrix bituminite, bituminite, VLM and liptodetrinite, and rare thucholites are present in the Xiamaling shales. The algal-derived OM decomposed gradually due to hydrocarbon generation at 300–350 °C. OM is mainly composed of the in-source solid bitumen in the artificially heated shales after 350 °C, similar to the overmature Precambrian to Cambrian natural shales, and the in-source solid bitumen gradually loses its mass with increasing thermal maturity. The in-source solid bitumen is derived from the thermal cracking of the retained oil or the direct conversion of algal-derived liptinite macerals (e.g., the bituminite) or their mixture. VLM in the Xiamaling oil shales can not be observed after 350 °C, but VLM is still present in the Alum oil shales. It can be inferred that there is a different source of VLM in these shales, and VLM in the Xiamaling oil shales contains more volatile material. The VLM in the Xiamaling shales may be biodegradation products of liptinites under anoxic environments. The origin of VLM in the Cambrian Alum shales requires further study to be verified, although it is certain that graptolites are not its sources. The reflectance of in-source solid bitumen (SBR o) increases with heat-treated temperature in both the Xiamaling shales and the Alum shales, but at different rates, which may be due to the difference of the original kerogen composition in these shales. VLM reflectance (VLMR o) and SBR o in sections perpendicular to bedding can be used to determine the maturation level of the Precambrian-Cambrian sediments. Their relation to equivalent vitrinite reflectance (EqVR o) can be expressed by the following equations: EqVR o = 1.07 × VLMR o – 0.18, EqVR o = 0.87 × SBR o + 0.25 (in the Precambrian sediments) and EqVR o = 1.15 × SBR o + 0.01 (in the Cambrian sediments). • The vitrinite-like macerals have different thermal evolution behavior and origin. • The formation mechanism of in-source solid bitumen has been determined. • Thermal maturity indicators have been studied for the Precambrian-Cambrian shales. • In-source solid bitumen is the predominant composition in the postmature shales. [ABSTRACT FROM AUTHOR]

The Mesoproterozoic Hongshuizhuang Formation (∼1.4 Ga) from northern China consists of organic-rich sediments which record important environmental and biological information from that time. Organic and element geochemistry techniques were used to assess the syngeneity and indigeneity of hydrocarbons, and the thermal maturity and biological sources of the organic matter in the Hongshuizhuang Formation. Results from a slice-extraction experiment indicate that the exterior of the core was subjected to contamination from drilling, sampling and storage, whereas the organic matter in the interior shale core slices was indigenous. The organic matter in the interior argillaceous dolomite core slices might have been affected by the migration of oil. It is important to remove the outside of the core prior to solvent extraction, as this increases the possibility of analysis of indigenous hydrocarbons. The source rocks of the oil are the black shales of the Hongshuizhuang Formation. Aliphatic and aromatic hydrocarbon distributions indicate that the Hongshuizhuang Formation samples have a low thermal maturity in the early to middle oil window (0.6–0.8 %Ro equivalent), based on comparison with middle Velkerri Formation rocks of about the same age in the McArthur Basin, northern Australia. These data are consistent with results based on the values of T max and protobitumen reflectance, and suggest that the samples contain well preserved organic matter that has not been thermally destroyed. The redox conditions indicated by the pristane/phytane (Pr/Ph) ratios are not consistent with other evidence. The correlations between n -C 20− / n -C 21+ , major elements and Pr/Ph indicate that the Pr/Ph ratios are influenced by biological sources and mineralogy, rather than by redox conditions. Pr and Ph may have been derived not only from chlorophyll-a, but also from archaea. The short and long chain n -alkanes in the Hongshuizhuang Formation were derived from anoxic clay-rich environments and oxic carbonate-rich environments, respectively. The biomarkers observed in the Hongshuizhuang Formation indicate that the organic matter was primarily derived from prokaryotes. [ABSTRACT FROM AUTHOR]