Preliminary Geologic Analysis of the Tar Sands Near Sunnyside, Utah
Published:January 01, 1987
Sandstones that crop out along the Roan Cliffs near Sunnyside, Utah, are estimated to contain approximately 6 billion barrels of bitumen, making it one of the largest deposits in the United States. Little is known of the geologic aspects, particularly sedimentology, mineralogy (especially clays), and diagenesis, of these sandstones. These aspects of tar sands must be characterized before the feasibility of bitumen recovery can be assessed.
Preliminary results of this ongoing study indicate that the sandstones were deposited in fluvial and marginal lacustrine environments. Sandstone geometry is dependent on depositional environment: Fluvial sandstones tend to be more extensive basinward but are less laterally extensive than the marginal lacustrine sandstones. The sandstones are mainly feldspathic arenites, containing both plagioclase and K-feldspar. Authigenic cements include calcite, dolomite, ankerite, siderite, and analcime. Authigenic clay minerals include kaolinite, illite, and smectite. The percentages of clays and cements vary considerably from sample to sample, particularly between bitumen-bearing sandstones and adjacent lithologies. Some of these authigenic minerals have replaced framework grains. Dissolution has produced secondary porosity, and the bitumen appears to occupy secondary porosity, preferentially in the coarser grained sandstones.
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.