THE ROLE OF GEOCHEMISTRY IN CHARACTERIZATION OF GAS SHALES AND PRODUCED GAS

The recent increase in exploration for shale gas has made extensive use of geochemical data such as TOC, Rock Eval parameters, and vitrinite reflectance. The basic concept behind the use of these parameters being determination of shales that are of sufficient maturity and with a high enough TOC content to have generated and retained sufficient quantities of gas for commercial production. Another geochemical approach that, to a large extent, has been overlooked during these recent high levels of activity, is the stable carbon and hydrogen isotope compositions of the hydrocarbon components in the gas.

This paper will review basic concepts of stable isotope geochemistry and how this approach can be applied to both exploration and production studies. From an exploration viewpoint the isotopic composition of natural gas components can be used to provide an indication of the maturity level at which the gas was generated; whether or not there are multiple sources of gas in a basin; and whether there has been a contribution from microbial gas. From a production point of view stable isotopes can be used to determine communication between fault blocks; presence of barriers within a producing horizon and in certain cases monitor the extent of commingled production.

Finally in parallel with work undertaken for many years in the oil source rock area, gas shales can be matured in the laboratory through the use of hydrous pyrolysis. The purpose of this approach is to undertake maturation experiments in the laboratory using immature samples of the shales to determine the maturity level at which the gas is generated and the isotopic composition of the gases. Information on the maturity and composition of the gases generated in this manner is extremely important for predicting areas of maximum gas generation in a basin.

Wherever possible examples of these applications will be provided in order to illustrate the utilization of the stable isotopes and the other geochemical parameters to characterize actual prospects. It should be noted that any geochemical information is but one part of the puzzle in the investigation of gas shales, and should be integrated with geological information, lithostratigraphic information, petrographic information and geophysical data.