Reservoir Heterogeneity Of Shallow-water Delta Front

The shallow-water delta front is the main sedimentary facies of oil and gas accumulation which is a sedimentary environment with stable structure,flat topography,shallow water,and abundant sources.The Chang-6 reservoir of the Upper Triassic Yanchang Formation in the area of Foguyuan of the Ordos Basin is the main oil layer.This article based on the investigation of 459 wells,consist of well logs and analysis data in the Foguyuan area.According to petroleum geology,lithofacies palaeogeography,well logging geology,reservoir sedimentology and other theories,combined with logging curve characteristics,through core observation,casting thin slices,scanning electron microscopy,mercury injection test,particle size analysis and other methods to conduct a comprehensive study.Comparing and dividing the Chang-6 strata in the Foguyuan area,study the sedimentary microfacies,reservoir characteristics and reservoir heterogeneity,and the main controlling factors of reservoir heterogeneity characteristics were analyzed which was considered that the compaction and cementation usually make the reservoir heterogeneity enhanced and the dissolution make the reservoir heterogeneity weakened.The specific conclusions are as follows:(1)The Chang-6 reservoir in the Foguyuan area develops sub-facies of the delta front.the sedimentary microfacies mainly consist of underwater distributary channels and underwater split bay.(2)Through rock core observation and lithology analysis,the rock types in the Foguyuan area are mainly feldspar sandstones,and the cements include chlorite,calcite and quartz.the sedimentary structures mainly develop parallel bedding,deformed bedding,staggered bedding,and vertical wormholes,etc.,see plant debris,plant stems;pore type mainly consists of intergranular pores,intragranular pores,dissolved pores,and fracture pores;mercury injection test results show that the reservoir pore structure is characterized by small pore size and fine throats,It has the characteristics of medium discharge pressure,slightly rough twist,good pore throat sortability,and general connectivity.It belongs to the small hole fine throat-small hole fine throat.(3)By analyzing the sedimentary microfacies of the shallow-water delta front,it is found that it controls the heterogeneity of reservoir lithology and physical properties.on the plane,the heterogeneity of the main sand bodies in the underwater distributary channel is relatively weak,and the edge of the river channel and the estuary sand dam have a strong degree of heterogeneity;the permeability of the mat sand is greater than that of the distributary channel,and the heterogeneity of the land bank is relatively low,and is strong toward the coast;the value of the permeability of the concave bank of the underwater distributary channel is greater than the value of the permeability of the convex bank;the heterogeneity of the upper reaches of the estuary dam is weaker than the downstream,and the value of the central permeability is greater than the permeability values of both sides.(4)The heterogeneity of the shallow-water delta front in the Foguyuan area is mainly controlled by factors such as sedimentary environment and diagenesis.Shallow-water delta front water is shallow and its flow rate is very sharp,strong hydrodynamic conditions,relatively uniform particle size in the submarine distributary channel,weak heterogeneity,micro-phase hydrodynamic conditions such as side margins of river channels,estuary dams,and underwater natural embankments are weak,the homogeneity is relatively strong;compaction and cementation in the diagenesis make the reservoir sand body dense and physical properties deteriorate;the intergranular dissolution pores and intragranular dissolution pores formed by secondary dissolution make the reservoir storage space larger and the reservoir properties become better,makes the reservoir more heterogeneous.