So what if we're hitting hard rock, we'll It can be a great sign. The hard rock could be the top of a oil reservoir...
Crude oil found in all oil reservoirs formed in the Earth's crust from the remains of once-living things. Crude oil is properly known as petroleum, and is used as fossil fuel. Evidence indicates that millions of years of heat and pressure changed the remains of microscopic plant and animal into oil and natural gas .
Roy Nurmi, an interpretation adviser for Schlumberger , described the process as follows: "Plankton and algae, proteins and the life that's floating in the sea, as it dies, falls to the bottom, and these organisms are going to be the source of our oil and gas. When they're buried with the accumulating sediment and reach an adequate temperature, something above 50 to 70 °C they start to cook. This transformation, changes them into the liquid hydrocarbons that move and migrate, will become our oil and gas reservoir." [ 1 ]
In addition to the aquatic environment, which is usually a sea, but might also be a river , lake , coral reef or algal mat , the formation of an oil or gas reservoir also requires a sedimentary basin that passes through four steps: deep burial under sand and mud, pressure cooking , hydrocarbon migration from the source to the reservoir rock, and trapping by impermeable rock. Timing is also an important consideration; it is suggested that the Ohio River Valley could have had as much oil as the Middle East at one time, but that it escaped due to a lack of traps. [ 2 ] The North Sea , on the other hand, endured millions of years of sea level changes that successfully resulted in the formation of more than 150 oilfields . [ 3 ]
Although the process is generally the same, various environmental factors lead to the creation of a wide variety of reservoirs. Reservoirs exist anywhere from the land surface to 30,000 ft (9,000 m) below the surface and are a variety of shapes, sizes and ages. [ 4 ]
Traps
A trap forms when the buoyancy forces driving the upward migration of hydrocarbons through a permeable rock cannot overcome the capillary forces of a sealing medium. The timing of trap formation relative to that of petroleum generation and migration is crucial to ensuring a reservoir can form. [ 5 ]
Petroleum geologists broadly classify traps into three categories that are based on their geological characteristics: the structural trap , the stratigraphic trap and the far less common hydrodynamic trap . [ 6 ] The trapping mechanisms for many petroleum reservoirs have characteristics from several categories and can be known as a combination trap.
Structural traps
Structural traps are formed as a result of changes in the structure of the subsurface due to processes such as folding and faulting, leading to the formation of domes , anticlines , and folds . [ 7 ] Examples of this kind of trap are an anti-cline trap , [ 8 ] a fault trap and a salt dome trap . (see salt dome )
They are more easily delineated and more prospective than their stratigraphic counterparts, with the majority of the world's petroleum reserves being found in structural traps.
Stratigraphic traps
Stratigraphic traps are formed as a result of lateral and vertical variations in the thickness, texture, porosity or lithology of the reservoir rock. Examples of this type of trap are an unconformity trap , a lens trap and a reef trap . [ 9 ]
Hydrodynamic traps
Hydrodynamic traps are a far less common type of trap. [ 10 ] They are caused by the differences in water pressure, that are associated with water flow, creating a tilt of the hydrocarbon-water contact.
Seals
The seal is a fundamental part of the trap that prevents hydrocarbons from further upward migration.
A capillary seal is formed when the capillary pressure across the pore throats is greater than or equal to the buoyancy pressure of the migrating hydrocarbons. They do not allow fluids to migrate across them until their integrity is disrupted, causing them to leak. There are two types of capillary seal [ 11 ] whose classifications are based on the preferential mechanism of leaking: the hydraulic seal and the membrane seal.
The membrane seal will leak whenever the pressure differential across the seal exceeds the threshold displacement pressure, allowing fluids to migrate through the pore spaces in the seal. It will leak just enough to bring the pressure differential below that of the displacement pressure and will reseal. [ 12 ]
The hydraulic seal occurs in rocks that have a significantly higher displacement pressure such that the pressure required for tension fracturing is actually lower than the pressure required for fluid displacement – for example, in evaporites or very tight shales. The rock will fracture when the pore pressure is greater than both its minimum stress and its tensile strength then reseal when the pressure reduces and the fractures close.
