Abstract:

To restore the post-Caledonian paleogeomorphy of the Dagang exploration area, an innovative "scaled stratigraphy" method, in which a regional substratum division coupled with the substratum correlation based on lithological composition and electric logs is conducted, is used to investigate the substratum outcrops of the Ordovician Fengfeng Formation in the area.The result shows that the Caledonian unconformity is of low-angle substrata of ten to dozens of meters thick and its paleo-geomorphology shows an overall northward and eastward rising, and that the outcropped substrata have a tendency of aging upward.This low-angle feature altered the conventional understanding of the Caledonian unconformity in the area and highlighted the influence of Caledonian movement on the Ordovician strata.This study also reveals that, within the Fengfeng Formation, the Substratum V controls the vertical distribution of carbonate reservoirs, which are considered quality reservoirs as they are sandwiched between karst with lumpy dolomite at the top and layered dolomite at the bottom, and continuously occur in central and southern Huanghua Depression.The Ordovician Fengfeng Formation bounded by the Caledonian unconformity as a whole, together with overlying coal seam sedimentation and underlying Ordovician reservoirs, forms a good source rock-reservoir combination for later generated oil or gas.

Key words: dolomite reservoir, scaled stratigraphy, unconformity, Fengfeng Formation, Ordovician, Caledonian period, Dagang exploration area, Bohai Bay Basin