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C.S.P.G. 1990 Convention, "Basin Perspectives"
Realtime Validation of an Oil- and Gas-Resource Assessment for the Offshore Gulf of Mexico [Abstract]
The accuracy of a forecast made in 1980 of the number and the sizes of oil and gas fields expected to be discovered in the Miocene-Pliocene and the Pleistocene trends in the offshore Gulf of Mexico was determined by comparing the predicted and the actual results from the drilling of 1,832 wildcat wells in the Miocene-Pliocene trend and 682 wildcat wells in the Pleistocene trend between 1977 and 1985. This forecast used a two-stage procedure that was based on a discovery process model and the concept of economic truncation. An estimate of the number of larger fields remaining to be discovered in each field size class (the untruncated portion of the field size distribution) was made by using a modified version of the Arps and Roberts discovery process model. The number of fields remaining to be discovered in the truncated portion of the distribution to the left of the mode, including fields containing as few as (116103m3) of oil equivalent, was estimated by using a geometric multiplier of the log-geometric distribution. This assessment of the undiscovered oil and gas resources was, in turn, the basis for the forecast of the number and the sizes of the fields that would be discovered in the future (Drew et al., 1982).
Between 1977 and 1985, 280 oil and gas fields were discovered in the Miocene-Pliocene trend; each field contained more than 729,000 barrels (116103m3) of oil equivalent. Collectively, these fields contained 17.0 trillion cubic feet (481.6109m3) of natural gas and 542 million barrels (86.2106m3) of crude oil and condensate (3.38 billion barrels (537106m3) of oil equivalent) based on 1988 reserve estimates. The 85 fields discovered during the same time period in the Pleistocene trend were estimated to contain 4.8 trillion cubic feet (136109m3) of natural gas and 240.1 million barrels (38.2106m3) of crude oil and condensate (1.04 billion barrels (165106m3) of oil equivalent).
According to our forecast made in 1980 for the period between 1977 and 1985 for the Miocene-Pliocene trend, 222 fields would be discovered during the period and would contain 1.79 billion barrels (284.6106m3) of oil equivalent; this is an underestimate by 1.59 billion barrels (252.8106m3) of oil equivalent in this trend (47% low). The forecast for the same period in the Pleistocene trend indicated that 79 fields would be discovered and would contain 713.2 million barrels (113.4106m3) of oil equivalent, whereas 1.04 billion barrels (165106m3) of oil equivalent were actually found. The forecast was 30 per cent low, based on 1988 reserve estimates.
This underestimation of the amounts of oil and gas discovered in both trends during the 1977-85 period can be attributed, for the most part, to the field growth phenomenon. The field size data used to calibrate the discovery process model for the 1980 forecast were taken from the FRRE file maintained by the Conservation Division of the U.S. Geological Survey (now the Minerals Management Service). These field size estimates were based on engineering calculations that, in turn, were based on incomplete data on reservoir volumes, numbers of reservoirs, and quality of the reservoirs. Statistical growth factors were not applied to estimate the contribution to field growth through the extensions and revisions processes. During the 1980-88 period, the reserves in the 293 fields upon which the 1980 forecast for the Miocene-Pliocene trend was based increased by 18.7 trillion cubic feet (529.7109m3) of natural gas and 1.57 billion barrels (249.6106m3) of crude oil and condensate as a result of extensions and revisions. The parallel growth in the Pleistocene trend amounted to 7.2 trillion cubic feet (204109m3) of natural gas and 698 million barrels (110106m3) of crude oil and condensate. The growth in reserves of the oil and gas fields discovered before 1976 in both trends during the 1980-88 period is, therefore, of the same numerical order as the volume of oil and gas discovered during the same period as viewed from the end of the time period. As suggested by Manger et al. (1985), it is clear that a correction for field growth must be calculated and applied to those oil and gas fields used to assess the sizes and the number of undiscovered fields before future discovery rates can be forecast.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
1 U.S. Geological Survey, Reston 22092
2 University of Delaware, Newark 19716
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