Favorable Reservoir Genesis Of Sandstone In Wenchang Formation Of Lufeng Sag

The Paleogene Wenchang Formation in the Lufeng Sag is an important exploration direction for the deep formation of the Pearl River Mouth Basin.Wells LF13-1 and LF14-4 have been drilled to confirm that the Wenchang Formation in the Lufeng Sag has a high-yield oil layer,but the formation mechanism and control factors of favorable reservoir are varied due to the deep burial,the complex process of sedimentary environment and diagenesis.The favorable mechanism of reservoir formation is a key problem to be solved urgently.Based on the knowledge of sedimentary petrology,petroleum and natural gas geology,reservoir geology,reservoir physics,tectonic geology and so on,this paper makes a systematic study on sedimentary facies,petrology,reservoir properties and diagenesis of sandstone reservoirs by means of core observation,thin casting sheet,scanning electron microscope,cathode luminescence,X-ray diffraction,uniform temperature of inclusions,electron probe and isotope analysis and so on,timing and quantitative restore of the two kinds of favorable reservoir pore evolution characteristics.Through the dissection of key wells,the genesis and main controlling factors of favorable reservoirs are analyzed,and favorable reservoir development models are established.The Wenchang Formation in Lufeng Sag mainly develops braided river delta and shallow lake lacustrine facies.Reservoir sand bodies are mainly developed in distributary channel and estuary dam microfacies,and high quality source rocks are developed in mud shallow facies of shore shallow lakes.The reservoir of Wenchang Formation is mainly quartz sandstone,and the cement are mainly calcite,authigenic quartz and authigenic kaolinite,with high composition maturity and structural maturity.There are various types of pores in sandstone.According to the genesis classification,the pore types are divided into residual primary pores,granular dissolved pores,and cementate dissolved pores,of which the contents are similar in general,The sandstone of the Wenchang Formation has an average porosity of 9.7%,mainly distributed between 9%-15%,and an average permeability of 11.3m D,mainly distributed between 0.1-10 m D,which is a medium-low porosity and ultra-low permeability reservoir.Reservoirs are classified into "residual primary pores + cement dissolved pores + granular dissolved pores" type A pore combined reservoirs and "residual primary pores + granular dissolved pores" type B pore combined reservoirs.During the evolution process,the initial porosity of the reservoir is 37.27%.In the mid-diagenetic stage A,the porosity of the type A reservoir is 16.19%,and that of type B reservoir was 17.59%.As of now,the porosity of the type A reservoir is 10.81 %,which has reached the middle diagenetic stage B,and the porosity of type B reservoir is 14.3%,which is still in the middle diagenetic stage A.The diagenesis types are various: compaction and authigenic mineral precipitation have destructive effects on the physical property,and the physical property has been significantly improved by hydrocarbon filling and dissolution.The lower limit of physical properties of favorable reservoir is determined: the lower limit of porosity is 10.5% and the lower limit of permeability is 3m D.The formation of favorable reservoirs is due to the development of distant high-energy facies sandstone,early hydrocarbon filling,dissolution and early uplift and shallow burial in sandstone,establishing two reservoir development models: “distant provenance + paleo slope + paleo-fault” type A reservoir development genetic model and “distant provenance + high structure,compositional maturity + early hydrocarbon filling” type B reservoir development genetic model.