Evolution of the Ordovician top boundary and its relationship to reservoirsʹ development, Ordos Basin

The control of structural evolution on the development of different types of Ordovician reservoirs in the Ordos Basin is discussed by studying the evolution of the Ordovician top boundary in geological times, and the mechanisms of formation and evolution of carbonate reservoirs are examined. The top boundary of the Ordovician is an important sequence boundary in the Ordos Basin. It represents not only the depositional change between lower marine carbonate rocks and upper clastic rocks, but also a depositional break caused by weathering and corrosion from the Late Calidonian to the Early Hercynian. It experienced three great structural changes in the geological history, which influenced the formation and conservation of the Ordovician reservoirs. Difference of lithology of the Ordovician top sequence between the east and west of the basin controlled the type of reservoir space and caused the formation of different reservoirs of corrosive pores. However, the later reversal of the sequence boundary causes the filling of early formed pore space in the eastern basin, and affects the space distribution of effective reservoirs.