Oil formation origin

There are four main components required for an existence of hydrocarbon reservoir:

Source Rock - rocks from which hydrocarbons have been generated or are capable of being generated. Source rocks are classified from the types of kerogen that they contain, which in turn governs the type of hydrocarbons that will be generated.

Type 1 source rocks are formed from algal remains deposited under anoxic conditions in deep lakes: they tend to generate waxy crude oils when submitted to thermal stress during deep burial.

Type 2 source rocks are formed from marine planktonic and bacterial remains preserved under anoxic conditions in marine environments: they produce both oil and gas when thermally cracked during deep burial.

Type 3 source rocks are formed from terrestrial plant material that has been decomposed by bacteria and fungi under oxic or sub-oxic conditions: they tend to generate mostly gas with associated light oils when thermally cracked during deep burial. Most coals and coaly shales are generally Type 3 source rocks.

Reservoir Rock - a place that oil migrates to and is held underground. A sandstone normally has high porosity in which oil and gas can be trapped. That is why sandstone is one of the common reservoir rocks. However, more than 5% of the world’s major hydrocarbon reserves are found within carbonates, limestone and dolostones.

Sealing Rock (trap rock): unlike a reservoir rock, which acts like a sponge, trap rocks act like walls and ceilings, and will not allows fluids to move through. The most common trap rock is shale.

Trapping Mechanism: is a set of conditions to hold the petroleum in a reservoir and prevent its escape by migration. Beneath the earth's surface, oil will ooze through rocks if there is enough space between them, but this oil will not accumulate into large quantities unless something traps it in a particular place. There are a variety of geologic traps, which themselves can be broken into categories:

1. Structural trap types: anticline, fault, salt dome trap –

a structural trap is a type of geological trap that forms as a result of changes in the structure of the subsurface

2. Stratigraphic trap types: unconformity, lens, pinch-out

Stratigraphic traps are formed as a result of lateral and vertical variations in the thickness, texture, porosity or lithology of the reservoir rock. A stratigraphic trap accumulates oil due to changes of rock character rather than faulting or folding of the rock. As an example, it is possible that a layer of rock which is sandstone at one location is a siltstone or shale at another location. In between, the rock grades between the two rock types. From the section on reservoir rocks, we learned that sandstones make a good reservoir because of the many pore spaces contained within. On the other hand, shale, made up of clay particles, does NOT make a good reservoir, because it does not contain large pore spaces. Therefore, if oil migrates into the sandstone, it will flow along this rock layer until it hits the low-porosity shale. Voilà, a stratigraphic trap is born!

3. Hydrodynamic traps

Hydrodynamic traps are a far less common type of trap. They are caused by the differences in water pressure that are associated with water flow, creating a tilt of the hydrocarbon-water contact.

Structural trap types

An anticline is an example of rocks which were previously flat, but have been bent into an arch. Oil that finds its way into a reservoir rock that has been bent into an arch will flow to the crest of the arch, and get stuck	Anticline
Fault traps are formed by movement of rock along a fault line. In some cases, the reservoir rock has moved opposite a layer of impermeable rock. The impermeable rock thus prevents the oil from escaping. In other cases, the fault itself can be a very effective trap. Clays within the fault zone are smeared as the layers of rock slip past one another. This is known as fault gouge.	Fault trap
Salt is a peculiar substance. If you put enough heat and pressure on it, the salt will slowly flow, much like a glacier that slowly but continually moves downhill. Unike glaciers, salt which is buried kilometers below the surface of the Earth can move upward until it breaks through to the Earth's surface, where it is then dissolved by ground- and rain-water. To get all the way to the Earth's surface, salt has to push aside and break through many layers of rock in its path. This is what ultimately will create the oil trap.	Salt dome trap

Additional materials to go through:

· General trivial: latest news in Kazakhstan & abroad, major events…

Дата добавления: 2015-11-14