ANALOGOUS STUDY BETWEEN DESICCATION CRACKS AND DIAGENETIC SHALE CRACKS

The diagenetic shale cracks and the desiccation cracks are caused by water evaporation. This brings a certain similarity in formation process, patterns, main influence factors between cracks. But at the same time, there are many differences, such as formation mechanism, scale of the patterns, plane location etc.

The desiccation cracks are mainly produced in argillaceous fine sediments, meanwhile shale generally consist of them. According to the study, the conversion of illite and montmorillonite is one of the important reasons for diagenetic shrinkage and its transformation is affected by multiple factors. As the depth changes, the illite content gradually increases, which has a greater impact on the cracks. Mineralogy plays a great role for crack formation for both types of fractures. Therefore, for the experiment the most common argillaceous rocks were researched on cracks development with an influence of main factors, such as temperature, clay content and thickness of the clay layer. The results showed that clay content has significant effect on cracks development. Wide desiccation cracks occur in mud that contains of montmorillonite, while the total absence of cracks or its slight presence were noticed in illite samples. The thick layer of the clay produces the wide cracks exponentially. And the temperature of 100 ºC and 230 ºC affect the occurrence the thin and dense mesh of desiccation cracks.

The formation of the first crack also depends on clay content and varies in time with in an order from early to late: montmorillonite, kaolinite, illite. Moreover, microscopic analysis in shale showed, there is a wide variety of micro-fractures patterns, but the type of three-armed and four-armed star are the most common. The same regularity was found in desiccation cracks, despite the fact that diagenetic fractures and desiccation cracks have a different origin. The angles of three-armed and four-armed cracks in shale tend to 90º-100º and 170º-180º, that is same in cracks in argillaceous sediments samples.