Surging prices for oil and natural- gas shales, in at least one case rising 10-fold in five weeks, are raising concern of a bubble as valuations of drilling acreage approach the peak set before the collapse of Lehman Brothers Holdings Inc.

Chinese, French and Japanese energy explorers committed more than $8 billion in the past two weeks to shale-rock formations from Pennsylvania to Texas after 2011 set records for international average crude prices and U.S. gas demand. As competition among buyers intensifies, overseas investors are paying top dollar for fields where too few wells have been drilled to assess potential production, said Sven Del Pozzo, a senior equity analyst at IHS, Inc. ...

The quirky nature of shale geology means the risks are high that an investment made in a sparsely drilled prospect will go bust, Nikhanj said. Rock density, porosity and pressure levels vary widely within each field, which means one parcel may hold enough fuel to justify prices of $30,000 or $50,000 an acre, while the adjacent land is almost worthless to drillers.