| The basement reservoir is one of the most crucial exploration targets in Qaidam Basin.However,the characteristics of basement reservoirs and accumulation conditions for natural gas remain to be tackled.This study thus presents Dongping and Jianbei as objects to figure out these questions.Basing on field survey,core description,thin section identification,isotope dating analysis,and the determination of major trace elements,we employed the logging and seismic data,cathode luminescence,scanning electron microscopy,and reservoir physical property testing to study the characteristics of petrology,geochemistry,and the reservoir controlling factors.The analysis of accumulation conditions regarding natural gas was proceeded through studying the paleo-tectonic background,weathering factor,evaluation of source rock,assessment of reservoir quality,scale of cap assemblage,faulting system,and the trap preservation,after which the hydrocarbon accumulation models were established.This study found that the characteristics of various lithology on the logging response are obviously different.This terrane was abundant with granitoids and genesis,including the granite,granodiorite,calcareous gneiss,and the biotite-plagioclase gneiss;also,a few schist and slate were distributed in study area.The gneiss and granite were developed mainly in Doingping area,and the granodiorites were emplaced in Jianbei area.The magmatism is high potassium calcium-alkaline series,which was characterized by enrichment in large ion lithophile and depletion in high field strength.Granite and granodiorite were emplaced in the Early Devonian and Late Ordovician,respectively.The metamorphic ages of gneiss,slate,and schist are Early Devonian,Late Triassic,and Late Jurassic,respectively.The reservoirs of basement rock are matrix micro-pores,dissolution pores,cleavage fractures,structural fractures,and dissolution fractures.Matrix micro-pores have the highest contribution to the total porosity of the study area,which include the primary matrix pores developed in iron-magnesia minerals and the secondary micro-pores abundant in clay minerals;multiple high-angle fractures and reticular fractures have improved the connectivity of pores and throats.The property of reservoir was affected by diagenesis,lithology,palaeogeomorphology,pre-existing faults,and weathering leaching.The reservoirs formed in gneiss,granodiorite,and slate are of the highest quality.The zone of ancient slope is most conducive to the preservation of weathering crust reservoirs.The extent of weathering and leaching suffered by the basement was controlled by the pre-existing faults,and the secondary dissolution and leaching belts were formed with the heterogeneity of basement.Combined with the effective types of reservoirs and characteristics of reservoir property,the evaluation standard of basement reservoirs in study area was constructed;the Dongping area where was abundant in gneiss and granitoids contains I-and ?-type reservoirs,and the Jianbei area where was emplaced by the granodiorite has I-type reservoir.The semi-weathering layers of weathering crust reservoirs are the best zones for the development of these reservoirs.The natural gas was from Jurassic hydrocarbon-generating sag in Pingxi,with high maturity of organic materials and medium potential of hydrocarbon generation.The reservoirs were overlain by regional gypsum-mudstone caps during the Paleogene.Due to the movement of Xishan,faults and unconformities are the main pathways for oil and gas transportation,indicating the direction of natural gas transportation and accumulation and forming two types of natural gas reservoirs. |
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