Changing Hydrocarbon-Producing Potential of the Cambrian Niutitang Shale: Insights from Pyrite Morphology and Geochemical Characteristics.

Abstract

The characteristics of shale gas enrichment conditions at different depositional positions of organic-rich shale in the Niutitang Formation of the Lower Cambrian of the Upper Yangtze in South China vary greatly. The study of pyrite provides a basis for the restoration of the ancient environment and a reference for the prediction of organic-rich shale. In this paper, the organic-rich shale of the Cambrian Niutitang Formation in the Cengong area is analyzed by means of the optical microscope, scanning electron microscope observation, carbon and sulfur analysis, X-ray diffraction whole rock mineral analysis, sulfur isotope test, and image analysis. The morphology and distribution characteristics, genetic mechanism, water column sedimentary environment, and influence of pyrite on the preservation conditions of organic matter are discussed. This study shows that the upper, middle, and lower sections of the Niutitang Formation are rich in pyrite (framboid, euhedral pyrite, subhedral pyrite, etc.). Meanwhile, the sulfur isotopic composition of pyrite (δ34Spy) shows a tight correlation with the framboid size distribution throughout the shale deposits of the Niutang Formation, and the average particle size (9.6 μm; 6.8 μm; 5.3 μm) and distribution range of framboids (2.7-28.1 μm; 2.9-15.8 μm; 1.5-13.7 μm) in the upper, middle, and lower sections show a downward trend. In contrast, the sulfur isotopic composition of pyrite shows a tendency to become heavier from above and below (mean = 0.25‰ to 5.64‰). Together with the covariant mode of pyrite trace elements (such as Mo, U, V, Co, Ni, etc.), the results showed significant differences in the oxygen levels in the water column. They show that the transgression led to long-term anoxic sulfide conditions in the lower water column of the Niutitang Formation. In addition, the main and trace elements in pyrite jointly indicated that there was hydrothermal action at the bottom of the Niutitang Formation, which led to the destruction of the preservation environment of organic matter and the decrease of TOC, which can also explain the reason why the TOC content in the middle part (6.59%) was higher than that in the lower part (4.29%). Finally, the water column became an oxic-dysoxic condition due to the decline of sea level, and the TOC content decreased (1.79%).