The Environmental Atlas of Abu Dhabi Emirate

75

O IL & N ATURAL G AS F ORMATION

T HE G EOLOGICAL P ERSPECTIVE Although the Arabian Plate region has remained tectonically stable throughout the Mesozoic and Cenozoic eras (the last 251 million years), slow yet continual sinking in the east of the region has allowed the

accumulation of almost ten kilometres of sediments. It is within this great thickness of sediments that the significant oil and gas resources of Abu Dhabi were formed and are trapped.

The sequence and timing of this deposition is important. Some periods favoured the accumulation of organic-rich sediments, such as mudstones and shales. These are known as source rocks and are capable of generating oil and gas (hydrocarbons). At other times, conditions favoured the deposition of sediments suitable for the storage of oil and gas, referred to as reservoir rocks. These rocks were deposited in the warm shallow waters of the ancientTethys Sea that once covered the region (255 to 20 million years ago), in conditions similar to the Arabian Gulf today. In addition to a source and reservoir, hydrocarbon deposits require some form of seal – an impermeable rock that prevents the oil and gas from making its way to the Earth’s surface and escaping. To accumulate hydrocarbons in significant quantities, a trap is also required in which they can collect in reservoirs over time. The timing of the trap formation is important. If the trap is not in place before the hydrocarbons are generated, then the oil and gas will continue their upward journey until they are lost at the surface.

The story of oil and gas formation starts with a sedimentary basin. During times of high sea level, this depressed area may be flooded and marine sediments will be deposited. If water circulation is poor, the basin floor becomes starved of oxygen. Organic matter in the sediment will be preserved to produce organic-rich source rocks. The weight of accumulating layers of sediment compresses the underlying sediments causing them to lithify: to form sedimentary rocks.

Timeless and Restless: What Lies Beneath? Since the early days of oil and gas exploration, the technologies available to explore beneath the Earth’s surface have improved dramatically. Seismic surveying transmits pulses of energy into the Earth to create images of the rock layers below the surface. Geoscientists study the properties of these rock layers to determine if they are likely to contain water, oil or gas. Geoscientists also lower various devices down boreholes to directly measure the physical and chemical properties of rocks. They may also extract rock sample cores. All of these analyses are used to

develop detailed computer-based reservoir models and to plan the development of the oil or gas fields. These technologies allow a better understanding of the complex geology beneath the surface, allowing the development of efficient cost-effective methods for oil and gas extraction.

3500 m

In hydrocarbons are found in stratigraphic and structural traps. Stratigraphic traps occur where a sedimentary rock layer prevents hydrocarbon movement. Structural traps form when sediment layers are deformed to produce a series of gentle folds with anticlines (peaks) and synclines (troughs). Some rocks do not fold easily; they instead fracture to produce faults. Sealed faults block the movement of fluids across or along the fault: they are important in the formation of traps. Unsealing faults allow fluid migration and may breach earlier seals allowing hydrocarbons to migrate. Abu Dhabi

2500 m

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The temperature in the Earth increases with depth. This geothermal gradient is a major control on oil and gas formation, determining when, and if, organic-rich source rocks will generate oil or gas through the process of maturation. During burial, the rock experiences increased temperature and pressure until it enters the oil window; a time when oil is produced and migrates out of the source rock. Increasing burial will carry the rock into the gas window as a second phase of lighter hydrocarbon production.

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Natural Capital › Oil & Natural Gas Formation

Most of the traps beneath Abu Dhabi formed during the Late Cretaceous when Earth movements deformed the sub-surface sediments. Later, the collision between the Arabian and Eurasian plates during the Neogene period further developed these traps, producing the broad gentle structures that define many of the oil and gas fields of Abu Dhabi today.

The Arabian Plate has remained within 30° north or south of the equator since Permian times. Warm, shallow tropical seas are ideal for the formation of carbonate sediments. Over time, these sediments accumulated to produce the thick sequence of carbonate rocks below the surface of Abu Dhabi. Hydrocarbons are produced frommore than 15 rock formations inAbu Dhabi ranging in age from the Late Carboniferous to the Miocene. However, the largest reservoirs lie within only two stratigraphic layers: the Arab Formation and theThamama Group.

A potential reservoir rock must possess good porosity and permeability. Porosity describes the holes in the rock in which hydrocarbons can be stored and is recorded as a percentage of the rock’s volume. Permeability allows fluids to flow through the rock and is measured in millidarcies (mD). Permeable rocks have well connected pores through which hydrocarbons can easily flow.