Oil and Gas Traps Most oil and gas deposits are found in sandstones and coarse-grained limestones. A piece of sandstone or limestone is very much like a hard sponge, full of holes, but not compressible. These holes, or pores, can contain water or oil or gas, and the rock will be saturated with one of the three. The holes are much tinier than sponge holes, but they are still holes, and they are called porosity. The oil and gas become trapped in these holes, stays there, for millions of years, until petroleum geologists come to find it and extract it. The oil is trapped inside the rock’s porosity. Oil Formation and Oil Movement The very fine-grained shale we talked about previously is one of the most common sedimentary rocks on earth. In many places, thousands upon thousands of feet of shale are stacked up like the pages in a book, deep underground. It These shales were deposited in quiet ocean waters over millions of years time. During much of the earth’s history, the land areas we now know as continents were covered with water. This situation allowed tremendous piles of sediment to cover huge areas. The oceans may have left the land we now live on, but the great deposits of shale and sandstone remain deep underground….right under our feet! The Tiny Gigantic Kingdom In the deep ocean, far from land, about the only sediment deposited is the fine-grained clastic rock known as shale. A lot of other material is deposited along with the clay or mud-sized sediments. We often think of sharks and whales as being the kings of the deep oceans. Actually, there are other animals that have established giant kingdoms in the sea…the largest and most impressive kingdoms of all! These animals are the various kinds of microscopic creatures….both plant and animal. Most of them would fit on the head of a pin. They are tiny, but there are uncountable trillions of them. When these creatures die, they sink to the bottom and become part of the sediments there that will eventually turn into shale. The animals die by the trillions and rain down on the ocean floor all the time. And since the beginning of life on earth, they have been living their exciting lives in the ocean, dying, sinking to the bottom, and becoming part of the once-living matter that is part of most shale rocks. It is the trillions of tiny animals that make up most of the gunk (the scientific name for this gunk is “ooze”) deposited on the ocean floor. It’s a very fine-grained goop containing a lot of organic material mixed with the clay-sized particles that form shale. It is called organic-rich shale. Later, when thousands of feet of organic-rich shales have piled up over millions of years, and the dead animal bodies are buried very deep (more than two miles down), an amazing thing happens. The heat from deep inside the earth “cooks” the animals, turning their bodies into what we call hydrocarbons……oil and natural gas. At first, the oil and gas only exist between the shale particles as extremely tiny blobs, left over from the decay of the tiny animals. Then, the intense pressure of the earth squeezes the oil and gas out of the shale, and the oil and gas fluids gather together in a porous layer and move sideways many miles. On their way, they may meet up with other traveling oil or gas fluids. Finally, the oil and gas may become “trapped” in a rock formation like sandstone or limestone….a hydrocarbon trap. The oil and gas stay there, under tremendous pressure, until the petroleum geologist comes looking for it. Without a trap, there is no place to drill. All oil and gas deposits are held in some sort of trap. The Two Types of Traps Structural Traps These traps hold oil and gas because the earth has been bent and deformed in some way. The trap may be a simple dome (or big bump), just a “crease” in the rocks, or it may be a more complex fault trap like the one shown at the right. All pore spaces in the rocks are filled with fluid, either water, gas, or oil. Gas, being the lightest, moves to the top. Oil locates right beneath the gas, and water stays lower. Once the oil and gas reach an impenetrable layer, a layer that is very dense or non-permeable, the movement stops. The impenetrable layer is called a “cap rock.” Stratigraphic Traps Stratigraphic traps are depositional in nature. This means they are formed in place, often by a body of porous sandstone or limestone becoming enclosed in shale. The shale keeps the oil and gas from escaping the trap, as it is generally very difficult for fluids (either oil or gas) to migrate through shales. In essence, this kind of stratigraphic trap is surrounded by “cap rock.” Here are four traps. The anticline is a structural type of trap, as is the fault trap and the salt dome trap. Image: 1. Oil Moving Through Pore Space In Sandstone 2. Tiny Microfossils Make Up the Sea-Floor Ooze 3. Structural Trap 4. Stratigraphic trap 5. Stratigraphic trap 6. Stratigraphic trap 7. Stratigraphic Problems When Drilling 8. Structural Problems When Drilling 9. Jaisalmer Basin Chauhan Ravindras photo. Chauhan Ravindras photo. Chauhan Ravindras photo. Chauhan Ravindras photo. Chauhan Ravindras photo. Chauhan Ravindras photo. Chauhan Ravindras photo. Chauhan Ravindras photo. Chauhan Ravindras photo.
Posted on: Mon, 10 Mar 2014 12:30:14 +0000
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