Research On Reservoir Characterization And Key Controlling Factors Of Ordovician Yingshan Formation Carbonate Reservoir In Bachu Region

Yingshan Formation has broad exploration prospects as well as has manyproblems in Bachu region. Researching on reservoir characterization and keycontrolling factors is beneficial for reservoir forecast in this region. Based on thecomprehensive analysis of core information, logging information,thin sectionanalysis information and geochemical test information of Yingshan Formation in thisregion, Comprehensive analysis of sedimentary facies, petrological characteristics,thecharacteristics of the reservoir space,and a combination of reservoir spacecharacteristics of Yingshan Formation and then reach the conclution: Developmentopen platform facies in the end of the early Ordovician in Bachu region as well asdevelopment of some restricted platform in the east of Bachu region. The reservoirspace of Yingshan Formation ise cracks and holes cracks. Then combined thediagenetic evolution sequence and tectonic evolution of Yingshan Formation,the keycontrolling factors are supergene karstification and cracks.Combined with the the ancient landforms and karst recognition flag and incontradistinction to Tabei and Tazhong karst,Yingshan Formation karst in Bachu hasthe following characteristics:①Middle Caledonian karst development of a wide areaand large depth,but the karstification feature is not relatively obvious incontradistinction to Tabei and Tazhong karst;②Karstification in Bachu region hasstrong superposition, especially in the western and southwestern;③Karst is controlledby palaeogeomorphology in in Bachu region, The karst development area is in thewest and southwest,as well as effecting by early fracture;④The distinction ofConventional logging curve in Karst reservoir is obvious between vertical vadosezone and horizontal subsurface zone. The entire karst has the features with generallyGR value increasedand the resistivity decreases.These features prove that karstreservoir has high shale content;⑤The Carbon and oxygen isotope values ofweathering crust has little difference on Ordovician limestone background values,Presumably because of thick early carbonate rocks influence the exchange of matter. Cracks are the main reservoir space and seepage paths in Bachu region. Non-structural cracks are weathering fissure, mainly developed in the unconformity andkarst scarp or near the cave.They are provide seepage channel for meteoric water, alsomakes its own dissolution and expand it into a reservoir space.This kind of cracks arefilled whit shale and organic,some of them are half-filling. Structural cracks aretelated to folds and faults. Bachu region structure is strong, particularly in the westand southwest, fractures are abnormal development. This type of cracks areelongation and less filling, they are has great contribution to the reservoir. Analysisbased on fracture development and its distance away from the fault zone location, Thecracks is developed close to fault and in the Y-shaped fracture, The cracks is notdeveloped away from the fault zone.Consolidated sedimentary facies, lithologic characteristics of reservoir space andits combination of features, the main control factor analysis, the Yingshan Formationin Bachu region favorable reservoir locate in karst and ancient fracture (earlyCaledonian) overlay area. Selibuya fault and Mazatage fault region are the bestreservoir.Selibuya fault area affected by the middle Caledonian I, II, III curtain karst,while the fracture developed in the early Caledonian,the fracture cracks and fractureprovide seepage channel for middle Caledonian karst meteoric water thusexacerbating the degree of karst. Mazatage fault area experienced middle Caledoniankarst, late Caledonian karst the early Hercynian karst superposition. It is more moredevelopment than Selibuya fault regional in karst, while the fracture developed in theearly Caledonian and then contribute to the role of the karst. At the same time, theseancient faults actived again in the Hercynian and Himalayan, and formed a largenumber of tectonic fractures, most of these cracks are not filled, thus they are greatlyimproved penetration of the reservoir.