Research On Sedimentary System And Squence Stratigraphy Of The Jurassic In Ordos Basin

The Ordos Basin is a large-scale craton superimposed basin locating on the west of the North China platform, which is rich in oil, natural gas, coal, uranium and many other energy resources minerals. Many distinct subjects and points of view are needed in order to deeply study the characteristics of the occurrence and distribution of these energy resources in the basin. The studies of sedimentology and sequence stratigraphy are important indispensable aspects. The method system that is suitable for depression basin sequence stratigraphy of the continental facies is applied, in order to carry out the partition of high resolution sequence stratigraphy of Jurassic in Ordos basin and study the composition and characteristics of long-term, mid-term and short-term cyclic sequence. Which is measure up to set up the sequence stratigraphic framework of Ordos basin and discuss the composing and spatial distribution of sedimentary system in sequence stratigraphic framework. The formation and gathering of coal, gas, oil and uranium are always related to the given sequence position or sedimentary system. Moreover, the formation of sequences are controlled by tectonism, supply of sediments, transformation of base level and climate etc. It will give us great help to illustrate the gathering and distribution laws of energy resources in terms of theory if we study the relationships between sedimentary system, sequence stratigraphy and certain minerals.According to 47 outcrops sections, and log formations of 325 exploitation wells, the sequence stratigraphy of Jurassic in Ordos basin is devided into a tectonic cycle, which is the stratum constrained between post-Indo-Chinese epoch to mid-Yanshan epoch, and is limited by unconformities both up and down. The interior of the sequence can be divided into three clear super-long-period cyclic sequences. The first super-long-period cyclic sequence (SLSC1) is made up of Fuxian and Yan'an Formation, while Zhiluo and Anding Formation constitute the second super-long-period cyclic sequence (SLSC2), and Fenfanghe Formation forms the third super-long-period cyclic sequence (SLSC3). The adjacent two super-long-period cyclic sequences are separated by unconformities. The interior of the sequence has the characteristic that it transits from the early tectonic active stage to the late stable stage. Fuxian Formation and Yan 10, the under parts of Yan'an Formation, represent the sedimentation in tectonic active stage; while the middle and upper parts of Yan'an Formation are the relative stable stage. The under parts of Zhiluo Formation represent the second tectonic active stage, and the upper parts and Anding Formation are the second stable stage. Fenfanghe Formation represents the third active stage. According to the principle of high resolution sequence stratigraphy, and considering tectonic movement, supply of sediments, climate and changes of lake level, Jurassic can be further divided into7 long-term cyclic sequences and 13 mid-term cyclic sequences. The first long-term cyclic sequence is made up of 1 mid-term cyclic sequences: Fuxian Formation is MSC1.Yan'an Formation is made up of 3 long-term cyclic sequences: LSC2—LSC4 which are further divided into 7 mid-term cyclic sequences: MSC2—MSC8. the LSC5 and H LSC6 long-term cyclic sequences are partly represented by Zhiluo Formation and Anding Formation, which are all divided into 2 mid-term cyclic sequences. Zhiluo Formation is MSC9—MSC10, and Anding Formation is MSC11—MSC12. The third long-term cyclic sequence is made up of one mid-term cyclic sequence, which is the MSC13 that Fenfanghe Formation represents. Each mid-term cyclic sequence can be divided into several short-term cyclic sequences that do not have comparability in the whole basin. The amount and characteristics of the divided short-term cyclic sequences are different in distinct regions. The Jurassic sequence stratigraphic framework is set up through two or more well tie contrast sections in the basin. There are comparisons of both boundaries and sequences for the sequence's comparison, as well as contrasting boundary with sequence.Through the multidisciplinary analyses of sedimentary structures, vertical secessions of log information, characteristics of rocks and minerals and fossils information, we can distinguish and measure off sedimentary facies and sedimentary systems of each formation in Jurassic. The Jurassic in study region can be divided into alluvial fan, river, lake and delta sedimentary systems. Arid fan takes the dominate place of alluvial fan,and particularize the humid fan developed in Yan'an Formation. River sedimentary system includes both braided stream facies and meandering river facies. Discuss the characteristics and interspace distribution of different kinds of facies, subfacies and microfacies in detail, as well as every facies' position and control mechanism in sequence stratigraphic array. In tectonic active stage, the early period of base level cycle, sediments accumulated quickly, and alluvial fan or braided stream was always developed, but their distribution area was limited. In the middle period of long-term cyclic sequence, as tectonic movement went into the relative stable stage, meandering river, delta and lake deposit developed. Draw plans to the sedimentary system measured in mid-term cyclic sequence, and depict the interspace distribution characteristics of sedimentary system. The contrastive study on different cyclic sedimentary systems set out the Ordos sedimentary basin's filling and evolvement history in Jurassic.We have to discuss the type and size of prototype basin in order to study the sequence stratigraphy and sedimentary system of Jurassic. The Ordos basin now is the relict basin having undergone tectonic movement and denudation. Resume Ordos prototype basin in Jurassic, through tectonic analyze and using sedimentary track record comparison around the basin. In Jurassic, the western, eastern and northern boundary of the basin had changed in varying degrees, except the south- Qinling orogenic belt rising in Indo-Chinese epoch, which is an always limited boundary. The western of the basin is limited by west-edge thrust nappe belt which formed in early and mid Jurassic. According to structural analysis and sediment comparison ,we judge the western of the basin is outside of west-edge thrust nappe belt .At the same time , the western of the basin is different in the north and south. Therefore in the early and mid Jurassic the western boundary was 40km west than today. The eastern boundary of the basin was not the Luliang Mountain now;and Luliang Mountain rose in late Jurassic, so it couldn't be effective in dividing the basin in early and mid Jurassic. Meanwhile, the relict stratum of Jurassic in Shanxi and He'nan Province can compare with the stratum in Ordos. So we can inferred from that the eastern boundary of the basin is on the east of Luliang Mountain. However, Taihang Mountain has already risen in Indo-Chinese epoch,so the farthest place of the east boundary can only be Taihang Mountain. Though the northern boundary of the basin had activities, but the range was small. We can consider it to be the same with today. The basin's main figure was nearly a square in early and mid Jurassic, but with latitudinal direction compression stress field in Yanshan epoch, the basin shrinked on latitudinal direction, and had little change on the north-south direction. So the basin turned to be oriented north-south rectangle.The Ordos basin has gathered coal, gas, oil,uranium and many other energy resources in Jurassic. The study of sedimentary system and sequence stratigraphy can post the distribution laws of energy resources from one side. The coal bed, especially the thick coal bed pertaining to a region, needs given particular structural setting, sedimentary environment and material basis. On the tabular surface, coal distributes mainly on the edge of the basin, especially gathering in the north and west of the basin. The thicknesses of coal bed minish or annihilate to the direction of "three Yan district"(Yanchang,Yan'an,Yanchuan). In the vertical section, coal bed locates on the late rising half cycle of base level cycle. Uranium outputs in both Yan'an and Zhiluo formation. On tabular surface, uranium distributes around the basin, and gathers most in north basin. In the vertical section, uranium locates on the early rising half cycle of base level cycle, and the avail sedimentary facies is river way sands. The oil has the characteristic of formation upside and save underside in Jurassic,which is favour of the gas and oil banking arena being controlled by sedimentary context, so as to form palaeogeomorphology channel draping oil deposit.