| Rift basins represent an important type of petroliferous basins in the world,and account for one third of the global petroleum resources.The steep slope zone of a lacustrine rift basin is generally characterized by widely developed glutenite reservoir,which constitutes one of the main petroleum reservoir types as well as important target for petroleum exploration in continental rift basins.The sedimentary evolution of the steep slope zone in lacustrine rift basins has been long concerned and regarded as an important scientific issue in the area of basin sedimentary dynamic and petroleum exploration.However,the sedimentary evolution and depositional model on the steep slope zone of lacustrine rifts in the early stage of rifting is still poorly understood,which restricts our understanding of this important scientific issue.Therefore,the study of the sedimentary evolution and depositional model on the steep slope zone of lacustrine rifts in the early stage of rifting can not only enrich the sedimentary theory of lacustrine rifts,but will also provide new ideas for the deep exploration on the steep slope zone of the continental rift basins in eastern China.This study takes the typical steep slope zone(the North Dongying Sag and the North & Middle Chezhen Sag)of the Jiyang Depression as an example,the depositional system and tectonic-morphology during the early stage of rifting in the North Dongying Sag and the North & Middle Chezhen Sag have been revealed using the integrated core data,3D seismic data,well-log data and on the basis of the analysis of seismic stratigraphy,structural geology,sedimentology,rheology and correlation with modern depositional systems.A new mechanism for the formation of oblique fan delta is proposed,three composite sedimentary systems are revealed and a depositional model is thus built on the steep slope zone of lacustrine rifts in the early stage of rifting.The following four main understandings can be acquired from this study.(1)Large-scale oblique fan delta has been found on the steep slope zone of lacustrine rifts during the early stage of rifting,which can enrich the depositional types on the steep slope zone of lacustrine rifts,and the universality of the formation of oblique fan delta is suggested.(2)A new mechanism for the formation of the large-scale oblique fan deltas on the steep slope zone during the early stage of rifting is proposed.It is considered that the paleo-troughs controlled by the boundary fault activity provide new accommodation for the development of oblique fan deltas during the early stage of rifting,the relay structures related with the segmentation characteristics of boundary faults provide transport conduits for the development of oblique fan deltas,and the fracture zone formed by the interaction between segments provides material basis for the development of oblique fan deltas.In addition,it is proposed that the drainage systems in the denudation area of modern rift basins enters the catchment basins through the relay structures on the steep slope zone,which may confirm the existence and development of oblique provenance during the early stage of rifting.Accordingly,a new depositional model for oblique fan delta on the steep slope zone of a lacutrine rift is proposed.(3)A new model for the early composite depositional systems on the steep slope zone in a lacustrine rift is suggested.Three sedimentary facies types,i.e.oblique fan delta,nearshore subaqueous fan and slump turbidite fan,are developed on the steep slope zone in the early stage of rifting,forming three types of depositional system,including oblique fan delta-deep water,nearshore subaqueous fan-deep water and nearshore subaqueous fan-slump turbidite fan-deep water system.In addition,it is founded that there is a spatial intersection between different depositional systems in the same period;resultantly,the composite model for depositional systems on the steep slope zone during the early stage of rifting is proposed,which can be sub-divided into four broad types,including oblique fan delta +oblique fan delta-deep water,oblique fan delta + nearshore subaqueous fan-deep water,nearshore subaqueous fan + nearshore subaqueous fan-deep water,and slump turbidite fan + nearshore subaqueous fan-deep water,respectively.Which may further enrich the sedimentary theory of lacustrine rift basin.(4)A new model of sedimentary evolution on the steep slope zone of a lacustrine rift during the early stage to the middle stage of rifting is established.Large-scale oblique fan delta and small-scale nearshore subaqueous fan are mainly developed during the early stage of rifting,while short-axis sources mainly exist and generate nearshore subaqueous fans,slump turbidite fans and small-scale fan deltas during the middle stage of rifting.The transformation of the types of sedimentary facies is mainly associated with the growth and splicing of the segments of boundary faults in the evolution of rifting,which leads to the transformation of transport conduits and flow properties.At the same time,it is suggested that the large-scale oblique fan deltas on the steep slope zone in the early stage of rifting are favorable reservoirs,which could contribute important targets for deep petroleum exploration. |
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