Abstract：Formation thickness is an important parameter, It is the basic work for explaining buried formation and construction correctly.The calculating methods in common use are constrained by the dig of formation and hard to calculate, and the deviation is obvious for un-horizontal formation.By analyzing the current calculation method for formation thickness, this paper put forward an easy operation method for true formation thickness calculating based on well logging-seismic combination.The map of formation true thickness that obtained with this method not only has accurate thickness value, but also has clear lateral trend.After practical application of the Ordovician in the Nanpu Sag show that the method has important practical value.

Keyword：formation thickness;true thickness;Nanpu sag;Ordovician;