Use of Dimethylbiphenyls for Assessing Paleogeothermal Gradients

Robert Alexander, Robert I. Kagi

Paleogeothermal gradients have been inferred for sedimentary sequences by detailed examination of the relative concentrations of dimethylbiphenyls in sediments. The procedure used is based upon laboratory measured kinetic parameters for the conversion of ortho-substituted biphenyls into fluorenes. Using these kinetic parameters and a value for the geothermal gradient, we can obtain a match between the calculated extent of reaction and the measured value from the sediments. These calculations trace the history of reaction during the various stages of subsidence of the sediment and give the average effective geothermal gradient.

A series of ortho-substituted biphenyls of differing reactivity have been used to assess geothermal gradients in different maturity zones. The most reactive system is suited to the pre-oil generation zone and the least reactive to the post-oil generation zone. Evidence is presented that these reactions are thermally induced and are not influenced by the nature of the mineral matrix. This approach has been used with success to predict the present-day temperatures and geothermal gradients of sedimentary sequences with a wide range of heating rates.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.