| In recent years,the global shale gas industry has developed rapidly,especially in North America,which has changed the world energy supply pattern.The potential of shale gas resources in China is huge,which has attracted the attention of the government and related enterprises.Shale gas reservoirs have these characteristics:high natural gamma,low acoustic velocity,low density and high resistivity.These characteristics are the main basis for identification and evaluation of geophysical techniques for shale gas exploration.Shale gas reservoirs are usually distributed in shale source rocks with thicker and wider distribution in the basin.The main source rocks in the study area are the Babaoshan Formation of Upper Triassic and Yangqu Formation of Middle and Lower Jurassic,which are all continuous terrigenous clastic deposits,there are no faults passing through the study area,the stratum occurrence is gentle,and the strata in the basin distribute steadily.The distribution area of dark shale in Babaoshan Formation of Upper Triassic in Babaoshan Basin is 830.57 km~2.The area of Yangqu Formation of Middle and Lower Jurassic is slightly smaller than that of Babaoshan Formation.The overall difference is not significant.The thickness of shale increases gradually from east to west.From the burial depth,the eastern part is inclined upward,the western part is inclined,and the burial depth increases gradually.The deep part of the western end of the valence zone may preserve better source rocks and have great potential for shale gas exploration.The dark shale in the lower part of Babaoshan Formation is mainly consists of grey-grey black siltstone and black shale with a single layer thickness of 0.20m-133.60m,which distributed in the study area is nearly 500 km~2.The number of shale layers in the upper section of Babaoshan Formation,upper and lower section of Yangqu Formation reaches 10,3 and 17 layers,respectively.The thickness of each layer above 15m is 16,and the mud to ground ratio is 27.57%,35.94%and 44.55%respectively.Wide field electromagnetic method is an artificial source frequency domain electromagnetic sounding method,which has the advantages of large exploration depth,wide observation range,high efficiency,high measurement accuracy and strong adaptability.This method can accurately calculate the wide-field apparent resistivity based on the electromagnetic field expression of bipolar source,and can observe in a wide area without limitation,and greatly increase the exploration depth at the same distance between transmitters and receivers.This method can effectively improve the efficiency and relative accuracy by measuring an electrical component or a magnetic component and obtaining multiple frequencies simultaneously.Wide field electromagnetic survey in Babaoshan Basin has remarkable effect on dark shale in this area.It can not only overcome the influence of topography and geomorphology and reduce exploration cost,but also meet the needs of multi-layer and large-area shale gas exploration in Babaoshan Basin.Wide field electromagnetic survey has been carried out in Babaoshan basin.956 physical points of wide field electromagnetic method have been completed,602 physical properties have been collected and measured,and resistivity and polarizability parameters have been measured.The average resistivity of mudstone siltstone,medium-grained sandstone and fine sandstone is 149-208?·m,which shows lower resistivity characteristics.The average resistivity of rock mass and other strata is more than 3,000 to up to 50,000?·m,which is 1 to 2 orders of magnitude higher than the lithology of target strata,indicating that this method has a significant effect on shale gas reservoir exploration.Finally,it is concluded that the Upper Triassic Babaoshan Formation and the Early-Middle Jurassic Yangqu Formation in Babaoshan Basin have good prospecting potential for shale gas. |
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