Distributional Features Of Sandbodies Within Sequence-stratigraphic Framework Of The Upper Triassic Yanchang Formation In The Jiyuan Area

The Upper Triassic Yanchang Formation,deposited in the Ordos Basin,is mainly composed of light-gray to gray-green medium-thick fine sandstone and siltstone,and grey mudstone interbedded with grey-black silty mudstone,clipping coal and oil bearing sediments characterized by black carbon mudstone,shale and oil shale.In this paper,we try to explore the spatial characteristics of the sand body in second-order sequences and third-order sequences in Yanchang Formation,as well as its indicative significance to sedimentary environment,by means of studying two connecting-well sections which transit the sedimentary source and a connecting-well sections which strikes along the sedimentary source.We propose that the Yanchang Formation comprises four second-order sequences(SSQ1-SSQ4)and eleven third-order sequences(SQ1-SQ11).The lake floods which can be regarded as the maximum flooded surfaces of second-order sequences were developed in Chang 9,Chang 7,Chang 4+5 and Chang 1.Due to the influence of tectonic uplift,SSQ4 preserved incompletely in the study area.And the other second-order sequences preserved completely.Due to the low base-level,SSQ1 and SSQ4 mainly developed the sand body combination types include distributary channel sand,subaqueous distributary channel sand and overbank sand.Due to the high base-level,SSQ2 and SSQ3 mainly developed the sand body combination types include subaqueous distributary channel sand,sheet sand and distal bar sand.The sand body along sedimentary source direction developed the progressive and regressive sequence with the lake basin extending or contracting.While in the direction perpendicular to the sources,the sand body can indicate the development scale and the space morphology.Third-order sequences have three types of cycle configuration:rising asymmetry cycle configuration,falling asymmetry cycle configuration and symmetry cycle configuration.The sand body combination types and space distribution are different in the three types of cycle configuration.The rising asymmetry cycle configuration mainly develops distributary channel sand and subaqueous distributary channel sand.The falling asymmetry cycle configuration mainly develops distal bar sand,mouth bar and sheet sand.The symmetry cycle configuration mainly develops subaqueous distributary channel sand,mouth bar and sheet sand.Jiyuan represents the semi-deep lake which is relatively higher than base level in the late Jurassic lake basin in Ordos.The distribution of sand in Jiyuan is closely related to the evolution of the whole lake.Accompanied by the expansion and atrophy of the lake basin,sandbody combination style characterized by progradation and retrogradation is developed inside Yanchagn Formation accordingly.Chang 6 sandbody and Chang 8 sandstone,together with the lake face mudstone on the beneath them,formed the high quality source-reservoir-cap assemblage which is suggested to be the main sedimentary layer for oil and gas exploration.