There are numerous mound-shaped reflections of Ordovician carbonate rocks bounded by T9 and T10 reflection waves on seismic profiles. Some of the mound-shaped reflections belong to reef dominated by algae; some represent salt mounds or residual hills of ancient weathering crust the Ordovician; and others represent diffracted waves and pseudomorphs caused by improper static corrections of terrain or weathered depth in the processing of seismic data. The Ordovician reef mounds are generally small in size and composed mainly bindstones; so, like weathered residual mounds, their porosity development depends mainly on secondary corrosion. But reef mounds may possess a better porosity because they were exposed and eroded repeatedly in their growth under the influence of sea level fluctuation. Both the reef mounds and weathering residual mounds are favorable sites for finding oil and gas.