Abstract:

Chang7 member of Yanchang Formation in Zhenbei area is one of the maintargets ofoil and gas exploration. The type and distribution of sand body are controlled by palaeogeomorphology. The recontructed palaeogeomorphology based on precise isochronous stratigraphic framework can provide basis for analyzing the genesis type and distribution of the sand body. Based on core, logging and seismic data, this paper established fine stratigraphic framwork under the guidance of high resolution sequence stratigraphy, and divided Chang7 reservoir into one middle-term base-level cycle and five short-term base-level cycles. An optimized back stripping method was used to restore the deposited terrain, and the paleogeomorphology map of the study area was drawn by short-term base level unit. The result shows that there mainly developed slope break in Zhenbei area, Chang7₃ is the maximum transgression, and turbidite fans appear in the deep lake in the northeastern part of the study area. From Chang 7₃ to Chang 7₁ stage, the base level decreases, the mouth bar and the slope break zone gradually advance to the lake, the angle of the break zone begins to decrease,and fluxoturbidite forms consequently; from late Chang 7₁ stage,the base level begins to rise, the slope break and the sandbody gradually retreat to the shore. Base level cycle change leads to geomorphic change, the origin and distribution of the sand bodies changes regularly, and it has an important significance in searching for oil and gas.

Key words: ultra-low permeability reservoir, fault-block reservoir, well pattern adjustment, remaining oil tapping, reservoir description