CBM reservoir-cap formation type and its gas controlling function in the eastern margin of Ordos basin

The preservation conditions of CBM are controlled by the reservoir-cap formation type to some extent. By analyzing the seal capacity of the seam roof and floor, CBM reservoir-cap formation type in the eastern margin of Ordos basin can be divided into 4 types. The first best reservoir-cap combination type is mainly developed in Taiyuan Formation in Baode-Lin Counties, and Shanxi Formation in Daning-Ji Counties, the former Formation is characterized by lagune-tidal flat facies, the latter Formation is shallow lake facies, which is favorable for CBM preservation. The second combination type is chiefly developed in Taiyuan Formation in Baode-Hequ area, Shanxi Formation in Shilou-Sanjiao area, and the above two formations in Hancheng-Heyang area, the sedimentation is of delta front facies, which is relatively good for CBM preservation. The third combination type developed in Taiyuan Formation in Sanjiao-Ji counties is continental shelf facies, and in Shanxi Formation in Baode -Lin counties is deltaic plain and flood-plain facies, these deposition types have weak sealing ability for CBM. The fourth and the poorest combination type is distributed in Shanxi Formation in the region north of Hequ area, whose deposition is upstream of braided river facies and alluvial fan facies, which is not favorable for CBM preservation. The reservoir-cap formation types are closely related to the gas concentration in the study area, so it is one important reference to forecast CBM abundance of the coal reservoir.