| Many investigations have been done on the formation mechanism and propagation law of desiccation cracks.Nevertheless,the formation mechanism of diagenesis micro-seams in shale is still unclear.Due to study propose,the SEM observation of marine,transitional and continental shale samples were observed on microfractures.Likewise,the most widely recognized clay minerals of shale(montmorillonite,illite,kaolinite)were experimentally studied to distinguish the main controlling factors of desiccation crack development,and show the impact of these factors on crack geometry.The most widespread diagenetic shrinkage cracks in shale depict evident similarity with desiccation cracks.Thus,to imitate the process of desiccation crack propagation,that can clarify the formation mechanism of shrinkage cracks,a diagenetic crack imitator machine and a laboratory oven were used.The results have shown that clay content,temperature,and thickness of the mud layer are the fundamental factors that control the crack occurrence and development.Moreover,crack width increases with increased montmorillonite content,and diminishes with increased kaolinite and illite content.Temperatures of 100? and 230? stimulate further desiccation cracks formation in pure mineral samples.Fractures width was noticed to increase with layer thickness.Moreover,92% of all samples of desiccation cracks depicted the three-armed pattern with angle distribution about 90-100 and 170-180 degrees.The water content during dehydration has an important role in the process of cracks formation that can be divided into four phases,where the great part of cracks number form at the initial and development stages.In nature shale diagenesis includes processes that take a great part in microcracks formation,namely under-compacted overpressure,cementation,recrystallization,mineral transformation,dehydration,hydrocarbon generation.Moreover,the origin of shale has significant impact on fracture propagation.Thus,marine shale can be described as a rock with high content of brittle minerals and organic matter and low clay content,that creates specific environment for a great fracture development.Transitional shale depicts a high content of clay and unstable minerals,and the cemented minerals are generally low,where bedding and diagenetic shrinkage fractures are more developed,and organic pores and cracks are generally not formed.Continental shale represents high clay minerals and relatively low brittle minerals content,and dominated by inorganic pores,with low development of organic pores and micro-fractures.The complex fracture network caused by fracturing is difficult.The results of SEM observation have shown the dominant present of(from highest to lowest number of cracks)undercompacted/overpressure cracks,then shrinkage,hydrocarbon generated,stress tectonic and mineral cracks.The bedding type was not observed in this investigation.Each type of fracture occurs according to its individual origin controlling by equation of strain and critical strain energy release rate.So that hydrocarbon generated crack forms under high pressure during organic matter maturation,shrinkage crack propagates during dehydration of intracrystalline,osmotic and pore water with increases of depth of burial.The driving force for compacted/overpressure fracture represented by high pressure in the pores in shale,that occurred after rapid sedimentation and water that also tries to release.The mineral crack occurs during chemical reactions during mineral transformation inside the mineral,as well on the borders between two reacted minerals.The main controlling factors for diagenesis fractures are mineral composition,that represents clay and brittle mineral content,total carbon organic contents,that is likely to be a dominant factor of microfracture development,maturity of organic matter,that creates not only anomalously high pressure and the local volume increase,but also fluids generation,and the shale sedimentary facies,that keeps all factors together and controls their variations. |
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