Heavy Oil Science Centre - Overburden

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Geological Terminology

Overburden

Oil industry term for all the layers between the surface and the "pay zones". Top soil, clay, rare earths, gravel, glacially deposited boulders, shale, lakes and rivers of fresh water, pools of salt water; dating from 65 million years ago to the present, they are all obstacles that have to be drilled through to reach the deposits of heavy oil which are produced to meet the needs of our modern society.

Pay Zones

Also called "reservoir rock", these are the sedimentary layers where oil is found in sufficient quantities to produce. These layers date from 144 million to 66 million years ago, a period known as the Lower Cretaceous.  Extending roughly from 500 meters to 700 meters below the surface (in the Lloydminster area) these layers contain sand, shale, sand, salt water, sand, heavy oil, and ... did we mention sand? These shallow depths mean that the oil is not at sufficient temperature to continue its chemical transition from a solid to liquid petroleum.  This is one reason the oil in this region is "heavy oil".

Source Rock

Oil industry term for the sedimentary rock layers where petroleum formed. Oil may or may not still be there as it tends to "migrate" upward. In the Lloydminster region, these layers date from the Devonian Period, some 410 to 360 million years ago.  In that time, warm tropical seas covered Alberta and teemed with life.  The sediments laid down, once they reached a sufficient depth to become heated, began the chemical processes by which petroleum is formed.

The Basement

Oil industry expression for the rock (mostly igneous) which pre-dates the proliferation of life on Earth. From the time the Earth formed (thought to be over 4 billion years ago), to the first beginnings of life as we know it (some 570 million years ago). Since petroleum is assumed by most scientists to be organic in origin, there is no point drilling into "the basement" to find oil.

 

See our Geological Ages section for more terms.