TYPES OF OIL AND GAS RESERVOIRS

ABSTRACT

The paper considers different types of oil and gas reservoirs based on the basis of storage and flow characteristics of reservoirs, and basis of reservoir geometry.

Keywords: reservoir properties, oil and gas reservoirs, the Meddle East trap, cap rocks, as sandstone, conglomerate.

Introduction

An oil and gas reservoirs are the oil and gas accumulation in an independent trap with a single pressure system and the unitary gas–oil interface and oil–water interface. In the light of development geology, an oil and gas reservoir have its geometric configuration and boundary conditions, storage and flow characteristics, and fluid properties.

Initial data

In accordance with different storage spaces and main flow channels of formation fluids, oil and gas reservoirs can be divided into the following types, shown in Table 1.

Table 1.

Oil and gas reservoirs types

1. Porous reservoir

The storage space and percolation channels are the intergranular pores. Therefore, the flow is called flow in porous media such as sandstone, conglomerate, bioclastic limestone, and olitic limestone reservoirs.

2. Fractured reservoir

Natural fractures are not only the main storage space, but also the flow channels. There may be no primary pores or be disconnected pores. Generally, the fracture porosity is not more than 6%. Tight carbonatite and metamorphic rock reservoirs, and mud shale gas reservoirs, are of this type of reservoir.

3. Fracture porosity reservoir

Intergranular pores are the main storage space, whereas fractures are the main flow channels. The flow is called flow in dual-porosity single-permeability media. Usually, the fractures extend for a long distance, but the pore permeability is very low. The Ranique carbonatite oil field of China and the Spraberry Trend oil field of the United States are of this type of reservoir.

4. Porous fractured reservoir.

Both intergranular pores and fractures are the storage space and the flow channels. The flow is called flow in dual-porosity dual-permeability media. Fractures grow fine, but extend for a short distance. The matrix porosity is low.

5. Combined fracture–vug–pore reservoir.

Fractures, vugs, and pores are both the storage space and the flow channels. All reservoirs of this type are soluble salina stone. The secondary pores are the main. This type of reservoir is also called a triple-porosity reservoir.

Classification of Reservoirs on the Basis of Reservoir Geometry Reservoirs can be divided into massive, stratified, fault block, and lenticular reservoirs in accordance with the geometry.

1. Massive reservoir

The reservoir has a large effective thickness (more than 10 m). The oil reservoir may have a gas cap and bottom water while the gas reservoir may have bottom water. The reservoir has a unitized hydrodynamic system and good connectivity.

1-Cap rocks    2-Reservoir rocks

2. Stratified reservoir

Most reservoirs of this type belong to an anticlinal trap with a complete structure and a unitized oil–water interface, see Fig (1) It has good stratification and a number of beds along a vertical section. Every individual bed has a small thickness. A bed with a thickness of 5–10, 1–5, and less than 1 m is called thick, medium thick, and thin, respectively.

3. Fault block reservoir

Fractures well developed in reservoirs of this type. The structure is cut into many fault blocks of different sizes. The areas of some fault blocks are less than 0.5 km². Vertically, there are many oil-bearing series of strata with a long total oil-bearing section

4. Lenticular reservoir

The geologic description of geometry for a sand body depends on the length-to-width ratio. A sand body with a ratio less than or equal to 3 is called lens. Lenses are scattered, and the major area is occupied by the pinch out region. When lenses alternately overlap each other, multiple reservoirs may appear on the vertical oil- and gas-bearing well section.

The Middle East reservoirs and traps:

The Middle East is renowned for its rich carbonate oil reservoirs. Such reservoir rocks are also found in North America and Western Siberia (mainly Paleozoic), Central and South America (mainly Mesozoic), and South East Asia (mainly Miocene). In the Middle East, however, carbonates were deposited on a long and wide shelf from the Permian to the Paleocene with insignificant hiatus. Even after the Arabia-Asia collision, carbonate sedimentation continued in a very shallow marine environment (an ongoing process in the Persian Gulf). Indeed, large oil fields In the Middle East have thick stacks of multiple carbonates pay zones.

Limestone and dolomite reservoirs of the Middle East have fairly good porosity and permeability.

Primary porosity has been well preserved in pack stones and grain stones such as those of the Late Jurassic Arab Formation widely spread in the Middle East. In Saudi Arabia’s Ghawar field (the world’s largest oil field), two producing members (C and D) of the Arab Formation, have thicknesses of 30m and 80m respectively, and a porosity of 20%. The same formation in the UAE ranges from 130 to 240m in thickness and 10-30% in porosity. Moreover, the Zagros deformation has created fracture networks enhancing permeability, especially in cemented limestones such as the Oligocene-Miocene limestones of south-west Iran (Asmari Formation) and eastern Iraq (Jerribe Formation), where their thicknesses range from 120-480m, and porosities from 8-24%. Apart from marine shale and marl cap rocks, many Middle East basins also contain evaporite beds, which are efficient seals because of their ductility.

Conclusion

Reservoirs can be divided into gas, condensate gas, volatile oil, conventional oil, high pour-point oil, and heavy oil reservoirs in accordance with the properties of reservoir fluids.

The continental reservoirs are divided into multilayer sandstone oil reservoirs, gas cap sandstone oil reservoir, low-permeability sandstone oil reservoirs, complex fault-block sandstone oil reservoirs, glutenite oil reservoirs, fractured buried-hill basement rock reservoirs, conventional heavy oil reservoirs, high pour-point oil reservoirs, and condensate gas reservoirs in accordance with the main characteristics of development geology and the development mode of the continental reservoirs.

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