Spatial location prediction of paleochannel scouring zones of Middle Jurassic Zhiluo Formation in the southern Shenfu mining area

Paleochannel scouring zones have been served as a common geological phenomenon in coalbearing strata,which can not only lead to the changes of geometric shapes of coal seams,but also affect the safe and efficient production of coal mines. Therefore,the accurate prediction of spatial distribution of paleochannel scouring zones has always been an important issue in the coal mine geology. At present,sedimentary analysis,geophysical prospecting and geostatistical methods are mainly used to predict the spatial distribution of paleochannel scouring zones. Due to many interference factors in underground geophysical prospecting and complex geological conditions affecting geological statistical mathematical models,the prediction of spatial distribution of paleochannel scouring zones by the geophysical prospecting and geostatistical methods are uncertain. The sedimentological analysis has been considered as the most direct and effective method to predict the spatial distribution of paleochannel scouring zones. The southern Shenfu mining area is one of the main mining areas of the Jurassic coal field in Northern Shaanxi. Sand bodies of the Zhiluo formation developed at the top of the Yan’an Formation in the southern Shenfu mining area are formed in braided river or braided river delta,and can lead to an obvious scouring effect on the minable coal seam. The coal mines in the southern Shenfu mining area have a high risk of water inrush with the paleochannel aquifers as water source,and the roof water disaster from the paleochannels has become an important disastercausing factor which threatens the safe and efficient production in the mining area. However,the distribution of the paleochannel scouring zones is unclear,leading to a great challenge to the prevention and cure of water disaster in the mining area. In view of this,based on field geological survey and core logging,the paleochannel scouring zones of the Zhiluo Formation in the southern Shenfu mining area have been identified and predicted by the sedimentological mapping of the sand dispersion system,coal seam thickness,and contact relationship between paleochannel sand bodies and underlying strata. It is found that the ancient channel in the lower part of the Zhiluo Formation is characterized by a large set of thick sand bodies,with a thickness of 30-70 m,which is composed of several positive rhythm cycles. The ancient channels have a significant effect on the underlying Yan’an Formation,resulting in the loss or thinning of the strata in Units V and IV of the Yan’an Formation. Distribution of sand dispersion system,coal seam,underlying strata units and lithology shows that the ancient channel scouring zones in the southern Shenfu mining area mainly develops along the HongjiannaoErlintuJinjie. The width of the complete scoured zones from Units V of the Yan’an Formation is 17-28 km,but the width of its part scoured zones is 37-46 km. Effects of the paleochannel scouring zones in the lower part of the Zhiluo Formation on coal mining in the southern Shenfu mining area are regional,and the prevention and cure of roof water disaster in the process of coal mining near the paleochannel scouring zones in the southern Shenfu mining area should be done well. This research will not only serve as a reference for the accurate prediction of distribution of paleochannel scouring zones in coalbearing series,but also can provide an important guidance for the safe and efficient production of the coal mines in the southern Shenfu mining area.