| The Ordos Basin is one of the large inland multi-layered petroliferous basins in my country,among which the Triassic Yanchang Formation in the basin has achieved huge benefits as a large-scale lithologic reservoir in the basin.The Hongdenan area is located in the middle-eastern part of the Tianhuan Depression in the southwest of the Ordos Basin.The administrative division belongs to Hongde Town in the north of Huanxian County,Gansu Province.Hongdenan area starts from Gengwan Township in the west,Qiaochuan Town,Huachi County,Shaanxi Province in the east,Huangmi Village in the north,and Haojiajian in the south.In order to meet the needs of exploration and development,this article applies theories of stratigraphy and sedimentology,and based on the research results of stratigraphic division and sedimentary environment in the Ordos Basin,through core observation,analysis and description,logging data collection,and various kinds of test analysis,which was conducted to study the stratigraphy,sedimentary facies and palaeogeographic evolution of Chang 7 reservoir in Hongdenan area of Ordos Basin,and the knowledge obtained is as follows:The stratigraphic division method based on the thickness principle of marker layer control,sedimentary cycle method,sedimentary strata,etc.,determined the top and bottom interface characteristics of the Chang 7 reservoir in the Hongdenan area,and divided it into three sub-segments,Chang 73,Chang 72,and Chang 71.The Chang72 sub-segment is further divided into long 722 and long 721 sub-layers,and the Chang71 sub-segment is divided into long 712 and long 711 sub-layers.Through stratigraphic division and comparison,the results show that the thickness of the Chang 7 oil layer group is 95-120m,of which the Chang 72 sub-member is relatively thick,between25-55m,and the thickness is generally more than 35m,the thickness of the length 71and the length 73 is between 20-45m.The long 722 and long 721 small layers generally have a thickness between 15-30m,and the two small layers are not much different.Combining the regional paleo-flow direction data,through the analysis of the source analysis methods of the Hongdenan area’s lithic composition content,the enrichment law of rare earth elements,etc.,the comparison with the parent rock in other areas in the basin and the periphery of the basin,The study indicated that the source of Chang7 period in Hongdenan area comes from the southwest of the basin,and its parent material is mainly granite and alkaline basalt with shallow metamorphism of the Longxi ancient land.Based on the lithological assemblage,sedimentary structure and geochemical test and analysis results of Chang 7 oil-bearing group in Hongdenan area,it is pointed out that the sedimentary environment in this area is a deep lake to semi-deep lake,with sandy clastic flow and turbidity current deposition.The gamma curve of sandy debris flow is mainly box-shaped,and the curve value is generally low,between 41-125 API,and the teeth characteristics are weak.The corresponding lithology is mainly gray and dark gray fine sandstone with a small amount of powder.Sandstone,the sedimentary structure is mainly a block structure.The sedimentary thickness of a single-stage sand body is usually greater than 0.5m.Irregular argillaceous tearing debris and argillaceous boulders are seen in the massive sand body.The sand body is in sudden contact with the upper and lower mudstones.Irregular contact surface,common flushing surface,heavy load mold,flame-like structure,etc.The characteristic of turbidity current on the gamma curve is that the natural gamma value ranges from 100 to 225 API,the toothing phenomenon is obvious,and it has a distinctive bell-shaped curve characteristic;The lithology is mainly gray,dark gray fine-siltstone,siltstone,argillaceous siltstone,The core section is rich in sedimentary structures,with cross bedding,sand-line bedding,parallel bedding and other sedimentary structures.The turbidite content is high.The single-stage sand body has thin sediments,low grain size,and thickness less than 0.5m;In addition,the rock assemblages on the core section are mostly belt-shaped,laminar sandstones and mudstones are parallel and interbedded,and an incomplete Bauma sequence can be seen.The impression method was used to identify the bottom shape of the lake basin during the initial deposition of Chang 7 in the Hongdenan area,On the basis of the bottom shape analysis of Chang 7 Lake in Hongdenan area,Combining the formation thickness contour map,sandstone thickness contour map and mudstone thickness contour map of Chang 73,Chang 722,Chang 721,Chang 712 and Chang 711 in Hongdenan area,With the idea of"single factor analysis,multi-factor integration"map compilation,the paleogeographic evolution characteristics of Chang 7 reservoir group in Hongdenan area are analyzed.The results show that the Hongdenan area during the Chang 73 period was mainly semi-deep lake-deep lake muddy deposits,which was less affected by gravity flow,Chang 722,Chang 721 and Chang 712 are greatly affected by gravity flow,the difference is that the influence of the Chang 722 turbidity current is slightly greater than that of the sandy clastic flow,while the influence of the Chang 721 turbidity current is much greater than that of the turbidity current,there is little difference in the development of sandy debris flow and turbidity current in Chang 712 period.The influence of gravity flow in Hongdenan area of Chang 711 period is slightly better than that of Chang 73 period,but worse than that of Chang 722,Chang 721,and Chang 712.The development of turbidity current is slightly better than that of sandy clastic flow.According to the distribution of fine sandstone,siltstone,mudstone and the lithofacies paleogeographic map of each stage（Chang 73,Chang 722,Chang 721,Chang 712,Chang 711）of the Chang 7 oil layer group in Hongdenan area,and combining the lithofacies and paleogeographic environment characteristics of each layer in the oil layer group and the thickness distribution of fine sandstone,the area where sandy clastic flow is developed and the thickness of fine sandstone is greater than or equal to 5m is judged as a favorable exploration area.According to the distribution map of the favorable exploration areas of Chang 73 sub-member,Chang 722 sub-layer,Chang 721 sub-layer,Chang 712 sub-layer,and Chang 711 sub-layer,the Chang 721 small layer of the Chang 7 reservoir in Hongdenan area has the best development of sandy clastic flow.The fine sandstone with a thickness of more than 5 m is widely distributed.It is the small layer with the highest exploration and development value of the Chang 7 reservoir in this area,followed by the Chang 722 and Chang 712 sub-layers.Although the areas with exploration value are not as extensive as the Chang 721,they are better than the Chang 73 sub-member and the Chang 711 sub-layer.The exploration and development value of the Chang 73 sub-member and the Chang 711 sub-layer is lower... |
