The invention relates to a method for detecting free gases in a stratum. In the method, an exploratory well and a development well are used. The method comprises the following steps of: carrying out
well logging in the exploratory well and the development wall to obtain
well logging data; then calculating the Lame constant and the
shear modulus of an underground rock stratum according to the
well logging data; taking the Lame constant, the
shear modulus and the density as
hydrocarbon detection factors and determining threshold values of the
hydrocarbon detection factors; and directly detecting gases, such as free
natural gas,
carbon dioxide and the like with economic values, in the stratum, by using the
hydrocarbon detection factors and the threshold values thereof. The method disclosed by the invention is applied to certain
coal bed gas exploration area and used for detecting superficial free
natural gas reservoir stratums which interdependent and coexistent with
coal bed gas. The method disclosed by the invention is used for detecting superficial
free gas reservoirs by using the well
logging data observed in the well and has the advantages of
low observation environment
noise, reliable data and easiness in implementation. When the invention is implemented, field
engineering cost is not increased generally; even the field
engineering cost needs to the increased, the increased cost is the acquisition cost of
transverse wave speed at most. Therefore the invention has the advantages of low cost.