Characteristics Of Bedrock Reservoirs In The Eastern Segment Of Altun Piedmont

Based on the field outcrop survey,core observation,section identification,scanning electron microscopy and other laboratory analysis,combined with conventional logging,image logging,pore infiltration test data,etc.,main characteristics and evaluation of bedrock reservoirs in the eastern segment of the Altun Piedmont are studied.The results show that the lithology of the bedrock reservoirs in the eastern segment of Altun Piedmont mainly includes magmatic rocks and metamorphic rocks,the magmatic rocks are mainly granite,and the metamorphic rocks are mainly gneiss.The types of reservoir space mainly include incompletely filled fractures,dissolved pores and matrix micropores.The maximum porosity of bedrock reservoir is 9.76%,the minimum is 0.004%,and the average is 1.663%?3.844%.The maximum permeability of bedrock reservoir is 33.239m D,the minimum is 0.002m D,and the average permeability is0.02m D?3.836m D.The physical properties of gneisses are better than those of granites as a whole.There is no obvious correlation between porosity and permeability.Magmatic weathering crust reservoir and metamorphic rock weathering crust reservoir are mainly formed under weathering.The weathering crust of bedrock can be divided into four layers according to weathering degree named soil layer,weathered layer,semi-weathered layer and unweathered layer.In order to facilitate the study,the semi-weathered layer can be further divided into dissolution zone of semi-weathered layer and disintegration zone of semi-weathered layer.The differences in mineral composition,geochemical response,conventional logging,reservoir space type and characteristics between the two kinds of weathering crust reservoirs are summarized.There is little difference in the vertical distribution of magmatic minerals.The content of mafic minerals and clay from shallow to deep in metamorphic rocks decreases as a whole.It can be discovered from Si O₂/Al₂O₃ that the bedrock in the study area is mostly in semi-weathering state,the Si O₂/Al₂O₃ of granite is between 5.13 and 6.5,which is relatively stable,and the metamorphic rocks are between 3.01 and 6.53,which have great changes.The reservoir space type of dissolution zone of semi-weathered layer of magmatic rock is fracture-vuggy type,and the disintegration zone of semi-weathered layer is fracture type;the reservoir space type of dissolution zone of semi-weathered layer of metamorphic rock is vuggy type,and the disintegration zone of semi-weathered layer is vuggy-fracture type.The average porosity of metamorphic rock is 3.429%and the average permeability is 1.424 m D in the dissolution zone of semi-weathered layer,2.940%and 8.294 m D in the disintegration zone of semi-weathered layer.The correlation between porosity and permeability in the disintegration zone is greater than that in the dissolution zone.The average porosity of magmatic rock is 4.027%and the average permeability is 2.390 m D in the dissolution zone of semi-weathered layer,4.076%and 3.625 m D in the disintegration zone of semi-weathered layer.The correlation between porosity and permeability in the disintegration zone is better than that in the dissolution zone of magmatic rock and semi-weathered layer of metamorphic rock.The development model of weathered crust reservoirs of magmatic and metamorphic rocks are established.The dissolution zone of semi-weathered layer is the most important reservoir of metamorphic rock weathered crust reservoir,it is the main factor of stable production,followed by the disintegration zone of semi-weathered layer;the disintegration zone of semi-weathered layer is the most important reservoir of magmatic rock weathered crust reservoir,followed by the dissolution zone of semi-weathered layer.Reservoir performance of magmatic weathering crust reservoir is better than that of metamorphic weathering crust reservoir.The main controlling factors of bedrock reservoirs include lithology and mineral combination,paleogeomorphology,pre-existing faults and fracture zones,late magmatic intrusion and hydrothermal action.Combined with the physical properties and the main controlling factors of the bedrock reservoir,the evaluation criteria of bedrock reservoir controlled by many factors is put forward.The reservoir is divided into four categories:good,average,fair,poor.The results of evaluation show that the study area is mainly composed of type II reservoirs,mainly found in granitic gneiss,followed by granite,plagioclase gneiss,feldspar hornblende schist,etc.The Dongping nasal and Niuzhong-Niubei slopes are mostly type I reservoirs,the Jianbei slope,Niudong nasal and Lengbei slope are mostly type II reservoirs.