The Research Of High Resolution Sequence Stratigraphy And Sedimentary Facies In The Middle-upper Submembers Of Member 3 Of Shahejie Formation, Liuxi Oilfield

Subtle reservoir of Member 3 of Shahejie Formation has been a high point of the oil and gas exploration in Liuxi Oilfield. Certain achievements of research have been made. However, due to the extremely complex structure, particularly the role of faulting, many problems still exist,including the layering confusion, difficulty of distinguishing the structure and deposition, unconspicuous lithology and electrical characteristics, broken blocks and unconspicuous controlling role on the deposition of palaeostructure. In this paper, the research of high-resolution sequence stratigraphy of the middle-upper submembers of Member 3 of Shahejie Formation in Liuxi Oilfield has some theoretical and practical significance.On the basis of the former layering plan and using high-resolution stratigraphy principles and research methods, a new plan of stratigraphic correlation was set up.The middle-upper submembers of Member 3 of Shahejie Formation in Liuxi oilfield were divided into three oil groups named uniformly S3-â… , S3-â…¡and S3-â…¢oil group, which were further divided into different oil layers.According to the U-shaped asymmetric seismic reflection characteristics, oblique progradational reflection configuration or progradational infilled reflection configuration on the seismic profiles and the logging data, incised valley and its infilling deposition were indentified for the first time at the bottom of the middle-upper submembers of Member 3 of Shahejie Formation in Liuxi area.The incised valley deposits developed mainly during the earlier period while long-term base-level cycle rose and were divided into channel and overbanking subfacies. The channel subfaices mainly included channel microfacies. The overbanking subfacies was divided into flooding plain and overbanking bar microfacies.With observation of the core and the analysis of logging data and seismic data in detail, and in the combination with regional geological background, the characteristics of rocks and lithofacies, lithofacies assembly, primary sedimentary structures(scour, bedding and characteristic contemporaneous deformation structure) and logging facies were studied. Incised valley, braid delta and lacustrine facies were indentified. The braid delta included delta plain, delta front and prodelta subfacies,and they were further divided into a few of microfacies.Shore-shallow lacustrine subfacies was further divided into shore-shallow mudstone and sandstone bar microfacies. Depositional models of the incised valley and braid delta were summed up. The sedimentary facies maps of each oil layer were compiled and their distributary characters were studied.The characteristics of sequence boundaries and flooding surfaces were studied in detail with core data, logging data and seismic data. A long-term deeper-upwards half-cycle,five mid-term base-level cycles and a few of short-term cycles were identified in the studying section.Taken the short-term cycles for example, they were divided into asymmetrical deeper-upwards type (A type),asymmetrical shallower-upwards type (B type) and symmetry type( C-type).The causes, characteristics and locations of each type were discussed in detail. The controlling factors including synsedimentary faulting,climate and the source of supply,lake-level fluctuation and palaeogeomorphology were discussed.The medium-term sequence boundaries and flooding surfaces were chosen as the marks of stratigraphic correlation to set up the high resolution sequence stratigraphic framework of medium-term cycles.Small-scale incised valley sediments developed while S3-â…¢-6 oil layer deposited in the southern part of Liuxi Oilfield.Braid delta developed during the phases while S3-â…¢and S3-â…¡oil layers deposited,with the northeast and eastern source of supply. Sediments of braid channels and underwater distributary channels mainly developed in the lower part of the S3-â…¢and S3-â…¡oil groups.With the rising of the base level, the scale of braid delta decreased gradually, and delta front mainly developed while the upper parts of the S3-â…¢and S3-â…¡oil groups deposited. Shore-shallow lacustrine facies developedom in S3-â… oil group.