Study On Fan Delta Reservoir Architecture And Numerical Simulation

Fan delta deposits are widely developed in the Mesozoic-Cenozoic continental rift basins in China,forming important reservoir types.The typical tectonic setting proximal to the source brings up the depositional characteristics of rapid accumulation vertically and adjacent development laterally.Therefore it is difficult to characterize this type of reservoir in detail.Centered on sedimentary characteristics,architectural model,evolution law and controlling factors of fan delta,the thesis firstly focuced on the prototype model study of fan delta by making a contrastive study of steep slope fan delta and gentle slope fan dlta using Cretaceous fan delta outcrop of Luanping rifted basin;then on the basis of ourcrop study,the subsurface characterization of fan delta reservoir architecture was carried out;and also numerical modeling of fan delta lobe and fan delta formation was did and an analysis of controlling factors was made by way of process study step by step;finally,as a supplement,diagenesis study of fan delta reservoir was made and a discussion of how diagenesis make a difference to the sand bodies connection and heterogeneity was also given.Three typical fan-delta outcrops with different tectonic positions in Luanping basin were selected as the target and fan-delta deposition prototype model research were carried out.Then the microfacies composition and sedimentary characteristics of different fan delta subfacies comprising fan delta plain subfacies,fan delta front subfacies and pro-fan delta subfacies of three typical fan delta outcrops were compared in detail.On the basis of this,sedimentary outcrop microfacies models of steep-slope fan delta and gentle-slope fan delta were established respectively according to the tectonic paleotopography slope.The results show that steep-slope fan delta plain has a narrow range and is dominated by debris flow,and the braided channel is developed in fan delta front and the erosion and scouring phenomena are common,and highdensity turbidite is developed in the deep lake area;whereas the gentle-slope fan delta plain is broad,and little muddy matrix is contained in the conglomerate,the front is undeveloped and characterized by banded and isolated sandy channels,mainly developing low-density turbidity currents caused by slope failures.Based on the study of subfacies sedimentary characteristics,three level-sixth-level architecture units consisting of single fan,compound genetic unit and internal accretion unit and other units were identified in the composite fan delta sedimentary sequence,and the evolution characteristics of composite channels from plain subfacies to front subfacies to the distal end were illustrated in detail from the aspects of morphology,occurrence,arrangement,etc.The channel of the fan delta plain mainly comprising sandy conglomerate has low mud content,and its interface is sandy conglomerate eroded cutting surface,and the accretionary body in the compound channel is mainly vertical stacking;and in the fan delta front,each single channel in the distributary channel has a higher lateral migration velocity and the high frequency swing of the channel causes the deposition width to increase.And compared with the distributary channel in plain,the distributary channel in front has a higher mud content and poorer sorting;In the distal part of lake basin the distributary channel is characterized by low ratio of width to thickness,thinner lithology,single internal structure and little or no contact between the channel sand bodies.Based on the information about microfacies composition,sandbody structure and classification pattern revealed by the Luanping field outcrops,taking a set of fan delta sedimentary reservoirs of Xing'anling in Sudeerte oilfield of Hailaer Basin as an example,combined with the core,logging,and analysis test data,fan delta reservoir architecture analysis based on well data was carried out,then with the idea of reservoir hierarchy analysis,the sedimentary reservoirs in the study area were divided into several levels and the classification standard and recognition characteristics of different levels of architecture units were clarified.The study on fan delta reservoirs shows that the subfacies of fan delta can be divided into three levels,ie single microfacies unit,single fan body and compound fan body according to medium-short term cycle interface and mudstone of lake flooding period.Single microfacies unit can show three kinds of superposition modes,namely close superposition,mosaic overlay and separated superposition based on the position of fan body cycle and internal unit composition differences,which can be corresponding to the evolution characteristics of composite channel from the proximal end to the distal end in the outcrop model respectively.Using the electrofacies characteristics of the single fan in different positions of various fans,four kinds of six-level single fan architecture units were identified.The range of logs from the fans in the proximal end to those in the front to those in the distal end decreases gradually,and the combination forms change from box-type-box-type/bell-type/funnel-type-bell-type/funneltype/finger-type.In the study area,four types of composite fan combination models were identified,ie layered superposition,staggered superimposition,primary-secondary superimposition and interactive superimposition through the identification of single fan lateral boundary and the comparison and combination with adjacent wells.On the basis of the above study,with the combination of fan delta architecture classification system and sequence stratigraphic unit division system,based on the control of sequence structure on sedimentary architecture,the controlling of multi-level factors on the shapes,distribution and contact of architecture units with different levels were illustrated.Under the control of the medium-term base-level cycles,seven-level composite fan units can exhibit the accumulation characteristics of progradation,accretion or regression.Under the control of short-term base-level cycles,sixlevel single-body displays small-scale and high-frequency progression or retrogressive feature and shows the corresponding change in terms of development scale,sand body thickness and sand ratio etc to the the short-term base-level cycle.Ultra-short-term base-level cycles only exhibit the autocyclicity sedimentary attributes with energy change and show the migration,proliferation and extinction of a single river channel unit or other single structural unit.Combined with the research results of outcrop and subsurface fan delta reservoirs,using the 3D sedimentary simulation software-Dionisos,with the method of computer simulation,the nature and mechanism of fan delta growth and development were reflected.Fan-delta modeling of single-point provenance shows that fan-delta grows in the order of axial-vertical-lateral direction.During the fan body progression,the slope adjustment of the mud and sand resulted in the lateral migration of the fan.The superposition model of delta lobes of different scales in different time periods was constructed through the multi-source fan delta modeling.Combined with the evolution of base-level cycles,a complete fan-delta depositional system framework was established.The adjacent fan lateral margin develops fan delta bay and is dominated by fine-grained sediments,when the source supply is strengthened,the lateral accretion of fan bay is common.As a result,thick sand bodies can also be developed in fan bay,forming the sheetlike sand mat.The modeling results of the multi-source fan delta can be compared with the subsurface fan-delta sedimentary strata developed in Sudeerte oilfield.Finally,through the design of four groups of fan-delta controlling factor comparison experiments,this paper is focused on the effects of clastic composition,lake slope,supply rate and water depth on the fan delta scale,structure and shape in order to provide a way to better guide related fan delta reservoir bodies prediction.The experimental results show that the coarser clastic is,the slower the velocity change of fan delta front is and the smaller the curvature of the fan delta front appearing as straight shape;otherwise the velocity change will be faster and the curvature will be larger and tends to be fan-shaped.Meanwhile,coarse fan deltas are often accompanied by high-density hyperpycnal flows,while fine fan deltas only develop turbidites caused by failures.The lake slope directly controls the distribution form of the fan delta,and the axial elongation of the steep-slope fan delta is obvious,showing a tongue-shape distribution,while the gentleslope fan delta tends to be more fan-shaped.The frontal margin of the steep-slope fan delta is larger,while the plain facies of the gentle-slope fan delta is more developed.The supply rate of sediments directly controls the evolution rate of fan delta,the faster the supply rate is,the larger the elongation rate and the broadening rate of the fan will be,and the change of the spreading rate is particularly significant.The lake depth controls fan delta front angle,lateral edge angle and their stability.When the front angle is greater than the repose angle of sediments,the stability of fan delta front will be destroyed and gravity-controlled slope failures will be prone to develop.Finally,the paper is focused on the study of the favorable reservoir space distribution,which is more meaningful to the reservoirs.As a supplemental study to explore the diagenesis effect on the modification of fan delta reservoirs with complex lithlogies,with the comprehensive combination of core observation,thin section identification,physical property testing,the study on fan delta reservoir diagenesis was carried out.Four types of diagenetic facies were identified in the fan delta reservoir of Sudeerte oilfield,namely strong compaction diagenetic facies,weak compaction-calcite medium-strong cementation diagenetic facies,medium compaction-analcime / feldspar strong dissolution diagenetic facies,medium compaction-analcime / feldspar medium-strong dissolution-kaolinite strong filling diagenetic facies.And the corresponding reservoir characteristics are characterized respectively.Through the logging identification of diagenetic facies and logging constraints the spatial distribution characteristics of diagenetic facies and its modification significance to the multi-level fan heterogeneity and connectivity were analyzed.The result shows that after the complex diagenetic processes,due to the development of strong compaction diagenetic facies and weak compaction-calcite medium-strong cementation diagenetic facies,the single fan-body originally connected laterally show strong heterogeneity and effective reservoirs only be developed in some areas.Moreover,the effective reservoir fractions are gradually reduced with the increasing burial depth;For the upper and lower stacking composite fans,the large development of the strong compaction diagenetic facies and weak compaction-calcite mediumstrong cementation diagenetic facies constitutes a dense diagenetic interlayer,thus resulting in the original connected two fan no longer connected or only longitudinal local connectivity.