Lacustrine Sedimentary Characteristics of the Pliocene Xiayoushashan Formation in Xianshuiquan Area,Western Qaidam Basin:A case study from cored interval of well Xiandong 1
[Objective]Exploration practice of the Qaidam Basin show that the remaining petroleum in the Qaidam Basin is mainly concentrated in the western area,where relatively little exploration has been carried out due to the complex landform and the presence of typically low-permeability unconventional reservoirs.Accordingly,to increase petroleum exploration in the western Qaidam Basin.[Methods]A comprehensive study of the data from sedimentary structure observation,rock slices,X-ray diffraction whole-rock analyses and seismic data,among other techniques,was carried out on an interval of core from the Pliocene Xiayoushashan Formation in well Xiandong 1.[Results and Conclusions]The rock was found to comprise gray-black mudstone and shale interbedded with thin-to-medium mas-sive fine-grained greywacke.The top and bottom of the sand bodies are in abrupt contact with the mudstone.The lower boundary of the sand bodies is obviously an uneven erosional surface,and the mudstone and shale beneath the ero-sion surface exhibits deformed bedding.The sand bodies are sandy debris flow deposits formed by re-transport.The dark-gray to gray black calcareous mudstone and shale contains interbeds of calcirudite bearing shell-terrigenous de-bris and siltstone,and a silty mass with obviously deformed bedding.These laminae are clearly seen and have an ob-vious deformation direction caused by clastic slump.The Pliocene Xiayoushashan Formation in well Xiandong 1 is mainly the dark gray-gray black mudstone and shale interbedded with siltstone bearing dolomite,calcite and clay.The mudstone and shale contains horizontal rhythmic bedding.The siltstone has developed slightly dipping cross-beds with more clay laminae,climbing-ripple lamination and coarsely interlayered bedding,which are mainly from semi-deep lake deposits interbedded with weaker bottom flow deposits derived from sediment gravity flows.In addition,the deeper-water areas of the Altun Mountain front appears to have developed slump-sediment gravity flow sedimentary reservoirs similar to well Xiandong 1 at a different period of basin development,but further research into this aspect is needed.
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