Sedimentary microfacies study of the transgressive tract of the forth-grade sequence1 of No.2 and 3 sections in west clinoform region of north Songliao Basin

To forecast hydrocarbon reservoir, studies have been carried out about the characteristics of sedimentary systems, sedimentary facies and sedimentary mocrofacies of the transgressive tract of the forth-grade sequence1 of section No.2 and section No.3 of Qingshankou Formation in the west clinoform region of north Songliao Basin. In the course of the research, we have applied synthetically the up to date theories and methods of sedimentography, sequence stratigraphy, high resolution sequences stratigraphy and geophysics, using cores, logging, well logging, seismics, analysis assay data and paleontology data. The transgressive tract of the forth-grade sequence1 corresponds to the upside part of the No.4 sand beds set of Gaotaizi reservoirs. The distribution area of the transgressive tract of the forth-grade sequence1 is relatively extended. It is mostly composed of delta facies and lacustrine facies in plane. It develops delta estuarine bar and distal end bar microfacies in its north and south-western boundary area. It develops long-zonary shore-shallow lacustrine sand bar with abundant shore-shallow lacustrine mixing beach sediment in most other area where it distributes sporadically shore-shallow lacustrine sand bar and muddy beach. These sandstone bodies are bedded in the middle of condensed sections of overlying rock and underlying sequence, which are advantaged development location of strata-lithology reservoirs.