Research On Palaeokarstification Of Ordovician Carbonate Rock In The East Lungu Region, Tarim Basin

Ordovician carbonate rock in the east Lungu region is divided into two partsincluding Lunnan30 zone and Lungu38 zone in the thesis. It makes use of drilling,logging and rock core datum to explore palaeokarstification characteristic anddistribution law of Ordovician carbonate rock in the east Lungu region, to providetheory basis for reservoirs evaluation.There are three kinds of Ordovician carbonate rock palaeokarstification in theeast Lungu region including syngenesis palaeokarst in middle Caledonian,hypergenesis palaeokarst in early Hercynian and subsequent burial palaeokarst.Hypergenesis palaeokarst contribute on carbonate rock reservoirs at most.Syngenesis meteorie water solution hole and fissures consist in middleOrdovician Yijianfang Formation platform margin facies oolitic limestone,bioclastic limestone, grain limestone at east of the Tumuxiuke Formation pinch-outboundary. Yijianfang Formation form kinds of dissolution pores, hole and fissuresas the result of sea level change and exposure dissolution. Irregular poers, smallhole, moldic, interparticle, intraparticle, vadose silt filling, beetling calcite cementand crescented calcite cement are main dissolution. Karst zone thickness is aboutforty-seventy meters forming available reservoirs regionally.Burial palaeokarst is selectable dissolution by organic acid which leadscompact rock to dissolution vug, fracture and stylolite fissures for carbonate rockrelation with gas movement, sprinkling distribution on rock core, exceptional wellexist only.Hypergenesis palaeokarst usually manifests distinct vertical zoning rule whichdivides into epikarst zone, vadose zone and phreatic zone. Epikarst zone lies on theweathering crust surface, rock is cataclased by weathering eluviation and directrunoff, transported stonecast to weathering crust face of low-lying landform, formsthe karren, dissolution cavern, fissures, wash, funnel and sinkhole, etc. Its filling issurface eluvium material and cavity wall collapsed material. Vadose zone is belowweathering crust face and on water table. Ground water along terrane's fractureeffluent downwards mainly, give priority to vertical karst with high angledissolution fracture, stylolite fissures, vertical shoestring minitype vug distributedon fissures and stylolite, filled compound by calcite, clay and some other differentorigin vadose sand. Phreatic zone upper limit is water table at lowest, its groundwater is controlled by pressure gradient and flow along with horizon, horizontalcave, underground river and dissolution vug, fracture develop segment is the mostdistinct karst character by far.According to genesis and identification of hypergenesis palaeokarst, this textdivision of single well vertical karst zone, characters of vug develop segment andlog response is proceeded for fifteen wells in the east Lungu. Entire verticalcharacter is that epikarst zone is below plane of denudation in fifty meters,dissolution vug and sinkhole develop highly. Vadose zone is below plane ofdenudation in hundred meters with high angle dissolution fracture and shoestringdissolution fracture, cavity, vug distribute along fracture and stylolites. Dissolutionvug of phreatic zone is usually horizontal, develop large dissolution cave.Hypergenesis palaeokarst develops on the Ordovician buried hill peak, a set ofintegral weathering karst vertical serial exists inYijianfang Formation and YishanFormation of Lunnan30 zone, vadose zone and phreatic zone lie in west ofTumuxiuke Formation pinch line. Lungu38 zone atmosphere fresh water eluviateLianglitage Formation knollenkalk segment by fault belt and fracture of buriedhill peak, thickness of vadose zone is very thin or less development, phreatic zonedevelops in landscape orientation without exception.The development mode of hypergenesis palaeokarst is that, karst highlands ispreponderate in vertical dissolution cave and miniature horizontal cave, karstslopes is represented by large horizontal dissolution cave, karst basin only exist afew of dissolution vug and is discharge area of karst developmental plase.Ordovician Yishan Formation, Yijianfang Formation and LianglitageFormation limestone is fit for dissolution largely and speed with ancient weatheringkrust, but Sangtamu Formation is mainly mottle clastic rock interstratify limestone,Tumuxiuke Formation is mudstone interstratify limestone, that goes against karstdevelopment. So sediment and lithofacies is basic condition of karst development.Tectonic pattern and fracture character is important condition of karst development,tectonic pattern controls palaeogeomorphology characters, rupture and fractureprovide available channels and increase soluble rock's dissolution acreage.Palaeogeomorphology is key factor of karst development, the east Lungu locates intransition area of karst slopes and karst basin, northwestward is high andsoutheastward is low entirely, karst reservoirs available development area is karstslopes.The types of Ordovician carbonate rock reservoir space include cave, vug andfracture three types in the east Lungu, according to combination character ofporosities and fractures, reservoirs can be divided into cave reservoir, vug reservoir,fracture reservoir and fracture-vug reservoir four types. Each hypergenesispalaeokarst's karst zone have itself reservoir character. Epikast zone exist lots ofweathering fracture, dissolution fissures and hole. Vadose zone forms fracture-vugreservoirs or fracture reservoirs, there are isolated big dissolution cave once in awhile. Phreatic zone usually forms excellent vug reservoirs or cave reservoirs.reservoirs development position is consistent with karst zone on lengthways section,the depth of excellent reservoirs development is consistent with karstificationdevelopment.