Reservoir Architectural Heterogeneity And Its Influence On Fluid Flow Of The Ahe Formation In The Dibei Area

The Dibei area is located in the Yiqikelike structural belt of the Kuqa Depression.The Lower Jurassic Ahe Formation is one of the main hydrocarbon-bearing formations.Its hydrocarbon-bearing property is not controlled by local structures.The Ahe Formation,a tight sandstone reservoir,is highly heterogeneous,resulting in unhomogeneous oil and gas distribution pattern.However,current studies on the influence of reservoir architectural heterogeneity on fluid flow are limited.In this thesis,the lithological and diagenetic characteristics of the sandstone reservoir of the Ahe Formation are deciphered by using cores,logging and laboratory data.The sedimentary and internal differential diagenetic features are descripted by analyzing digital outcrop images photographed by UAV and satellite images in order to establish a conceptual model of the architectural heterogeneity of the Ahe Formation.Finally,Petrel and Eclipse software are used to carry out geological modeling of reservoir heterogeneity and fluid flow simulation in order to reveal the influence of reservoir structural heterogeneity on fluid flow.This study shows that the Ahe Formation is dominated by feldspathic lithic sandstone,and mainly contains six types of lithofacies,including massive bedded sandy conglomerate,trough cross-bedded sandstone,tabular cross-bedded sandstone,massive bedded sandstone,parallel bedded fine-grained sandstone and dark-color mudstone.These lithofacies constitute five lithofacies associations in vertical direction,which are river channel,inter-channel bay,river lateral margin,braid bar and abandoned river channel.Compaction,cementation and dissolution occur in the reservoir of Ahe Formation.The reservoir is strongly compacted,with significant cementation within the parallel bedded fine-grained sandstone and the massive bedded sandstone and significant dissolution within the tabular and trough cross-bedded sandstones.The 1^st-8^th order bounding surfaces are systematically identified based on Miall’s architectural surface classification scheme^[22].The heterogeneity of a single sandstone body bounded by 2^nd and 3^rd surfaces is controlled by the scale and location of the thin muddy interlayer.The heterogeneity of a sandstone body bounded by 4^th and 5^th order surfaces is indicated by the difference of cement content and grain size.The heterogeneity of sandstone body bounded by the 6^th order surface is indicated by the difference of sedimentary facies,appearing as differences in sandstone types and lithofacies associations.In vertical direction,the Ahe Formation has a five-storey architecture,which is characterized by migrated-isolated to lateral migrated to vertical aggradated to oscillated-vertical aggradated to oscillated.The five storeys correspond to the five sandstone groups of the Ahe Formation（J₁a¹,J₁a²,J₁a³,J₁a⁴,J₁a⁵）.The five groups are different in the diagenetic features.The fifth sandstone group develops thick migrated-isolated sandstone bodies,with an average apparent dissolution rate of 26%.The fourth sandstone group develops lateral accreted sandstone bodies,moreover,the scale and location of mudstone interlayers are variable.The sandstone body of the third sandstone group is vertically aggradated,and the apparent cementation rate of the tight calcareous sandstone within can reach 30%.The second sandstone group mainly develops oscillated-vertical accreted sandstone bodies.The first sandstone group develops thin oscillated sandstone body,and the apparent compaction rate is generally above 70%.All diagenetic processes are well developed in the second and fourth sandstone groups.Furthermore,four types of interlayers are developed in the Ahe Formation,which are regional mudstone interlayer,lenticular mudstone interlayer,calcareous interlayer at channel margin as well as calcareous interlayer in inter-channel area.Based on the identified bounding surfaces and physical characteristics of typical samples,lithofacies and property models of typical outcrops and stratigraphic sections are established,and numerical simulations of fluid flow are carried out.The simulation results show that the sedimentary architecture of the sandstone body plays an important role on the physical properties and fluid distribution of the reservoir.The lenticular mudstone interlayer and inter-channel calcareous interlayer are developed at the 2^nd and 3^rd order bounding surfaces,which have limited control on the fluid distribution.The channel-margin calcareous interlayers are developed adjacent to the 4^th and 5^th order bounding surfaces,which have a significant influence on fluid flow,especially the 5^th order interface.The stacking pattern of sandstone jointly determines the connectivity.The lateral migrated and the migrated-isolated sandstones have poor connectivity.Fluids within flow a short distance and consume a long time.The oscillated and oscillated-vertical aggradated sandstones have better connectivity.The vertical aggradated sandstone has the best vertical and lateral connectivity.In addition,vertically,hydrocarbon within the Ahe Formation is mainly concentrated in the first to third sandstone groups.The distribution of favorable reservoirs in the study area is controlled by the 4^th and 6^th sequence boundary.It is considered that the third sandstone group is the most favorable reservoirs of the Ahe Formation based on the analysis of various optimized parameters.Based on the statistical analyses of sand-to-ground ratio,physical property,thickness of favorable stacking type and favorable lithofacies of the third sandstone group,a favorable zone analysis of the reservoir is performed.The fuzzy mathematical comprehensive evaluation method was used to establish the reservoir evaluation criteria of the Ahe Formation,which as a basis of determining favorable reservoirs within the third sandstone group.