| Themes > Science > Earth Sciences > Geology > Oil and Gas > Hydrocarbon Exploration > Geologic Maps |
Geologists draw dozens of types of geologic maps. They want to show the earth as it is deep underground. Geologists spend most of their time working with the maps they make. The map at the right is called a "structure map". It is drawn on the top of an oil zone that is approximately 8000 feet deep. The map is about two miles across. This is an old map from the 1950's that was drawn with pen and ink. The PG picks the top of the oil zone in every well that is drilled. She knows the elevation of the ground at the drilling site. For example, if the elevation of the ground is 1000 feet above sea level, and the top of the oil zone is found at 7700 feet, she subtracts 7700 feet from 1000 feet to get a "subsea" elevation of -6700 feet. These subsea elevations are posted on the structure map. Then "contour lines" are drawn on the map to create her picture of the underground structure. In this case, the structure is shaped like a broad dome...or hill...with the top of the hill at -6550 feet and the base of the hill at -6800 feet. So, the top of the hill is about 250 feet higher than the base! But remember, this "hill" is located more than 1 1/2 miles below the surface of the earth. |
| Most maps are
now created by the geologist with the aid of computers. Here is a
different type of map called an "isopach (eye-so-pak) map",
constructed over a small gas field.
The squares (or "sections") are one mile on each side. On this map, the petroleum geologist has contoured the thickness of an individual sandstone. This sandstone is about 45 feet thick in the middle (the hottest pink area), and thins to 20 feet or less around the edges of the gas field. |
| Below is a colorful map (an isopach, like above) contoured on one of the Springer sands in Oklahoma. This particular sandstone was deposited in the ocean, as a sand bar. Land was toward the northeast, and the deep ocean was toward the southwest. Now this sand bar is 14,000 feet deep (nearly three miles), and has been transformed into a really nice gas reservoir in Caddo County, Oklahoma. |
| One more map. This one shows an ancient stream or river channel. The sandstone is about 30 feet thick in the middle of the channel. Production charts have been placed on the map. These show clearly and graphically how much oil and gas was produced from each well over the years. The large red numbers indicate the amount of produced gas. For example, the Soar 1-18 (top middle) has produced 3,267,524 thousand cubic feet of gas (abbreviated 3,267,524 MCF). An easier way to express this huge number is 3.2 billion cubic feet, or 3.2 BCF. |
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