The High Frequency Sequence Framework And Interstratum Karst Reservoirs Development Mechanism Of Lower-middle Ordovician In Tabei Area,Tarim Basin

Stable Ordovician carbonate deposits develop in the Northern Tarim Basin,in which the largest carbonate karst type oil and gas field were found.On the basis of field outcrops,drilling core,well logging,seismic data and sample test,the third and fourth order sequence boundaries were identified in the lower-middle Ordovician.And the high-resolution sequence framework was built,in which the developmental mechanism of interstratum karst reservoirs was studied.It is hard to identify the lower level sequence boundary.In the outcrop,the Handheld X-ray fluorescence(HXRF)instrument and MGP HDS-100 G instrument were used,and the meter cycle stacking pattern was analyzed.Paleontologys and geochemical characteristics,well logging and seismic response characteristics were conducted to identify the lower level sequence boundary.The high-resolution sequence framework,including three third sequences and six fouth sequences,was built on the basis of the identification of the sequence boundaries and marine flood surfaces.In the sequence framework,the type facies indactor lithology deveope,including stromatolite,algae lamellar limestone and algal mound limestone,which reflects the tidal,lagoon and open platform setting,respectively.On the basis of the lithology sets,five types of meter-scale cycle models were grouped into peritidal sequences and subtidal sequences.On the basis of cycle stacking patterns,vertical lithofacies variations,and accommodation variations were shown from Fischer plots.The lower sequence set of Yingshan Formation is peritidal facies-dominated sequences and characterized by a progressive decrease in accommodation space.It indicates a longer-term fall in sea level.On the contrary,the upper sequence set of Yingshan Formation is subtidal facies-dominated sequences and characterized by the accommodation space with a progressive increase.Interstratum reservoirs refer to the reservoirs that develop not in the buried hill area,and are related to the unconformity,palaeogeomorphology,and fracture.And it has the characteristics of buried deep,wide distribution and varied controlling factors,including the reservoirs of syngenesis palaeokarst,interlayer karst,bedding karst and fault-controlled karst.Forming conditions of the fracture-cavity reservoirs was analyzed in the high-resolution sequence framework.The mainly influence factors include sequence boundary,phreatic aquifer and meter cycles.Interstratum reservoirs are mainly the bedding karst,interlayer karst and syngenetic karst under the condition of the Medium and long term angle unconformity,which develop in the upper part of Yingshan Formation,while interlayer karst and syngenetic karst related with the micro-angle or parallel unconformity mainly develop in the lower part of Yingshan Formation and Penglaiba Formation.The karst develop in three evolution stages in Middle and Lower Ordovician,including the interlayer karst and syngenetic karst in the Middle Caledonian,bedding karst and fault karst in Later Caledonian and Early Hercynian,hydrothermal karst in the Later Hercynian.The bedding karst is mainly controlled by the unconformity,phreatic aquifer meter cycle.The syngenesis palaeokarst controlled by the high-energy facies,and the interlayer karst controlled by unconformity are the basis of the early karst,while Bedding karst is the key to form scale karst reservoir.According to interstratum reservoirs model,the favorable distribution area was evaluated and predicted.