Paleogeomorphic Characteristics And Its Control On Sand Bodies Of Chang 7 Member Of Triassic Yanchang Formation In Ordos Basin

The Ordos Basin is a large inland petroliferous basin with the most complete Mesozoic preservation and the most abundant energy minerals in the central and northern regions of China.It is also the second largest sedimentary basin in China and the largest petroliferous basin in the central region.The Triassic Yanchang Formation in the basin is the earliest discovered strata of onshore oil and gas in China,and it is also the earliest profile developed and studied in the continental Triassic strata in China.Among them,the Chang 7 member is an important layer of gravity flow development,and it is also a key exploration layer of shale oil in the basin.At present,the gravity flow deposition in the deep water area of the lake basin has achieved milestone results in terms of gravity flow type,genetic mechanism,sedimentary transport process and sedimentary model.With the deepening of exploration,there are great differences in the genetic mechanism,sand body type and distribution law between the thick continuous sand bodies of Chang 7₁and Chang 7₂sub-member and the thin sand bodies of Chang7₃sub-member,showing different characteristics.It is urgent to carry out research on the genetic mechanism of sand bodies to clarify the formation mechanism of shale oil layers in Ordos Basin.In this paper,through the analysis of core data,drilling data,logging comprehensive interpretation and other data in the study area,combined with the characteristics of marker layer and sedimentary cycle,the fine stratigraphic division and correlation of Chang 7 section of Triassic Yanchang Formation in the study area are carried out.On this basis,through the data collation of wells in the study area,the paleogeomorphology of Chang 7₃,Chang 7₂and Chang 7₁of Yanchang Formation in the study area is restored.The results show that the Chang 7 paleogeomorphology of the Triassic Yanchang Formation in the basin is an asymmetric depression with a gentle east and steep west.The Chang 7 paleogeomorphology in the study area is divided into seven microgeomorphic units:highland,slope break zone,plain at the bottom of the lake,deep depression at the bottom of the lake,ancient channel,ancient ridge at the bottom of the lake and ancient uplift at the bottom of the lake.Through the core observation and description of the core section of the key wells in the study area,combined with logging,drilling,analysis and testing data,the Chang 7sand body is divided into five types of sand bodies:underwater distributary channel sand body,estuary sand dam sand body,sheet sand body,sandy debris flow sand body and turbidity flow sand body.Seven vertical combination types of sand bodies are established:multi-stage superposition type of underwater distributary channel sand bodies,multi-stage superposition type of sheet sand bodies,mutual superposition type of underwater distributary channel sand bodies and sheet sand bodies,mutual superposition type of underwater distributary channel sand bodies and estuary dam sand bodies,multi-stage superposition type of sandy debris flow sand bodies,multi-stage superposition type of turbidity current sand bodies and multi-stage superposition type of sandy debris flow sand bodies and turbidity current sand bodies.In the near-source area of the lake basin,sand bodies are developed on a large scale,mainly sandy debris flow sand bodies.As they move towards the center of the lake basin,the scale of sand bodies gradually decreases,and sandy debris flow sand bodies gradually transform into turbidity current sand bodies.In the central region of the lake basin,turbidity current sand bodies are dominant.During the sedimentary period from Chang 7₃to Chang 7₁,the lake basin gradually shrank,the turbidity current sand bodies y decreased,the sandy debris flow sand bodies increased,the sand bodies scale gradually increased,the sand bodies thickness increased,and the connectivity improved.The paleogeomorphology has obvious control effect on the genetic types and distribution characteristics of sand bodies.The sand bodies of the traction flow genetic type are developed above the slope break zone,showing a banded or ring-shaped distribution.The sand bodies of the gravity flow genetic type are developed under the slope,showing a ring-shaped or block-shaped distribution.In addition,the ancient landform unit also has a controlling effect on the sand bodies,and the ancient highland is the source of the sand bodies of the gravity flow genetic type;the slope break zone provides unloading area for sand bodies;the ancient channel provides the transport channel and sedimentary area for the gravity flow type sand bodies,and the lake bottom ridge and the lake bottom uplift control the transport direction of the gravity flow type sand bodies.