Consultant, Denver, CO
A relatively new technique provides a means for determining the continuity of gas distributions on both a reservoir and regional scale. Gas samples needed for this determination can be collected simply and inexpensively either while drilling or later during completion or production. Evaluation of stable carbon isotopes for the hydrocarbon gases in these samples gives information on: 1) compartmentalization for reservoir modeling and development drilling, 2) regional gas continuity for planning exploration strategy, 3) the number of possible source systems, and 4) the likely depth of gas generation.
Use of this technique for determining continuity of gas distributions is straightforward. If the carbon isotopes for methane, ethane, propane etc. are similar from one gas to another, it is likely the gas is continuous. Conversely, if the isotopic data exhibit notable differences, the gas distribution is discontinuous. In a production context, isotopic data can be used as input for reservoir modeling and to help in more effective development well placement. Similarly, when exploring for basin-centered gas, strategy will differ significantly depending upon whether the gas distribution is discrete or continuous. Additional exploration applications include identifying the number of source systems in an area and evaluating gas generation depth. For example, in the Uinta Basin, isotopic information was used to identify different hydrocarbon gas source systems. Finally, if gas isotopic data and maturity measurements indicate generation occurred below existing production, then the potential exists for deeper, hydrocarbon-filled reservoirs. To summarize, isotopes represent a powerful additional tool for finding and developing hydrocarbons.