The Research On Sedimentary System Of The Chang6Interval Of Yanchang Formation In Huanxian-Qingcheng Area, Ordos Basin

On the basis of existing exploration and research with predecessors, as the guiding ideology of sedimentology, and combined with geophysical and geochemical data, treating Chang6interval of the upper Triassic Yangchang Formation in Huanxian-Qingcheng area, Ordos Basin as the study object, this paper analyzes the depositional environment in detail, determines the source direction, and clear up the sediment types and their characteristics, specifies different deposition of shoreline and deep lake line. At the same time, it constructs a depositional model, and eventually establish the distribution of sand bodies of the studied area.On the basis of regional geological setting, light mineral, rock debris and heavy mineral analysis, it’s considered that the area of Huanxian-Qingcheng is mainly affected by the northeast, northwest, southwest and south of the four source control during Chang6depositional stage of Yangchang formation. And the northeastern provenance is master,(West) south of the secondary source area. Source supply is relatively stable during each period.Previous research data based, core data, petrological characteristics and logging facies integrated, it establishes the two deposition system types in Huanxian-Qingcheng area:"distal gentle slope depositional systems, meandering river delta-lacustrine depositional system" and "near-source steep slope depositional systems, fan delta-lacustrine depositional system", including:a meandering river delta facies, fan delta facies and lacustrine facies. Meandering river delta and fan delta deposit delta front mainly. Meandering river delta front includes underwater distributary channel, tributary bay, estuarine sand bar microfacies; fan delta front subfacies deposite underwater distributary channel, tributary bay and the front sheet sand microfacies; lacustrine facies include coastal-shallow lake, semi-deep lake-deep lake subfacies, and semi-deep lake-deep lake based on semi-deep lake-deep lake mud and gravity flow microfacies.In the context of water receding of the depositional period of the Chang6interval of Yangchang Formation, a detailed analysis of the distribution of the shoreline and deep lake line in the studied area is given. It indicates that it’s still in the underwater environment in Huanxian-Qingcheng area at the end of the depositional period of the Chang6interval, but the contractive deep lake line in the work area has gradually taken shape:deep water area covered of90%distributing with the shape of continuous "W" at the time of the Chang63interval, but only in northwest and western parts of the shallow water; The northeast deep-lake boundary had retreated to the north of Baibao in the period of the Chang62interval with the "Z" shaped distribution, while it contracted to the centre of the lake in the west of the work area; The deep-lake line reached the west of Baibao at the time of the Chang61interval of Yangchang Formation, at the same time, the northwest of Huanxian had retreated as the area with shallow water environment. The deep lake boundaries of contraction rate of the northeast was significantly faster than the west. It indicates that northeast terrain is flat compared to the west.Based on the above, it’s constructed that the depositional model of the Chang6interval of Yanchang Formation in Huanxian-Qingcheng area, Ordos Basin. It mainly belongs to the "near source steep slopes-sham fan delta front-gravity flow deposits".It’s predicted that the distribution characteristics of sand bodies of the study area through the reserch of sedimentary facies. The analysis indicates that the detritus moved into the sham-water district of the lake basin through Baima, Baibao, Shangzhuang mainly with a long road which is provided by Yinshan, the north of Ordos Basin. And it provided the rich provenance for gravity flow deposits. At the time of the Chang63interval, The northeastern part of the sand bodies distributed by Baibao-Huachi-Yuele area, then to Caijiamiao area in the period of the Chang63interval, the southern area developed the deep-water sediments in Chengguan-Qingcheng area, connecting in Xuanma area with the sand bodies of northeast; Gravity flow of Huanxian and Heqi in the west of the study area extended farther.During the period of the Chang62interval of Yanchang Formation, the sand bodies of northeast sharply backed down to Yuele-Shangzhuang area, and had no connectivity with southern sand bodies, while the southern and western sand bodies far promoted and gathered in Heqi-Xuanma area, and it extended southward of Heqi area. At the end of the period of the Chang6interval, the sandy content of the study area greatly reduced, the northeastern and the southern sand bodies re-engaged with a narrow band; By this time, the sand bodies in the western region existed with the form of fan-delta-skirt as a single entity spreading along the line of Huanxian-Heqi-Tongchuan area. All in all, sand bodies mostly stripped with poor connectivity during the whole period of the Chang6interval of Yanchang Formation in Huanxian-Qingcheng area, Ordos Basin.