Structure Characteristics Of Platform Margin And Distribution Of Reef-shoal Facies In Typical Areas,Tarim Basin

The great oil-gas exploration breakthrough of reef-shoal facies reservoirs in the Cambrian-Ordovician carbonate platform margin of Tarim Basin has made platform margin reef-shoal facies become a hot spot in recent years.As we all know,the inner structures of platform margin are closely related to the shape,distribution and vertical stacking relationship of reef-shoal.Using seismic data,drilling and core data as well as the theories of sequence stratigraphy,sedimentology to analyze the structure characteristics of Cambrian-Ordovician platform margin in the typical areas(TaBei and Gucheng uplifted areas),identify the vertical stacking relationship,development periods and the distribution of platform margin internal seismic structures,predict the distribution of the reef-shoal facies reservoirs.The seismic reflection structural features of the platform margin reef are moundy or lentiform in the external form,while the internal forms are intermittent,clutter or blank reflection.Study suggests that the 3 stages moundy clutter reflection with progradation are 3 stages Algal mounds and sand cutting shoals in Early Cambrian in Tabei area;in Middle Cambrian,there are 5 stages reflection structures,corresponding to 5 stages reef depositions in the platform with the later reef-shoal bodies stacking on the top of the early reef-shoal bodies and migrating towards the basin;in the late Cambrian,there are less reflection structures in the platform margins and the reef-shoal bodies are eroded mightily;in the Early-Middle Ordovician,the reflection structures in the platform of the Penglaiba Formation are mainly intermittent,while part of the Penglaiba Formation,there are subtle ups and downs in the reef-shoal bodies;in the Yingshan Formation,there are 5 retrogradation reflection structures,representing 5 stages reef-shoal bodies,with the rising of the sea level,reef-shoal bodies migrating towards to the platform.In Gucheng area,the platform margin evolved from gentle sloped nonfringe platform margin to gentle sloped weak fringe platform margin and steep sloped fringe platform margin.Cambrian platform margin is mainly the mound seismic reflection configuration,the inner is weak amplitude or blank reflection.The early and middle Ordovician platform margin is mound and interrupted reflection,well drilling confirms these seismic structures are grain-shoal and algal mound.Reef-shoal reservoirs in Tabei area mainly distribute along the YQ6 well-TS1 well-YangWul well,including 9 periods of reef-shoal bodies in the Cambrian platform margin,early Cambrian platform margin develops 3 periods,middle Cambrian develops 5 periods,and 1 period in late Cambrian.The Ordovician platform margin is identified 6 periods reef-shoal sedimentation,among them Penglaiba group and Yingshan group have 1 period and 5periods reef-shoal reservoirs respectively.The 10 periods reef-shoal bodies from early Cambrian to middle Ordovician present the style of progradation,The 5 periods reef-shoal bodies in Yingshan group present the style of retrogradation.The distribution of platform margin reef-shoal reservoirs in Gucheng uplifted area is along the ManCanl well-GuCheng4 well and develop 5periods reef-shoal sedimentation.Early Cambrian,middle Cambrian,late Cambrian and Penglaiba group,Yingshan group develops 1 period reef-shoal respectively.The reef-shoal bodies in early Cambrian and middle Cambrian present the style of progradation,but The next 3 periods reef-shoal bodies from late Cambrian to Yingshan group present the style of aggradation.