Impact of Realistic Shale Properties on Exploration-Scale Vertical Migration Modeling

Dempster, Kelly, William Almon, William Dawson, Michael Hertle, ChevronTexaco, Bellaire, TX

Basin modelers routinely incorporate data and interpretations from other disciplines into their models: structural and seismic interpretation for past and present geometry, log analy­sis for lithologic content, sequence stratigraphy for depositional framework and lithologic continuity, biostratigraphy for ages, and geochemistry for source rock properties, thermal calibration, and migration indicators.

Seal behavior has historically been treated differently: instead of adjusting the model inputs to reflect measured values, model results have usually been compared after the fact to observed seal properties. Default shales in basin modeling programs have been designed to be excellent top seals and to hold large hydrocarbon columns. In the early basin models for the deep-water prospects in West Africa, these shales have been mixed with silts and placed within a high-resolution stratigraphic framework in an attempt to predict observed hydrocarbon distribution. This empirical method was not satisfactory, because upward mobility of the hydrocarbons in early simulations was not as great as observed in on-struc-ture wells. It also led to variability of results due to differing modeler skills and experience regarding parameter changes.

The lack of fit to observed occurrences of hydrocarbon required ChevronTexaco to develop a methodology that incorporates measured shale/seal properties into existing mod­els. Analyses of marine shales reveal six distinct groups, based on fabric and textural varia­tions. Each type has a different compaction rate, which influences seal character. This vari­ation is described in terms of porosity-depth and porosity-effective stress relationships, porosity versus permeability, and capillary entry pressure distributions. A regional database relates mudstone properties to sequence stratigraphic position and depositional setting. Data relating interfacial tension to composition, as well as temperature and pressure, are also captured.