A study on microscopic characteristics of stylolites caused by pressure dissolution effect in carbonate rocks

Carbonate rocks is one of the important types of hydrocarbon source rocks. Stylolites as a kind of important geological characteristic of carbonate rocks，it is a kind of geological phenomenon extensively developing in... [ view full abstract ]

Carbonate rocks is one of the important types of hydrocarbon source rocks. Stylolites as a kind of important geological characteristic of carbonate rocks，it is a kind of geological phenomenon extensively developing in carbonate rocks caused by pressure solution, and it is the important part of organic matter enrichment and geological fluid migration in carbonate rocks. This project focus on choosing the Erodos and the Bohai Bay Basin Cambrian-Ordovician carbonate rock samples to study. By observing microscopic characteristics of casting thin sections of the rock samples to analyze stylolites’ sedimentary petrology and mineral petrology characteristics and significance of hydrocarbon generation in the carbonate rocks. When we observed the carbonate rock slices under microscope, result shows that the main mineral composition in stylolites is dolomite, the filler mainly is black pitch, earthly yellow argillite and organics, etc. Under polarizing microscope when we turn on reflected light black substance was found to contain large amounts of pyrite. It shows the characteristics of basal cement and the mixed base support between particles. This is important evidence of large amounts of organic matter enrichment in stylolite. It provides basis for stylolite matter as source rock to generate oil and gas. However, the main mineral composition of matrix around stylolite is calcite. By contrast, the dolomite and black organics and asphalt that enrich in stylolite are rare. When turn on the reflected light under polarized microscopic we found it contain small amounts of pyrite, it shows features of contact cementation or mosaic cementation, line contact or bump contact. This is an important evidence for stylolite’s pressure solution causes. In the process of stylolite formed, fluid carry the soluble carbonate migrate to matrix, when conditions change the dissolved ions will recrystallize, the matrix around the stylolite will become denser. The phenomenon has been observed also provide evidence for this theory. In addition, regardless of the stylolite or the surrounding matrix, the color blue part, which means the pore that has not been filled is few, the most part the original pore has been filling completed, the lithology is compact. It also affirmed the significance of pressure solution after sedimentation to stylolites’ formation.