Origin Of The Salt Rock Of Shahejie Formation And Its Relationship With Source Rock In Dongpu Depression

The area of Dongpu Depression is 5300km~2,which belongs to the smaller basin.The development of salt rocks distribution characteristics and occurrence of oil and gas resources has significant differences in the north and South sag.The salt rock is all developed in the northern region.The oil reserves in the northern region accounted for 80% of the whole basin's oil reserves and the natural gas reserves accounted for more than 90% of the whole basin's natural gas reserves.It can be seen that the development of salt rock has certain relation with the generation and accumulation of oil and gas resources.It is of great significance to establish the origin model of saline salt in the salt lake basin and explore the interaction between salt rock and source rock through the in-depth study of the paleoclimate,paleo environment,material sources of salt rocks,and the origin model of salt rocks in the salt rock formation period as well as the problems such as the occurrence and distribution of source rocks.Based on a large number of core observations,drilling and logging data,combined with the petrology,mineralogy,and geochemical analysis data of salt rocks,the author studied the paleoclimate and environment during the formation period of salt rock,discussed the origin of salt rock,analyzed the quality and evolution characteristics of source rocks and the coupling relationship between salt rock and source rocks,made a clear understanding of the effect of salt rock on hydrocarbon source rock.A study on the sedimentary environment of the Shahejie Formation during the salinization period in the Dongpu Depression through the methods of petrology,mineralogy,paleontology,and Sr/Ba,B/Ga,(V+Ni+Mn),Fe/Mn,Sr/Cu,Mg/Ca,V/(V+Ni),REE content and chlorine isotopic variation characteristics,etc.The results show that the saltification period of Dongpu Sag is a reductive environment of deep-water deposition with high salinity,reaching the standard of brackish-salt water or Saltwater.Trace elements and sporopollen fossils indicate that the salt rock development stage in the study area is a warm and humid environment.The analysis shows that the development of salt rock is related to the tectonic activity,and the relationship between the development of salt rock and the tectonic activity shows that the tectonic activity of the salt rock development layer is stronger than the freshwater sedimentary stage,and the salt rock in the zone of strong tectonic activity is thicker.The distribution pattern of rare earth elements shows that light and heavy rare earth fractionation is obvious,light rare earths are relatively rich,heavy rare earths are relatively depleted,and there is a clear positive Eu anomaly,consistent with the characteristics of rare earth elements in hydrothermal fluids.The distribution difference between the north and the south of the salt rock is due to the different forms of tectonic activity in the north and the south,and the main tectonic activities in the South and middle parts are mainly volcanic activity,and the northern part is the brine upwelling.Upwelling hot brines continue to provide salt rock material sources for the basin and maintain a deeper state of the lake water.The erupted magma evaporates large amounts of water during the cooling process,resulting in increased salinity of the lake.The seismic data show that the topography of the North South decomposition area is high,which is a "ancient beam",and the terrain is high and is not conducive to the exchange of North and South waters,so the development of the north and South salt rocks is different.The paleoclimate of the Shahejie Formation deposited on the salt rock is warm and humid,the depth of lake water is deep,and the interbedded with salt rock are dark grey,brown mudstone and oil shale.In lateral distribution,salt rocks develop in sedimentation subsidence centers on both sides of the central uplift zone of the sag,sandstones distribute around the salt rocks,and the same layer does not contact with each other;at the development time,salt rocks are deposited in the middle of two depositional cycles.During this period,the Depression was deep or ultra-deep water deposition.Through the above research,it is considered that the salt rock in Dongpu Depression is formed in deep water environment,and the salt-forming substances are hot brines that rise in the deep underground.Stratified brine is the origin model.Salt rock and source rocks are all distributed in the sedimentary period of the Member 3 and Member 1 of Shahejie Formation in the northern part of the Depression.Therefore,whether in the formation or the horizontal distribution range,both have a good coupling relationship.By studying the quality and thermal evolution characteristics of source rocks,it was found that the organic matter abundance of the source rock in the salt rock development area basically reached a good-better standard,and the kerogen type was dominated by type I and II1;but in the areas without salt rock development,the source rock is a better-worse standard,and the kerogen type is mainly Type II2 and Type III.The quality of the source rocks in the salt rock development areas is obviously superior to the source rock in the salt free areas.