Study On Reservoir Characteristics Of Jurassic Lianggaoshan Formation In The Quangan Area Of Central Sichuan Basin And Prediction Of The Advantageous Reservior Area

Jurassic Lianggaoshan formation of Guangan area is one of main oil formation in central Sichuan basin. Because shallow bury, good oil mass and low exploration cost, Jurassic Lianggaoshan formation is of favorable oil exploration potential in Guangan area. Due to the diageneses, the reservoir has low porosity, low permeability and heterogeneity,which is very hard to explore.Therefore,it is necessary to make research on Lianggaoshan formation of the jurassic reservoir characteristics in the area.Based on the theories and technologies of sedimentology, reservoir geology,petrology, seismic stratigraphy and reservoir evaluation,researches are carried out on several aspects such as reservoir petrologic characteristics,reservoir sandbody distribution characteristics, diagenesis type, reservoir pore types and evolution, physical propertycharacteristics and its controlling factors, the feature of fracture development and so on.The main results are as follows:Comparing the dry well(Guang-100 well,Xian-9 well,Dacheng-5 well) and oil well(Xi-56 well) on sedimentary micro-facies and petrologic characteristics.The present paper has found that the sand in the area of Guang-100 well was characterized by small grain size and high shale content, less primary pore, almost no secondary pore;but in Gong-17 they are low in mud content , relatively coarse in grain size, occasionally some secondary pores.So Lianggaoshan formation of Guangan area belongs to pore -fracture oil reservoir, and Gongshanmiao area is fracture-pore oil reservoir.By the methods of core observation, thin section appraisal, X-diffraction analysis, and scanning electron microscope, it is concluded that reservoir lithology is mainly silt-fine sandstones, the feldspathic lithic sandstone and the feldspathic-quartzose sandstones, featured by extremely low porosity and low permeability. They are extra-low porosity and extra-low permeability reservoir. But the good fracture effectively increase reservoir physical properties, forming low permeableporous -fracture oil reservoir.Based on analyzing previous data and core observation, the sedimentary facies during the sedimentary period of the Lianggaoshan formation in this area is the shallow-lake subfacies and semideep subfacies. The shallow-shore lake bar microfacies is the favorable sedimentary facies belt of sand. Using well logging informations for statistical analysis sands distribution, it is found that sand bars are more developed on the positive local structure.The diagenesis in this area mainly comprises compaction, cementation, metasomatism, dissolution and tectonic effect. On the basis of X-diffraction clay mineral analyses, compaction degree, particle contacting relation, porosity types, and authigenic mineral developing degree, the reservoir is in the stage B of middle diagenesis period. The strong compaction and cementation caused the very low porosity and permeability. But fracturing improved reservoir properties of clastic reservoirs, and it is advantageous to the increase of porosity in the certain degree by internconnecting matrix micropores.The paper has identified the fractures by use of the correlation analysis method and found that the low vertical coherence zone in section can be interpreted as the fault and fracture zone. The gobbet low correlation unconventionality in the plane is the main exhibition form of favorable fracture area,which develop mainly in the structural distortion,near the fault and the fault disappearing belt.Because the Lianggaoshan Formation oil reservoir belongs to structural lithologic reservoir, integrating the advantageous distribution of sandstone with favorable fracture area, is is concluded that the advantageous reservoir area is the area of well Guangan-103 to well Guangan-127, and the relatively favorable area is the one of well Xian-4 to well Guangan-9, well Guangan-17, well Dacheng-2.