Chapter 2:
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 Chart courtesy of US Geologic Society During the Permian Period 225 million years ago, the world's continents were still connected together in a single super continent called Pangaea. As the sequence of maps on the following page illustrates, Pangaea fragmented into several pieces, each piece being part of a mobile plate of the earth's outer crust called the lithosphere. These pieces later became earth's current continents. The time sequence shown through the maps reveals how the continents arrived at their current positions. Throughout 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 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, which was thinner than the continents, spread out between the plates. While the rate of movement is very slow over the time span we are considering, the results are dramatic. The theory that explains these plate movements is called plate tectonics.
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The two basic types of crust are oceanic and continental. While the oceanic crust is thin - about five to seven miles thick - and composed of heavy igneous rock that formed from magma flows, the continental crust is 10 to 30 miles thick. As a result, 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 water. 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 and gasSince 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 between, and water on the bottom. page 6 |
Chart courtesy of Earth Science World and Exxon-Mobil 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. In some instances, oil and gas may be trapped under a layer of sediment that deposited down into a basin, later migrating up from the source rock to reservoir rock.
Chart courtesy of Earth Science World and Exxon-Mobil page 7 |
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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 (see pinchout below). Permeability and Porosity of Rock StructuresPetroleum deposits are called reservoirs. These are trapped layers of sandstone or limestone, or dolomite. Exploration and production companies are most interested in reservoirs that have good permeability and porosity. As we have seen, both oil and gas co-exist in their natural state with grains of sand, pebbles, rocks and boulders in a rock layer. 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 pours within the rock are connected to each other. In the formation below, porous areas are in blue. page 8 |
Porous areas are in blue Chart courtesy of Earth Science World and Exxon-Mobil Petroleum porosities are measured in percent with the average reservoir ranging from seven to forty percent. Permeability is measured in units named darcies and the number of darcies range variously throughout each reservoir from millidarcies to over forty darcies. Our accumulated knowledge about rock structure and formation processes is helpful if not essential in determining where to drill for oil.
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