RESERVOIR ROCK
Geologists know that certain kinds of underground rock formations are formed when pressure builds up along
fault lines or along the edges of continental plates. Gas is more likely to be found when these reservoir rock
formations are present because they form traps for the gas. Many of these formations are visible on the surface.
Four common underground formations which help trap natural gas are:
- FOLD TRAP: This kind of formation is caused when pressure and heat cause a section of rock to fold upward
in a dome shape.
- SALT DOME: These occur in areas where deep waters of land locked seas became over saturated with salts and
other chemicals.
- FAULT TRAP: This is formed when rock layers broke apart along a fault line and moved up or down along the
edge of the break.
- UNCONFORMITY TRAP: This is formed when earth movements cause sediments to erode or not form where scientists
would expect to find them.
Often geologists use aerial photography and satellite imagery to give them the big picture of what is below
the surface.
The main component of natural gas is probably formed when organisms decay on the muddy floor of a sea or lake.
When the mud hardens, the organisms are encased. The rock that results is called SOURCE ROCK.
Natural gas is held in and above reservoir rocks. RESERVOIR ROCKS are porous and permeable, allowing for absorption
of liquids and passage of natural gas. In the western United States, DAKOTA SANDSTONE is the reservoir rock formed
in the Cretaceous Period.
Natural gas and oil stop their movement toward the top of the earth's surface when they meet an impermeable
layer of rock. This impermeable CAP ROCK forms a barrier, causing gas and oil to pool beneath it.
 TALL
TALES
Tall Tales often are stories based on fact, but are full of imaginative exaggeration. The purpose
of a Tall Tale is to entertain as well as to educate.
Think about a little sea horse millions and millions of years ago. What would happen when he
sank to the bottom of the sea? What if he slowly turned into natural gas?
Use what you have learned about Reservoir Rocks to weave a Tall Tale of the sea horse's adventures
in the rocks.
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