Section V. Exploration Histories
Published:January 01, 1987
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.
Figures & Tables
Exploration for Heavy Crude Oil and Natural Bitumen
Gross volumes of oil, which must be kept in mind to address the volume/size framework, may be thought of in order from largest to probably smallest volumes as follows: (1) generated; (2) dissipated; (3) degraded/ partially preserved; and (4) trapped and conventionally producible. Basic knowledge of these volumes may be from greatest to least in essentially reverse order.
The 332 largest known accumulations (less than 1% of the total number) account for more than three-quarters of the known 7.6 trillion bbl of oil and heavy oil or tar in more than 40,000 accumulations in the world. About 2.4 trillion bbl of estimated undiscovered conventional oil added to the known volume of 7.6 trillion bbl yields a total of 10 trillion bbl known or reasonably estimated. World-wide cumulative production of about 500 billion bbl of oil accounts for only 5% of the gross.
Oil in place must be estimated for conventional oil fields before comparison with heavy oil and tar accumulations. The size range of accumulations considered in the size distribution of the 332 largest known accumulations is from 0.8 to 1850 billion bbl of oil. The smallest conventional fields in the distribution are about 1 billion bbl because the size cut-off is 0.5 billion bbl of oil recoverable. The size distribution of the 332 largest known accumulations approaches log normal and is overwhelmed by the largest three supergiant tar deposits that hold nearly half of the total 5495 billion bbl.
Globally, the largest three accumulations, all heavy oil or tar, are in South and North America; the two largest conventional oil fields are in the Middle East. Prudhoe Bay and East Texas fields rank 18 and 34, respectively, in descending size order.