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AAPG Bulletin

Abstract


Volume: 57 (1973)

Issue: 4. (April)

First Page: 726

Last Page: 740

Title: Entrapment of Petroleum in Isolated Porous Bodies

Author(s): Caswell Silver (2)

Abstract:

Isolated traps form a place for hydrocarbon accumulation that precludes water when the supply of oil and gas is adequate. Water is precluded by capillary displacement pressure in the preferential occupation of a porous mass completely surrounded by a less permeable matrix (preferably unoxidized organic shale, as a source bed). Such an isolated porous mass is designated herein as an "isolani." The isolani concept is useful in the understanding of many stratigraphic accumulations of hydrocarbons and their subsequent preservation. This concept explains the closed areas of low potential coincident with many large gas deposits.

Isolated reservoirs can be identified by the primary characteristics of the isolani, the occurrence of anomalous sinks or swells in a potentiometric surface, or the presence of little or no bottom or edge water. The secondary characteristics are the presence of water updip toward the outcrop in the stratigraphic unit, the absence of water downdip, and the general irrelevance of structure. Where water does occur with hydrocarbons in an isolani reservoir, it may take up a minor part of the entire reservoir system in contrast with structural traps or ordinary stratigraphic traps, wherein water is the main component of the system downdip in a basinward direction.

Isolani traps are an important class of stratigraphic reservoirs, forming very large fields in the basin structural lows. Examples are some of the fields in Cretaceous sandstone of the San Juan and Denver-Julesburg basins; the Muddy sandstone of the Powder River basin; the Cherokee sandstone of eastern Oklahoma and Kansas; the Atoka limestone and Morrow sandstone of the deeper part of the Delaware basin; the shoestring sandstones of Michigan; the fractured Niobrara shale of Colorado, Wyoming, and other parts of the Rockies; and some of the older fields of Pennsylvania, West Virginia, and Ohio.

Isolani traps can be divided broadly into primary and secondary types, depending on the coincidence of their formation with the completion of either sedimentation or compaction. Primary isolani are largely filled with oil. Secondary isolani can be oil filled, but more commonly are filled with gas. The primary isolani generally are related to the larger shale units by inclusion, and most secondary isolani are found in juxtaposition to the larger shale units. Certain stratigraphic models can be identified broadly with either type. Primary isolani contain oil toward the basin center and gas toward the outcrop or high on the limbs; secondary isolani generally contain gas wherever found. Such variations can be a guide to exploration.

Isolani-type fields can occur anywhere in a basin at any depth, but are most conspicuous in the basin structural lows. Because isolani traps are dependent on stratigraphic models and not structure, there are vast untested areas of basins without structures which are excellent targets for oil and gas deposits.

Isolani have produced some of the largest gas fields found in the United States. Because isolani so commonly are large in areal extent, they represent an important target for future petroleum reserves, especially gas.

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