OIL PRIMER

Characteristics of Reservoir Rocks Origin and Accumulation of Oil and Gas Petroleum Traps

Oil and Gas Reservoirs

Oil does not occur in underground lakes or rivers, contrary to what some people think. Instead, crude oil and natural gas occur in buried rocks. Not just any rock can hold hydrocarbons, however. To serve as an oil and gas reservoir, rocks have to meet several criteria.

Characteristics of Reservoir Rocks

• Nothing looks more solid than a rock. Yet, choose the right rock, (a piece of sandstone or limestone) and look at it under a microscope. You will see many tiny openings. Geologists call these tiny rock openings pores.

• A rock with pores is referred to as porous. This means it has tiny holes through which oil may flow. Reservoir rocks must be porous, because hydrocarbonscan occur only in pores.

Characteristics of Reservoir Rocks

A reservoir rock is also permeable. That means its pores are connected. If hydrocarbons are in the pores of a rock, they must be able to move out of them. Unless hydrocarbons can move from pore to pore, they remain locked in place, unable to flow into a well. A suitable reservoir rock must therefore be porous, permeable, and contain enough hydrocarbons to make it economically feasible for the operating company to drill for and produce them.

Origin and Accumulation of Oil and “Gas”• Charcoal is a form of the element carbon. Carbon is

in all common fuels. Fuels are materials that give off heat when they are burned. A fuel may be a solid, a liquid, or a gas. Fuels that come from the remains of living things are called fossil fuels. Wood, coal, natural gas and the liquid fuel petroleum are common fossil fuels. We are only going to discuss petroleum here.

• Petroleum is formed within the crust of the earth. It is also called crude oil. Scientists think that the bodies of prehistoric sea animals and plants became trapped in sediments. Over time, heat and pressure changed them into crude oil and natural gas. Crude oil and natural gas are usually found together in the crust of the earth. To get the oil and gas, it is necessary to drill into the earth's crust.

Petroleum Traps

• A hydrocarbon reservoir has a distinctive shape, or configuration, that prevents the escape of hydrocarbons that migrate into it. Geologists classify reservoir shapes, or traps, into two types;

structural traps: a deformation in the rock layer that contains the hydrocarbons. Examples; fault traps and anticlinal traps.

stratigraphic traps:form when other beds seal a reservoir bed or when the permeability changes within the reservoir bed itself.

combination trap:this happens when more than one kind of trap forms.

Examples of Petroleum Traps

Examples of Petroleum Traps

Examples of Petroleum Traps

WHERE TO DRILL?• The company decides where to drill by considering several factors. The

most important is that the company knows or believes that the oil exists in the rocks beneath the site.

• Geologists do experiments and tests on the site that give them information to make a guess about where the oil is on the site. Sometimes it really does come down to an educated guess. The drill site is only as good as the data the geologist has found.

• Legal and economic factors are also important in the selection of a drilling site. For example, the company must obtain the right to drill for and produce oil and gas on a particular piece of land. Also the company must have money to purchase or lease the right to drill and produce. It also must have the money to pay for the drilling.