Problems frequently encountered in evaluating tar sand resources-the South Texas San Miguel deposit

Good porosity and fluid saturation values are essential for accurately delineating original oil in place (OOIP) and evaluating the effectiveness of in situ recovery methods. Whereas the techniques for measuring and calculating these parameters from cores or downhole logs are well established for most conventional oil and gas resources, significant errors and inconsistencies can result if the same procedures are used to evaluate unconventional resources, such as tar sands and super-heavy oil deposits, thus adding more fuel to the age-old controversy that exists between core and log experts. For these resources, both evaluation techniques are not only useful but absolutely necessary, because neither alone will consistently yield good results. This paper discusses some of the problems frequently encountered in evaluating unconventional resources where the hydrocarbon density and viscosity, and the formation matrix properties, are significantly different from those used in standardizing traditional core and log analysis procedures. It uses, for example purposes only, a 2° API (1093 kg/m3) gravity resource located in Texas and known as the San Miguel tar sand deposit. One concludes that in dealing with unconventional deposits it is best to anticipate evaluation problems right from the start. Resolution of the problems is an evolutionary process that will likely not be perfected until a good number of wells have been drilled. Recognizing this ahead of time is important because it requires that the resource delineation and formation evaluation programs be properly integrated. Early in these programs all emphasis should be placed upon maximum data collection and retention in a form in which it can be reanalyzed at a later date with modified techniques. Resolution is further expedited by working closely with the core analysis and logging companies in the area. This assures that any improved evaluation techniques will be properly implemented.