The effect of microbial alteration and diagenetic evolution on clay minerals transformation in lacustrine basin

Clay minerals transformation has received much attention because of its impact on hydrocarbon generation, sediment physical properties, rock compressibility, and fault frictional characteristics. However, the role of microorganisms in facilitating clay minerals transformation during the early diagenetic stage in lacustrine basins was less studied, as previous works have focused mainly on marine and marine-continental transitional environments. This study focuses on the Cretaceous Mingshui Formation (K₂m) to Shahezi Formation (K₂s) mud shales in the Songliao Basin because of their high clay mineral contents. X-ray diffraction analysis, major element analysis, sulfur isotope analysis, and gas chromatography–mass spectrometry (GC–MS), along with a kinetic model of smectite-illite transformation, were used to investigate the variations in the content and characteristics of the transformation continental mud shale clay minerals during sedimentation and diagenesis. The findings indicate that the predominant clay minerals are smectite(S) and illite(I) in shallow buried strata (<1000 m), whereas I and illite-smectite mixed layers(I/S) are the main components in more deeply buried strata (>1000 m). Microorganisms such as methanogens and sulfate-reducing bacteria promoted the transformation of smectite to illite in mud shale from the early diagenetic stage in the semideep to deep lake sediments. However, such transformation was not observed in the semideep lake to shallow lake transitional sediments, as the oxygen and weak oxidizing sedimentary environment did not facilitate such microbial processes. In this work, two models for clay minerals transformation were proposed: (1) a clay minerals transformation model that is solely controlled by abiotic factors (such as temperature and pressure) at basin margins, and (2) a clay minerals transformation model that is collaboratively influenced by biotic and abiotic factors at basin sedimentary centers. The results of this study contribute to the understanding of clay minerals transformation under biological influences, elucidate microbial roles in promoting smectite-illite transformation during early diagenesis, and enhance the transformation sequence of clay minerals in lacustrine basins.