Thucholite in Mesoproterozoic shales from northern north China: Occurrence and indication for thermal maturity

•Thucholite was formed by radiation-induced polymerization of synsedimentary algal mats.•The 1.4 Ga Hongshuizhuang Formation is still in the oil window.•Radioactive heat has limited effect on the thermal maturity of organic matter.

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 (1.4 Ga) 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 1.4 Ga 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.