Classification of types of Oil and Gas accumulations and Traps

Oil and gas trapped in the anticline

Oil and gas fill the pores, vugs, and fractures in reservoir rocks. Usually, not the entire reservoir but only its part called "trap" is filled with petroleum. There are numerous classifications of traps depending on their relationships with the reservoir, lithology, rock origin, tectonics, trap size and shape, and the type of energy distribution. Criterion of usefulness of a classification or a definition is its applicability. Particular purpose, limited in scope, gives rise to incomplete classifications. The development of a natural classification for the traps and/or reservoirs is very difficult, first of all, due to the lack of knowledge about the formation of hydrocarbon accumulations at high pressures and temperatures.

The most definitions of a trap include the following concepts:

• Trap is a part of the reservoir where oil and gas can accumulate.  
• Where relative equilibrium of movable components is maintained by the gravity.
• Where the potential of oil and gas locally reaches a minimum.
• Which is limited by a closed equipotential surface. 

With increasing temperature and pressure, the effect of gravity on the fluid distribution decreases. The existing trap classifications can be divided into three groups.

The first and the most common classification group is called the "Oil and gas classifications". Yet, their is no stress on the parameters (characteristics) of the accumulations but instead of the crustal elements in which they are housed, i.e., traps (their genesis, structure, shape, and type of reservoir).

The second classification group associate the trap shape with the conditions of its formation: folds, faults, stratigraphic unconformities, lithological barriers, and combinations of the above.

Finally, the third group of classifications stressed the hydrodynamic head.
In every case, the trap is considered to be a part of the reservoir, with substantial emphasis on its geometry, which is undoubtedly important when selecting locations for exploratory wells. The accumulation represents a temporary accumulative stage against the background of migration, and the trap forms such part of the Earth's crust (reservoir), where this accumulative stage may occur under certain geologic environments (including the thermobaric and thermodynamic conditions). Not considering the genetic issues, the trap should be characterized by the

• Type of the reservoir it is associated with (including the lithology).
• Shape, lateral extent, and height in relation with the positions of possible hydrocarbon cross-flows.
• Type of the barrier restricting the migration, which at the same time defines the accumulation boundaries. 

Any trap classification reflecting these conditions can be considered practical. The accumulation of hydrocarbons, filling the trap, is an elementary accumulation of hydrocarbons, the major parameters of which are their quality and quantity and the accumulated energy. Besides, additional parameters must provide necessary information for a successful appraisal and subsequent development of the accumulation. Such parameters include the degree to which the traps are filled and the nature of the gas–oil–water contacts. The former depends on the environment of trap formation and the tectonic evolution of the basin. In many cases, certain patterns in the degree to which the traps are filled can be identified in a basin for a particular stratigraphic interval. This may be successfully used in exploration and estimation of resources of a particular stratigraphic sequence. The locations of the gas–oil–water contacts are important during the exploration and development phases. The knowledge of the tilt of oil–water contact, if present, may prevent drilling of unnecessary wells. The task gets more complicated if there are no definite contacts, replaced by transitional zones. Besides, the contour must be closed (also against a barrier). If the amount of hydrocarbons in an accumulation is sufficient to conduct production, the accumulation is called commercial. This concept is uncertain and depends on many factors, such as the geographic location, available infrastructure, depth, quality of hydrocarbons, and the presence of beneficial or harmful admixtures. In the US greater effort is devoted to the development of small accumulations than in Russia. 

The reasons for that include the following: 

• A more advanced oil and gas-producing equipment in the US enables to produce economically from the smaller accumulations. 
• Smaller accumulations are neglected in Russia, and only larger accumulations are developed because of economics and lower demand of oil than in US. 

The classification of the accumulations by the average rate of production is also quantitative. The class boundaries are tentative and may be changed to fit a specific situation for a particular petroliferous province. It is important to keep in mind the fact that the flow rates reflect not only the natural conditions of the accumulation (lithology, fluid properties, energy of accumulation, etc.), but also the technology (development techniques, well facilities, etc.). Besides, the boundary between the low-rate and non-commercial classes is determined by the economical–technical reasons. The price of the oil and gas determines the position of such boundary, and the political situation (e.g., embargo) may impose the development of even very small accumulations.

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