Sandbody Architecture And Remaining Oil Distribution In Shallow Water Delta Reservoirs

Shallow water delta reservoir is an important reservoir type in oil and gas fields of China.It is of great significance to study the sandbody architecture and remaining oil distribution.Taking the 4^th Member of Quantou Formation in the south of Songliao Basin as an example,based on the collection and analysis of logging data,seismic data,core data,test data and oilfield production data,the stratigraphic division and correlation,sedimentary characteristics of shallow water delta,reservoir sandbody architecture and remaining oil distribution in the study area have been completed by comprehensively applying the principles of sedimentology,high-resolution sequence stratigraphy,reservoir geology and reservoir engineering.The results showed that during the deposition period of the 4^th member of Quantou Formation,the terrain was gentle,the climate changed from hot and dry in the early stage to warm and humid in the late stage,and the retrograde shallow water delta was formed under the condition of overall water inflow.The Baokang-Tongyu water system in the southwest and the Changchun-Huaide water system in the southeast formed the heavy mineral assemblage of"Zircon+Garnet+Tourmaline".The influence of southwest water system on the study area was gradually increasing.The study area developed upper plain subfacies in the early stage,upper plain subfacies and lower plain subfacies in the middle stage,and front subfacies in the late stage.The 4^th member of Quantou Formation can be divided into 1 long-term base level cycle,4 medium-term base level cycles and 13 short-term base level cycles.The four mid-term cycles were all asymmetric mid-term base-level cycles（type A）that become"deeper"upwards.The short-term base level cycles are dominated by type A₁ and A₂,and contain a small amount of type B₂and C₁.The forward modeling of sedimentation showed that the shallow-water delta sedimentation was controlled by factors such as lake level changes,accommodating space changes,provenance supply,topographic slope and lithological composition.Distributary channels can be divided into main distributary channel,sub main distributary channel,terminal distributary channel and flank distributary channel according to their location and origin.There were three superimposed styles of single sandbodies in distributary channel:isolated,spliced and cut superimposed.During the depositional period of the 4^th member of Quantou Formation,the lake level rose gradually with the A/S ratio,and the superimposed pattern changed from cut superimposed to isolated.The width of meandering distributary channels in shallow water delta plain was 150m～400m,and the thickness of sandstone was 4m～10m.Straight underwater distributary channels were developed in the front of shallow water delta.The width of underwater distributary channels was 150m～350m,and the thickness of sandstone is 3m～8m.The end of underwater distributary channels were often sheet sand.The point bar of meandering distributary channel was composed of one or more lateral layers and lateral bodies,with an angle of 5°to 8°and the straight underwater distributary channel was composed of one or more nearly horizontal accretion layers and accretion bodies,with an angle of 0°to 3°respectively.The remaining oil distribution was affected by sedimentary evolution,single sand body,architecture of single sand body,sedimentary rhythm,microstructure,well pattern density and injection production well pattern perfection.The sedimentary evolution controlled the type and scale of distributary channel,and then affected the distribution of remaining oil.Different types of single sandbodies and different superimposed relationships of single sandbodies were easy to form seepage barriers,and then influenced remaining oil enrichment.The occurrence of lateral and accretion layers made the remaining oil in the distributary channel enriched by stages.There were main three types of remaining oil distribution,sedimentary evolution control type,single sand body control type and single sand body architecture control type.In addition,there were micro fault control type and injection production relationship imperfect type.