There are many oil showings in Cambrian and Ordovician systems in North China

which have drawn the attention of petroleum geologists for years.In this paper

lithologic identification is made up of 8 formations:the Houjiashan Stage

the Mantou-Xuzhuang Stage

the Changxia Stage

the Upper Cambrian

the Lower Ordovician

the Lower Majiagou Stage

the Upper Majiagou Stage and the Fengfeng Stage.Based on formation thickness and paleontologic data

paleogeographic changes are reviewed.Marine transgression of limited extent took place along the depression zone in the inner flanks of the Inner Mongolia

the Jiaoliao and the Huaiyang oldlands during the Houjiashan age.Clastic rocks and mudstones were well developed at the outset.Till the Changxia age carbonate rocks began to be widespread.During the late Cambrian

dolomites occured progressively from the north and reached their greatest thickness to the south

accompanied by a gradual lowering of horizons.In the Early Ordovician

the southern boundary of the sea basin shifted northward

but it expanded again during the early Middle Ordovician

more dolomites among carbonates were deposited.Clay and gypsum beds became widespread during the late Middle Ordovician when the sea area being contracted.All the isopach contours extend in an E-W direction.