The Sedimentary Facies And Environment Evolvement In The Forepart Of Late Trias, In Longdong Area, Ordos Basin

Along with the discovery of Xifeng oil field in recent years, the favorable oil-bearing layers had been found in Yanchang formations of Banqiao, Zhenyuan and Heshui in Longdong area, these indicated fully there were tremendous prospect of exploration and appraisement in Longdong area. In order to improve the drilling and exploration's successful probability, ancient lithofacies and sedimentary environment in early Yanchang of late Trias were recovered, especially Chang8 and Chang7 period's, by using the outcrop sections, the directions of paleo-river, light and weight mineral, logging and seismic facies, fossils, sedimentary structures and the theories of modern sedimentology. According to these researches, Longdong area formed the southwest edge of Yanchang lake in early Trias, and there were two different sediments source in Longdong's west and south area. The sediments come from west source area were mainly clastic rocks and carbonate rocks formed in early Paleozoic, and these sediments from Kongtong hill to Zhenbei area formed the fan delta's sedimentary system. The sediments come from south source area were mainly seashore clastic rock of Qin Mountain formed in Indosinian epoch, and these sediments from Ruishui River to Xifeng area formed the braided stream delta sedimentary system. According to the research of fossils, sedimentary structures, and logging facies etc., eleven sedimentary sub-facies were sorted. By the researches of the mainly oil-bearing formations' sedimentary evolvements, Chang7 and Chang8 period, which belong to Yanchang lake's development period, were undercompensation sedimentation, And these were benefit for hydrocarbon's sediment. Chang8 formation's rock were mainly sandstone, while Chang9 and Chang7 formations' rock were mainly mudstone. On the whole, Chang9, Chang8 and Chang7 formations formed several combinations, such as oil source upper and reservoir lower, oil source lower and reservoir upper etc. The characters of structural evolutions of Longdong area in late Trias determined the deep south and shallow north, dissymmetrary Yanchang lake. On the control of the Yanchang lake' south cutbank in Longdong area, the sandstone became narrow, and extended like finger. Base on these conclusions, chang81,chang82,chang71,chang72 and chang73 layers' distributions were studied. And in all facies, the underwater riverbed microface of fan-delta front subface and the underwater riverbed microface of braided stream delta front subface were most advantageous. At the same time, Baima's north area and Heshui-Banqiao area were forcest as most advantageous aims in recent years, and Huanxian-Heqi area as potential advantaged aims.