Significance of Gravity Segregation in Heavy-Oil Distribution, Forest 'A' Reservoir, Parrylands Field, Trinidad, West Indies
Published:January 01, 1987
Richard B. McCammon, Warren I. Finch, John O. Kork, H. Rambarran, 1987. "Significance of Gravity Segregation in Heavy-Oil Distribution, Forest 'A' Reservoir, Parrylands Field, Trinidad, West Indies", Exploration for Heavy Crude Oil and Natural Bitumen, Richard F. Meyer
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Parrylands is a multiple pay oil field with production from unconsolidated sand reservoirs of late Tertiary age in the Southern basin of Trinidad.
Stratigraphy and sedimentology of the formations penetrated indicate three cycles of delta progradation resulting in the deposition of the Cruse, Forest, and Morne L'Enfer formations. Primary production is obtained from the deeper Cruse Formation whereas the overlying Forest and Lower Morne L'Enfer formations contain heavy oil and tar sands.
The Forest 'A' Reservoir represents a delta fringe/barrier bar system containing approximately 100 million barrels of heavy oil in place. Within the Forest 'A' Reservoir, the oil occurs in north-northwest-trending barrier bars on the crest and flanks of the Lot 1 anticline. Geochemical evidence suggests that the oil was generated from a Cretaceous source and migrated along major faults into upper Tertiary reservoirs, where it occurs as a heavy degraded oil. Gravity segregation occurred resulting in 16-19°API crude in the crestal area of the anticline and 10-12°API crude on the lower flanks of the anticline. The higher API gravity crude is produced by primary methods while that of lower gravity can only be produced commercially by thermal methods.
This observed phenomenon of segregation within the same reservoir is significant in identifying primary production in areas of heavy oil. This can be done by carefully mapping and correlating structure, API gravity, and other characteristics of the crude.
The economic considerations of this technique in exploration and exploitation of heavy oil cannot be over-stressed since a (favorable) cash flow can be generated prior to intensive capital investment for thermal enhanced oil recovery.
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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.