The Generation of Oil and Gas ProspectsRead Sections 1.1 and 1.2
What is an Oil and Gas Prospect•An idea.•An idea about where on and how deep in the planet a new deposit of oil and gas might be found.•There is no way to know for certain if the idea is correct without drilling a well.Drawing from State of LouisianaMap from NOAA
Oil and Gas Source Rocks•The hydrocarbons must first be created.•The source is thought to be organic material that has collected in mud at the bottom of ancient seas.•Over long periods of time the mud beds are slowly buried and become deeper in the earth.•With burial, the pressure and temperature of the bed increases.
Oil and Gas Source Rocks•The increase in pressure and temperature slowly cooks the organic material (now called Kerogen) into hydrocarbons.•If heating never exceeds 140 deg F, the kerogen forms an oil shale.•Oil likely cooks out between 140° and 320° F.•Natural gas cooks out between 320°and 395° F.Higher temperature favors the break downof the kerogen into smaller compounds likenatural gas.Image from State of Colorado
Oil and Gas Migration•Once formed, the oil and gas are slowly squeezed out of the kerogen layer and enter more permeable rock.•Now it is free to migrate to shallower depths because it is less dense than the ancient sea water that fills the pores of the overlaying rock.•Thus, it rises by gravity.•Now that the hydrocarbons are out and moving what stops them?
Oil and Gas SeepsOil Oasis in Kurdistan (Walter Kessinger)Oil seep Ventura, Ca.(State of California)Pitch lake, Trinidad, West IndiesHydrocarbons may reach the surface and form seeps.
Anticlines as Hydrocarbon Traps•Traps are places in the earth that stop or greatly slow down the migration of hydrocarbons.•An anticline is a classic trap where an underground hill is over lane by impervious shale.•This type of trap can “fill up” allowing excess oil and gas to continue migrating elsewhere.
Faults as Hydrocarbon Traps•Faults cause earth layers to move and may juxtapose a permeable layer against an impermeable one.Figure from W Virginia geology dept.
Stratigraphic Hydrocarbon Traps•Stratigraphic traps are changes in the layer opposed to the surrounding layers. A buried sand bar is a good example, or a reduction in rock quality.Figure from bcexploration
Stratigraphic Hydrocarbon Traps•Salt domes are special types of structural traps.Figure from Univ of Wyoming
Geologic Investigations•Geologists use a variety of tools to try and detect traps in the folds of the earth.•These could be a detailed investigation of the surface for signs of what might be below ground.•Many of the original oil discoveries world wide were drilled near seeps.