Sedimentary Characteristics And Paleo-environment In Zhifang Fomation Of The Middle Triassic In Ordos Basin

During the past decades,with the steady progress in the exploration process in Ordos basin,showing Zhifang Fomation has some exploration potential,gradually Zhifang Fomation was put into the schedule of oil and gas exploration and development.Based on the prophase investigation and combined with a large amount of field and laboratory research,this paper has conducted the system research to Zhifang Fomation development characteristics,provenance direction,sedimentary system and distribution rules and paleo-environment of the basin.Combined with field outcrops and coring interval observation of 13 focus drilling well of Zhifang Fomation,and making full use of the drilling and logging data,contact relation between strata and stratigraphic distribution feature of the Zhifang Fomation have been described detailly.The contact relationship between Zhifang Fomation and underlying Heshanggou Fomation is parallel unconformity or conformity,while with the Yanchang Fomation is parallel unconformity.The strata thickness of the Zhifang Fomation in study area has the feature of "the south strata is much thicker than the north ",which reflects the ancient landform pattern with the feature of "the north landform higher than the south" before the sedimentary of the Zhifang Fomation,and the Zhenyuan-Zhengning-Tongchuan area is the deposition and settlement center of Ordos basin.We studied the provenance of Zhifang Fomation through the paleo-current analysis,light and heavy mineral analysis,grain size analysis.Zhifang Fomation is considered to be with a characteristic of multi-provenance,multi-river system.The deposition of Ordos basin is conctroled by the provenance of northwest and northeast,southwest provenance as the secondary provenance,south provenance was relatively weak and had little influence on sedimentary framework of basin.Five units sand bodies developed in study area and had a certain continuity.Based on the provenance analysis,through the observation of field outcrops and drilling core,starting with the macroscopic facies mark and the microscopic structure characteristics,we identified the sedimentary facies of the Zhifang Fomation in the research area.During the sedimentary period of Zhifang Fomation,there are four types of sedimentary system in the research area,such as alluvial fans?river?lake delta and shore-shallow lacustrine,with shape of ring belt.The periphery of research area developed the alluvial fan and river facies as it's near the provenance.From the provenance to the center of the lake basin delta plain and delta front are widely distributed.The area of Zhengning-Tongchuan mainly developed the shore-shallow lacustrine sediments in the southern area of Ordos basin.Comprehensively using the major,trace element ratio method and sporopollen analysis method,we studied the paleo-environmental conditions during the deposition of Zhifang Formation,such as the paleo-climate,paleo-salinity and paleo-water depth.During the deposition of Zhifang Formation,the paleo-climate condition is warm and humid;the lake water is mainly terrestrial freshwater,while brackish water exists in local area.Affected by the water depth and the hydrodynamic condition,the whole lake basin shows oxidative or redox transitional environment,the max paleo-water depth is 82 m.We also studied the paleo-environmental vertical evolution of Zhifang Formation,and it is found that the changes of the paleo-climate,paleo-salinity and paleo-water depth are similar,which have a good correlation.We believe that the periodic change of paleo-climate is the reason for the changes of the lake water salinity and water depth periodically.