Oil and Gas Handbook

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Geology

 

Chapter 2

Oil and natural gas are products of plant and animal remains or organic material. For the most part, these plants, corals, and animals lived in seas. Over millions of years, the remains settled on the ocean floor together with sediment which washed down from exposed earth and rock. In later periods, these layers of organic material and sediment were covered by more sediment which, as a result of time and pressure, converted these layers to sedimentary rock filled with hydrocarbons. To get a clearer picture of how this process occurred we need to understand some geological history.

The earth is believed to have formed some 4.6 billion years ago from a cloud of cosmic dust and ice. As the planet pulled itself together by gravity, compressing matter ever more densely, it became molten. The heaviest components, such as nickel and iron, sank to the center to form the planet's core while the lighter materials, including silicon, aluminum, magnesium and other light elements, solidified into a thin rocky crust. Water vapor moved to the surface to form oceans.

Over time the earth's crust has become thicker and more stable. Today, the planet can be viewed as a system of plates that fit together like a puzzle which slowly move and change shape.

Pangaea

As recently as the Triassic Period 225 million years ago, the Earth's continents were still connected together in a single super continent called Pangaea. Pangaea fragmented into several pieces, each piece floating on a molten layer. These mobile plates of the Earth's outer crust, lithosphere, have positioned themselves to form the Earth as we know it today.

During this 225 million year period, the continents and oceans experienced additional changes. As the earth's plates moved against one another, the forward or leading edges either lifted up on top of the next plate or moved below it causing edges to crumple and increase in height thus producing the mountains we see today. As plates separated, magma rose from the mantle and solidified in the rift forming mid-ocean ridges. The new crust, thinner than the continents, spread between the plates. While the rate of plate movement is very slow (averaging perhaps the speed of a growing fingernail), over the time spans we are considering, the results are dramatic. The theory that explains these plate movements is called plate tectonics.

The two basic types of crust are oceanic and continental. While the oceanic crust is thin, about 5 to 7 miles thick and composed of heavy igneous rock that formed from magma flows, the continental crust is 10 to 30 miles thick. Because of these differences, the continents tend to float, rising high above sea level as in the mountainous regions. These continental mountains were gradually worn down by rain and the action of ice, and the freed particles of rock were carried to the sea where they were deposited in thick sedimentary beds, cemented together by minerals and the pressure of more sediment deposited above.

At various times (some spanning tens of millions of years) much of North America was covered by seas. This occurred in warmer periods when the polar ice caps were much smaller and consequently held less water. This phenomenon of changing sea levels helps explain why oil and gas deposits are present far inland from any existing ocean.

The Right Conditions for Oil & Gas

Since sediment and deceased sea organisms are heavier than water, they naturally migrate toward lower areas or basins in the sea. These lower areas were caused by tectonic action between the plates and eroded valleys that were created in colder periods before the rise in ocean levels submerged them.

As these ocean basins gradually filled with layers of sediment, the weight of the newer layers increased on the layers below. This weight or pressure created friction and heat and began the process of converting the organic material to oil and gas. The story becomes more complicated because, along with organic material, salt water was invariably captured in the source rock. Under the weight and pressure of subsequent sediment layers, all three substances attempt to migrate along a path. Since oil is lighter than water and gas is lighter than both, when a reservoir rock formation is found, it is stratified with gas on top, oil in between, and water on the bottom.

In certain places, tectonic plate movement has caused the earth's crust to bunch up, creating folds or uplifts in rock strata. This movement also resulted in earthquakes that caused faults or fractures in the strata. These fractures and folds create the opportunity for oil and gas to move out of their source rock toward the surface. If the oil and gas make it to the surface, the gas is lost in the atmosphere while the oil ultimately evaporates. However, if the conditions are right, the hydrocarbons remain trapped under a layer of impermeable rock in another sedimentary rock called a reservoir.

Generally, oil and gas are found in a geologic structure called a "trap" that prevents the oil and gas from escaping. There are two general types of reservoir traps: a) structural and b) stratigraphic. Structural traps are formed by the deformation of the reservoir formation while stratigraphic traps are the result of an updip seal of porosity and permeability.

The anticline trap is formed by the folding of rocks into a dome. These anticlinal traps contain petroleum that has migrated from a source below. Further upward, migration of hydrocarbons was prevented by an impenetrable layer of rock above the reservoir. Fault traps are formed by the shearing and offsetting of rock strata. The escape of petroleum from a fault trap is prevented by non-porous rocks that have moved into position opposite the porous petroleum bearing rock formation. Dome and plug traps are porous formations on or around great plugs of salt or serpentine rock that has pierced or lifted the overlying rock layers. Stratigraphic traps are caused either by a nonporous formation sealing off the top edge of a reservoir or by a change in the porosity and permeability of the bed itself.


Elk Basin is a breached anticline

Permeability and Porosity

Petroleum deposits – reservoirs – are trapped in layers of sandstone, limestone or dolomite. Exploration and production companies are most interested in reservoirs that have good permeability and porosity. Porosity is a measure of the spaces within the rock layer compared to the total volume of rock. Though both are porous, a sponge is much more porous than a brick. And though both can hold water in their pores, the sponge has a much higher capacity for holding liquids. Permeability is a measure of how well liquids and gases can move through the rock and, thus, is a function of how well the pores within the rock are connected to each other.

Petroleum porosities are measured in percentages with the average reservoir ranging from seven to 40 percent. Permeability is measured in units named Darcies and the number of Darcies range variously throughout each reservoir from millidarcies to over 40 Darcies.

Knowledge of the trap and the reservoir rock's porosity and permeability are essential in determining where to drill and, if found, how much oil and gas can be recovered.

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