1995 National Oil and Gas
Assessment and Onshore Federal Lands
Donald L. Gautier, Gordon L. Dolton and Emil D. Attanasi
U. S. Geological Survey Open-File Report
For the 1995 National Assessment, geologists assessed the numbers and size distribution of technically recoverable undiscovered conventional oil and gas accumulations at the geologic play level. They also assessed technically recoverable unconventional resources in continuous-type oil and gas accumulations and technically recoverable gas in selected coalbeds. For this report, geologists allocated a portion of the assessed technically recoverable undiscovered conventional accumulations and the assessed unconventional (that is, continuous-type and coalbed gas) accumulations at the play level to Federal Lands. Although the 1995 National Assessment included projections of future additions to proved reserves from discovered conventional fields, called inferred reserves or reserve growth, data were insufficient to allocate projections of field growth of identified fields to Federal Lands, therefore, estimates of inferred reserves were not made.
Commodities assessed were crude oil, natural gas (associated and non-associated), and natural gas liquids from gas. All gas quantities are expressed as dry gas, that is, gas that has been stripped of natural gas liquids. Gas dissolved in geopressured brines and oil in tar deposits, and in oil shales are excluded. Gas from low-permeability "tight" sandstone reservoirs, oil and gas from shale reservoirs, and coalbed gas were specifically assessed. Only resources extractable through a well-bore were assessed.
Undiscovered technically recoverable resources are defined as estimated quantities of resources hypothesized to exist on the basis of geologic knowledge, data on past discoveries, or theory, and that are contained in undiscovered accumulations outside of known fields. Estimated resource quantities are producible using current recovery technology but without reference to economic viability. Posited undiscovered accumulation sizes include all components of field growth that might occur during field development and production. Conventional accumulations are oil and gas accumulations that have well-defined hydrocarbon-water contacts and seals that hold the hydrocarbons. Hydrocarbons can typically be extracted using traditional development and production practices. Accumulations assessed by geologists as occurring outside of existing fields were considered for the purposes of the economic analysis as separate and discrete new fields. Onshore and State offshore areas of the United States were divided into eight regions and further subdivided into a total of 71 provinces (figure 2). For the 71 provinces, about 460 conventional plays were assessed. The economic analysis was based on the characteristics of the mean of undiscovered conventional resources assessed at the province level.
Continuous-type accumulations are hydrocarbon accumulations that are pervasive throughout a large area or region and that do not owe their existence to the buoyancy of hydrocarbons in water as conventional accumulations do. Coalbed gas was assessed separately although it is also a form of continuous-type accumulation. In contrast to conventional accumulations, continuous-type accumulations have no downdip hydrocarbon-water contact. A dominant characteristic of the reservoir rock of a continuous-type accumulation is that it is everywhere oil-or-gas charged. Other geologic characteristics include positioning of the accumulation downdip from water-saturated rocks, low reservoir permeability, abnormal (high or low) pressures, and close association of the reservoir with the source rocks from which hydrocarbons were generated. These accumulations are contained in sandstone, siltstone, shale, chalk, or coal. Large portions remain undrilled and their areal extent and production properties remain uncertain. Play assessment methods that rely on the historical discovery size distributions are not applicable. New geologic and economic assessment methods were devised for assessing technically and commercially recoverable oil and gas from continuous-type plays. Economic evaluations were prepared at the play level and were also based on the characteristics associated with the mean values of the assessed resources.
In recognition of the uncertainties associated with the estimation of undiscovered accumulations of oil and natural gas, the estimates of technically recoverable resources are presented as a range of possibilities: a low case having a 95 percent probability of that amount or more occurring, a high case having a 5 percent probability of that amount or more occurring, and a mean case representing an arithmetic average of all possible outcomes. Finally, the estimates are reported as fully "risked" estimates, a category which includes the possibility that some areas may be devoid of oil or natural gas accumulations in the sizes assessed.
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