Evaluating Bedrock Reservoir And Analysing Reservoir-forming Conditions In Dong Ping Area Of The Qaidam Basin

The Qaidam basin exploration began in 1950s,has experienced 50 years of history of the oil and gas exploration,has also found a total of 25 small and medium sized conventional oil and gas fields,In recent years,explorers increase the Qaidam Basin bedrock exploration efforts,various degrees of industrial oil and gas flow in the bedrock strata of many oil and gas fields.2011 drilling wells in the eastern section of the Altyn mountain Dongping area,Dongping 1 wells rock weathering test yield up to 110 thousand in the bedrock 3159-3182m m,2011 drilling wells Dongping 3 in which multiple layer systems see industrial oil and gas flow,the wells rock weathering test yield up to 36 thousand in the bedrock 1856-1870m,which was found in the first self-contained East Ping rock gas reservoir.The hydrocarbon reservoir of conventional Sedimentary rocks have become increasingly difficult to meet the needs of economic development urgently needing new resources found by explorationists.The hydrocarbon reservoir of bedrocks having the high output and reserve is special and one of the main directions in the future for oil fields.The hydrocarbon reservoir of bedrocks have been found in many areas of home and abroad.Our country has made great achievements,but so far fails to find complete hydrocarbon reservoirs,especially bedrock gas reservoirs.The discovery of gas reservoirs of Dong Ping fills up the history of no gas reservoir in bedrocks in China.And it provides valuable experience for the future exploration and development.It has very important significance to improving the growth of oil and gas resources and the sustainable development of economy in China.Bedrock reservoir's lithology is complex,such as Granites and Gneisses.Structure of Its pores is diverse,and has the dual media of the fractural dissolution pore.Although our country has found the hydrocarbon reservoir of bedrocks,it has not done a further research.Based on the analysis of core data,logging data and assay analysis,this paper carries on systemic analysis to lithology,reservoir characteristics,vertical internal structure of bedrocks,evaluation of fracture effectiveness,reservoir forming conditions and patterns of the study area,and comes to the following understanding:(1)lithology of bedrocks in Dong Ping area mainly consists of two major categories:The first category is the intrusive igneous rocks.It is mainly Granites.Granites being blocky structure mainly develop in the Dong Ping 3 well area.Its color is mainly variegated and light red.Its mineral composition is mainly Quartz,Potassium feldspar,Plagioclase,Biotite,Hornblende and so on.The another category is metamorphic rocks.It is mainly Gneisses.Gneisses mainly develop in the Dong Ping 1 well area.Its color is various.Its mineral composition is mainly Feldspar,Quartz and all kinds of melanocratic minerals(Biotite,Amphibole,Pyroxene and so on).(2)According to conventional logging and imaging logging,different weathering structure layers can be identified and classified.Weathering crust of bedrocks in Dong Ping area is divided into three structural layers including the paleosol layer,the residual layer and the semi weathered layer.Weathering crust structure of Bedrocks determines that the semi weathering layer is not only the main developmental stage of the reservoir,but also the main position of oil and gas accumulation.Main factors affecting the formation of weathering crust is exposed bedrock weathering erosion time,phase of the ancient climate,overlying sedimentary environment control,different lithology and fracture structure control,the control of the ancient landscape.(3)The bedrock reservoir has double medium characteristics,both have matrix porosity,fracture porosity;both have pores,dissolution fracture;both pores,there are so many types of solutionfissure,influence of Dongping performance area of reservoir mainly reservoir space types:fracture and dissolution hole.The bedrock weathering cracks into dissolution fracture,weathering crack,cracks and joints crack schistosity.The bedrock dissolution pore are mainly intergranular dissolved pores and intergranular dissolved pore meat,Dongping 1 well area gneiss dark mineral content is high,prone to corrosion,a large number of developing crystal meat and dissolved pores,pores mainly developed at the top with semi weathering erosion.According to the characteristics of capillary pressure curves,the pore structure of Dongping bedrock 1 reservoir can be divided into three classes,class I pore structure distribution,reached 57.1%,followed by the type of pore structure,accounting for 28.6%,class III pore structure distribution is relatively small,accounting for 14.3%.(4)Based on the study of reservoir evaluation criteria for bedrock reservoirs at home and abroad,the evaluation criteria for bedrock gas reservoirs are proposed.Dongping bedrock 1 wells in the reservoir can be divided into I,II,III class,I class reservoir is good reservoir,fracture pore development,lithology of gneiss,the matrix porosity is greater than 4%,mainly distributed in the high parts of the structure,yield more than 10 x 104m3;class II reservoir is medium reservoir layer,crack-hole is developed,the lithology is granite,granite gneiss,matrix porosity2-4%,mainly distributed in the structure of the waist,yield less than 10×10~4m~3;class III reservoir with poor reservoir,fracture pore is not developed,lithology of granite and granite gneiss,matrix porosity 1-2%,mainly distributed in the the tectonic boundary of oil and gas shows.The study area is dominated by II,with less I.It is mainly distributed in the upper part of bedrock,and the lower part is mostly III.(5)The gas field of Dong Ping is a fractured gas reservoir controlled by structure,which has good conditions of oil and gas accumulation.To begin with,Dong)of two Jurassic(J1)depressions.Organic matter abundance of Source rocks is high,being the high quality for gas source rocks.Then,the reservoir is well,The reservoir is mainly Granites or Metamorphic rocks,and the thickness is large,and the development of cracks is well,and the permeability is good,and it is able to form the high yield's gas reservoir.Furthermore,a good cover is formed the soil layer and the eluvial.Finally,migration condition good,The guidance system of this area is mainly fractures,the unconformity,sand body and bedrock cracks.Faults and the unconformity are the most important factors for guidance.The Ping East fault is the most important guidance fracture for the East Ping gas reservoir.The regional unconformity of strata bottom boundary of the Tertiary system is important to migration and accumulation of oil and gas to the Dong Ping structure.(6)Research results with oil source characteristics,entrapment types,transportation system etc.consider that the characteristics of the hydrocarbon migration in Dong Ping region are Highly matural oil and gas of the Jurassic source rocks vertically migrate along fractures of the source rocks,and then laterally migrate along weathering crust and unconformity surfaces.Oil and gas firstly accumulate and form the reservoir in bedrocks and the Eogene System reservoir of the Dong Ping 1 well area,then adjust and migrate along the unconformity for the 3 well area,finally accumulate and form the reservoir in bedrocks and the Eogene System reservoir of the Dong Ping 3 well area.