| Unconventional reservoirs such as fractured mudstone reservoirs are drawing more and more attentions with the increasing difficulty of exploration. Mudstone cracks can be further divided into tectonic cracks and non-tectonic cracks. Non tectonic cracks can be defined as cracks which are induced by non-tectonic factors.Modern cracks of mud sediments contain mainly two types: desiccation cracks and synaeresis cracks. Abundant and effective researches have been done on them, however, most of which concentrated on only one type of cracks and lacked quantitative and comprehensive researches. This essay studied contemporary non-tectonic cracks in mud sediment of delta plain at Yellow River Delta. Subaqueous shrinkage cracks, desiccation cracks and mixed origin cracks are studied in detail at the aspects of quantitative classification, fractal features, planar and sectional growth pattern and development models. This essay has established the quantitative characterization and comparison standards of different non-tectonic cracks in modern mud sediment. Moreover, the formation mechanisms are studied for non-tectonic cracks in Gaolishan Formation of Carboniferous in Fenghuangshan profile in the city of Chaohu, Anhui, China.Non-tectonic cracks at Yellow River Delta can be divided into three types: Subaqueous shrinkage cracks, desiccation cracks and mixed origin cracks. Characteristics of each crack type in each typical work area are described in detail. The occurrence of many different crack types in this study area without significant microfacies changes can be attributed to the various volume and depth of overlying water body.Quantitative classification of first-grade crack unit area is applied to all study areas under the same criteria. The distribution of areas of subaqueous shrinkage cracks is the most centralized; desiccation cracks unit areas have the largest area span; while the mixed origin cracks fall in between the first two. Fractal dimensions are calculated for each crack area by the overlapping method. It is found that the fractal dimension rises from subaqueous shrinkage cracks to desiccation.Similar planar growth patterns of all kinds of cracks are observed: New crack bifurcating and original crack orientation changing angles are measured and angle development modes are established. A theory of shrinkage circle is adopted to demonstrate crack growth pattern. In vertical section, shrinkage sphere is used to illustrate the structure of bottom-up subaqueous shrinkage cracks, whereas there is a distinct difference from the V-shaped top-down desiccation cracks.Development models are established respectively for all types of cracks, in which relationships of mutual transformation of certain crack types are shown. Generally, from deep to shallow areas, subaqueous shrinkage cracks, mixed origin cracks and desiccation cracks develop in sequence, and some of them can transform if affected by weather. It is possible that more types of mixed origin cracks appear under more complicated conditions.The second part of this essay focuses on the non-tectonic cracks in Gaolishan Formation of Carboniferous in Fenghuangshan profile in the city of Chaohu, Anhui, China. Lithology, strata, and sedimentary environment are summarized. Then formation mechanism of Gaolishan Formation cracks is studied by analogy of those in documentation and researched at Yellow River Delta. The causes of desiccation, earthquakes activities and burthen are denied respectively, and finally, it is suggested that the Gaolishan Formation cracks are formed at the sediment-water interface.Although there seem to be numerous studies on spatial distribution of non-tectonic mud shrinkage cracks, convincing theories of factors affecting crack development is still yet to be discovered. It is recommended more studies of crack simulation experiment on certain crack development factor(s) to be done in order to gain a better understanding of non-tectonic shrinkage cracks.
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