ORIGINS OF PETROLEUM. During certain geologic ages, when the climate was suitable, petroleum began as organic material derived from plants and animals which grew in abundance. As these organisms went through their cycles of growing and dying, buried organic material slowly decayed and became our present day fossil fuels: oil, gas, coal and bitumen. Oil, gas and bitumen were dispersed in the sediments (usually clay-rich shales). Over millions of years, these organic-laden shales expelled their oil and gas under tremendous pressures from the overburden.
The oil and gas migrated into permeable strata below or above them, then migrated further into traps that we now call reservoirs. It’s interesting to note that the word “petroleum” is derived from the Latin words for “rock” (petra) and “oil” (oleum), indicating that its origins ie within the rocks that make up the earth’s crust. These ancient petroleum hydrocarbons are complex mixtures and exist in a range of physical forms — gas mixtures, oils ranging from thin to viscous, semi-solids and solids. Gases may be found alone or mixed with the oils. Liquids (oils) range in color from clear to black. The semi-solid hydrocarbonsare sticky and black (tars). The solid forms are usually mined as coal, tar sand or natural asphalt such as gilsonite. As the name “hydrocarbon” implies, petroleum is comprised of carbon atoms and hydrogen atoms bonded together; the carbon has four bonds and the hydrogen has one. The simplest hydrocarbon is methane gas (CH4). The more complex hydrocarbons have intricate structures, consisting of multiple carbon-hydrogen rings with carbon-hydrogen side chains. There are often traces of sulfur, nitrogen and other elements in the structure of the heavier hydrocarbons.
THE MIGRATION AND TRAPPING OF PETROLEUM
Oil is seldom found in commercial amounts in the source rock where it was formed. Rather, it will be found nearby, in reservoir rock. These are normally “sedimentary” rocks — layered rock bodies formed in ancient, shallow seas by silt and sand from rivers. Sandstone is the most common of the sedimentary rock types. Between the sand grains that make up a sandstone rock body there is space originally filled with seawater. When pores are interconnected, the rock is permeable and fluids can flow by gravity or pressure through the rock body. The seawater that once filled the pore space is partially displaced by oil and gas that was squeezed from the source rock into the sandstone. Some water remains in the pore space, coating the sand grains. This is called the reservoir’s connate water.
Oil and gas can migrate through the pores as long as enough gravity or pressure forces exist to move it or until the flow path is blocked. A blockage is referred to as a trap. Carbonate rock, limestones (calcium carbonate)
and dolomites (calcium magnesium carbonate) are sedimentary rocks and are some of the most common petroleum reservoirs. Carbonate reservoirs were formed from ancient coral reefs and algae mounds that grew in ancient, shallow seas.
Organic-rich source rocks were also in proximity to supply oil and gas to these reservoir rocks. Most limestone strata do not have a matrix that makes them permeable enough for oil and gas to migrate through them. However, many limestone reservoirs contain fracture systems and/or interconnecting vugs (cavities formed when acidic water dissolved some of the carbonate). These fractures and vugs, created after deposition, provide the porosity and permeability essential for oil to migrate and be trapped. Another carbonate rock, dolomite, exhibits matrix permeability that allows fluid migration and entrapment. Dolomites also can have fracture and vugular porosity making dolomite structures attractive candidates for oil deposits.
A significant portion of oil and gas production is associated with salt domes which are predominately classified as piercement-type salt intrusions and often mushroom shaped. Piercement-type domes were formed by the plastic movement of salt rising upward through more dens sediments by buoyant forces resulting from the difference in density.
Major oil and gas reservoirs have been found in recent years beneath horizontal salt beds. Until recently, it was a mystery what was b
eneath these extruded salt layers called salt sills, salt sheets and salt lenses. They could not be explored economically by drilling, and seismic interpretation through plastic salt was unreliable. Now, “sub-salt” formations can be evaluated through modern three-dimensional seismic analysis to identify potential reservoirs. Once likely formations are located, wells are drilled through the salt layer to determine if oil and gas deposits exist.
Structural traps result from a local deformation such as folding and/or faulting of the rock layers. Examples of structural barriers are anticline traps, fault traps and traps associated with salt domes (see Figures 1a, 1b and 2c). Stratigraphic traps are formed by geological processes other than structural deformation and relate to variations in rock properties (lithology).