Organic petrology of shales in the Mesoproterozoic Xiamaling Formation in the northern part of North China

The shales of the Mesoproterozoic Xiamaling Formation in the northern part of North China, which rank among China’s most ancient marine source rocks, are characterized by simple biotic sources, high organic matter abundance, and low maturity. However, there is a lack of detailed study on the morphologies and origins of macerals in these shales. This study delves into the composition, characteristics, and origins of macerals in these shales using organic petrological methods. The results highlight a complex maceral composition in the Xiamaling Formation shales, encompassing bituminite, lamalginite, vitrinite-like maceral particles, mineral-bituminous groundmass, and thucholite. Notably, microscopic examinations illuminate transitional shifts in the optical properties of vitrinite-like maceral particles, bituminite, and lamalginite. These shifts-a gradually decreasing random reflectance and progressively increasing fluorescence-are intimately linked to the origins of these macerals. As lamalginite undergoes gradually enhanced anaerobic microbial degradation, it evolves into bituminite and vitrinite-like maceral particles. Additionally, the organic matter forms thucholites after experiencing ionizing radiation-induced polymerization of radioactive mineral particles, which primarily comprise monazite and thorite. The thermal radiation of these particles appears to have a certain influence on organic matter maturity only in a limited range.