180 / 2022-02-28 11:57:46

Sedimentary characteristics and sand body distribution of retrogradational delta system controlled by paleogeomorphology: A case study of Cretaceous Qingshuihe formation in the southern margin of Junggar Basin

Junggar Basin, Qingshuihe formation, paleogeomorphology, slope breaks, sedimentary characteristics

In recent years, Well Gaotan 1 and Hutan 1 Well located in the southern margin of Junggar basin have successively obtained high-yield oil and gas flow in the reservoir at the bottom of Cretaceous Qingshuihe formation. The genesis and distribution characteristics of this set of reservoir need to be clarified. Based on the analysis of paleogeomorphology, combined with geological outcrops, cores, well-connected profiles and seismic profiles, this paper analyzes the types of sedimentary facies, the planar distribution of sedimentary facies, and the evolution of sedimentary facies in the Qingshuihe Formation. Retrogradational dalta sedimentary systems controlled by multi-level slope breaks are developed, and the delta front sand bodies of each stage are favorable reservoirs.

 Based on the interpretation of several 2D seismic profiles, this paper clarifies the paleogeomorphic features of the southern margin of the Junggar Basin before the deposition of the Qingshuihe Formation. The depositional center is located in the west of the middle section of the southern margin of the Junggar Basin. Paleomorphology is characterized by steepness in the south and gentleness in the north, and grade 2 slope breaks are developed in the gentle slope belt in the north.

 Affected by the paleogeomorphology, the Qingshuihe Formation in the southern margin of the Junggar Basin developed a north-south bidirectional sedimentary system, in which fan delta deposits developed in the south, and braided river delta deposits developed in the north. With the rise of lake level, the fan delta formed a double-layered sand body structure of "sand on the upper side and gravel on the lower side" during the regression process. The thickness of the conglomerate layer is 10-30m, and the thickness of the sand-mudstone layer is 20-30m. Braided river deltas form sandstone reservoirs with a thickness of 20-50m.

  According to the seismic profiles and well-connected profiles in the northern part of the southern margin of the Junggar Basin, it is clear that two slope breaks control the distribution of the three-phase delta sand bodies. The first-stage delta sand bodies deposited under the first-order slope break are distributed in the center of the lake basin.The second-stage delta sand bodies are deposited above the first-order slope break, and the second-stage delta front sand bodies can cover the first-stage delta plain. The third-stage delta sand bodies were deposited above the second-order slope break. The three-stage sand bodies are vertically superimposed, forming a sand body structure with a coarse-grained lower part and a fine-grained upper part. The sand bodies are contiguously distributed on the plane, especially the sand bodies in the northern braided river delta are more than 2000Km².

  In summary, the two types of delta-derived sand bodies at the bottom of the Qingshuihe Formation in the southern margin of the Junggar Basin are widely distributed, with a thickness of 20-50m. The sandstone formed by the braided river delta in the northern part of the southern margin is well sorted, which is conducive to the formation of relatively high-quality reservoirs. The conglomerate formed in the southern fan delta is poorly sorted, and relatively high-quality reservoirs can be formed locally under certain conditions.