| Since the Early Paleozoic time,the Yingshan Formation carbonate rocks of Ordovician age in Tazhong area have suffered from long-term multi-phased karst geological processes,such as sedimentation,exposure,corrosion-destroyingand corrosion-filling,associated with multi-phased sea level rising and dropping,resulting into multi-shaped,variable-size and karst complex in structuretexture.The tectonic setting,and sedimentary strata and exposure characteristics of the Yingshan Formation in Tazhong area were first summarized using the modern karst theory,paleokarstification conditions during the exposure period of the Yingshan Formation were analyzed,and palaeogeomorphologic and paleohydrodynamic conditions were restored using the “relic thickness trend surface and mould residual combination method”.Accordingly,on the basis of studies on karst morphology,and geochemistry of karst fillings,eogenetic karst of the Yingshan Formation in Tazhong area was delineated and identified,confirming that eogenetic karstification occurred in the Yingshan Formation in Tazhong area.Then,identification and statistics were made on the fracturecave spaces of eogenetic karst reservoirs,and reservoir type classification and evaluation were conducted.Additionally,as the Yingshan Formation fractures and caves in Tazhong area were seriously filled,the filling characteristics of karst fractures and caves,and the development and distribution laws of filled intervals and high-quality reservoir intervals were summarized.The results make a full understanding of the development and filling characteristics of eogenetic karst fractures,pores and caves in Tazhong area.In this study,an analogy was made between eogenetic paleokarst and modern eogenetic karst characteristics.The analogy results determine that Tazhong area has similar eogenetic karst to the Yucatan Peninsula in Mexico.Accordingly,guided by the modern eogenetic karst theory,it was determined that three phases of relative stable sea level occurred during the karst stage of the Yingshan Formation in the northern slope of Tazhong area,fractures,and pores and caves are developed within the mixed corrosion zone of three phases of sea level.Paleo-sea level change,paleo karstification types,faults and other factors control the formation and filling of fractures and caves.The following main conclusions were made in this study:(1)As affected by the Episode I tectonism,middle Caledonian Orogeny,the Yingshan Formation carbonate rocks were uplifted and exposed for 7-11 Ma,indicating an exposure soon after sedimentation under weak diagenetic conditions.During the exposure period,the Tarim basin was located near the equator(slightly southward),and covered by a tropical high-temperature,rainy climate,which provided a large number of soluble freshwater for the northern slope karst in Tazhong area.Additionally,sedimentary facies analysis indicates that the Yingshan Formation is a set of platform facies limestone and dolomitic limestone,and the Yingshan Formation carbonate rocks are classified into one class,two types and four subtypes in terms of karst formation.The Upper Ying-1,Lower Ying-1 and Upper Ying-2 Members are sparry grained limestone subtype,successive limestone type,and strong karst formations.The Lower Ying-2 Member is a limestone-interbedded-with-dolomite subtype,successive limestone type,and a moderate to strong karst formation.Above all,the development of strong karst carbonate rocks and massive amounts of soluble freshwater during the exposure stage as well as fractures developed during the Caledonian Orogeny are favorable conditions for paleokarst development.(2)Palaeogeomorphologic was restored using the “relic thickness trend surface and mould residual combination method” for the first time.The restoration results show that the paleo karst landform of the Yingshan Formation top is generally a large gentle karst slope higher in the south and west and lower in the north and east,with relative weak topographic relief,and slope of 1.0-2.05%,reflecting slow runoff flowing conditions,shallow underground water,and that karstification mainly occurred near underground water level.(3)The Yingshan Formation eogenetic karst in the northern slope of Tazhong area can be divided into four stages,i.e.,I carbonate synsedimentary karst stage,II shallow-covered penecontemporaneous karstification stage of the Yijianfang Formation,III totally-exposed karst stage of carbonate rock platforms,and IV transgression karst stage of Tage sea water.Using field outcrop,drilled cores,microscopic thin section observation,imaging logging,cathode luminescence and fillings geochemistry,etc.,eogenetic karst was comprehensively identified.The results show that:(1)I and II eogentic karst stages: pores are generally evenly distributed,and show corrosion selectivity to structuretexture;core observation indicates that pores have alveolar,lacery and graniphyric corrosion shapes,and microscopic observation indicates that pores include mould,intragranular corrosion and intergranular corrosion pores,with fibrous,foliated and indented cements developed;these cements show multi-phased zoning cathode luminescence.(2)III and IV eogenetic karst stages: the single pore is 0.1-3 m in size,pores are commonly filled with argillaceous matter,and show non corrosion selectivity to structuretexture,and in plan,are in meta-layered distribution.Imaging logging identification indicates that corrosion occurs as bands(5-15 m thick)vertically,composed of many small karst caves superimposing vertically;spongy,honeycomb and Swiss cheese corrosion pores are commonly observable along karst cave margins,and laterally karst caves are wide and large in quantity.The testing and analysis of petrology,electronic probe,REEs,trace elements,clay X-ray diffraction,calcite carbon and oxygen isotopes,and inclusions,etc.of fillings of karst caves indicate that fillings are mainly mixtures of corroded,dissociated grains and carbonate sands,mud and calcareous cements of the Yingshan Formation bedrocks,clay in fillings is of fresh water origin,pebbles are formed by corrosion of meteoric freshwater,and calcite is precipitated in sea water,indicating a freshwater or seawaterfresh water karstification environment.Accordingly,the existence of eogenetic karstification was comprehensively determined.(4)Karst fracture and pore spaces were identified using outcrops,cores,logging,seism and other methods.The results show that the Yingshan Formation corrosion fractures and caves in Tazhong area are generally smaller than those in the Tabei buried hill(the porecave height is mostly within 0.1-3 m,and <1 cm honeycomb pores are locally developed).Imaging logging displays the pores and caves are small,but in large quantity and large width/height ratio.Moreover,seismic flaky reflection shows these pores and caves are distributed over a large area,and it can be inferred that these pores and caves,in plan,are crosscutting tubular pores and caves.According to the identified fractures and caves,reservoir types were classified into cave,pore,fracture and facturepore types.The pore type and fracturepore type are main reservoir types.Reservoir evaluation according to physical property testing results indicates that the Yingshan Formation matrix in Tazhong area has extremely low porosity,but as affected by karstification,high-quality reservoirs are locally developed(I and II types).The fractures and caves in Tazhong area are commonly filled.According to drilling high GR logging curves and imaging logging analyses,61 high GR intervals of cave filling origin were identified.In combination with statistics of reservoir intervals(non filled intervals),vertically high-GR filled intervals are extremely developed within 0-40 m below the Yingshan Formation top,and 63% of reservoir intervals are mainly developed below high-GR filled intervals.Laterally,the upper high-GR filled intervals(within 20 m below the unconformity)are principally distributed in high positions and gentle slopes of karst,while the inner high-GR filled intervals(>20 m below the unconformity)lie in gentle karst slopes,and reservoir and filled intervals are developed in parallel with No.I Fault Belt or paleocoast lines.Therefore,the development of reservoir and filled intervals is controlled by paleogeomoprhology,faults and coastlines.(5)According to the development and filling characteristics,corrosion shapes and geochemical testing results of the Yingshan Formation fractures and caves of the northern slope of Tazhong,and the comparison with various modern karst types,eogenetic karst in the northern slope of Tazhong area is similar to that in the Yucaton peninsula in Mexico.The latter is a mixed water karst type controlled by coastal type sea level,and the former is mainly controlled by three phases of sea level of transgression.(6)Two models were established,i.e.,the karst development model of the I eogenetic karst stage controlled by the exposure of intraplatform psammitic beach facies,and the mixed water karst and filling model of the IV eogenetic karst stage controlled by three phases of relative stable sea level of transgression. |
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