What's Oil Doing in Signal Hill, Long Beach, California?
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to page one of Signal Hill
But we need more than just sandstones and shales to give us an accumulation
of oil, although we're off to a good start. We need a trap. For that,
we go back to talking about crustal plates in motion. As the Pacific and
North American Plates collided with one another in the distant past, sea
floor sediments became folded and crumpled. As a result, the Los Angeles
Basin, once at the bottom of the Pacific Ocean, rose from the water and
became dry land. At the same time, folds and faults developed in these
rocks.
Below, we see the structure of the rocks beneath the surface
at Signal Hill, Long Beach, California.
This is a petroleum geologist's dream. This is a map of an important rock layer at Signal Hill. The difference between this map and a topographic map (which shows you were hills and valleys are located) is that this map represents the elevations below ground. (Need help reading this map?)
The bull's-eye in the center is a high spot, and the structure slopes downward in every direction. The red lines indicate where there are faults that cut through the rock - places in the past where these layers of rock have broken and slipped past one another, in some places hundreds of meters. Technically, what this shows us is a combination trap, because it is a combination between the structure at Signal Hill (the anticline) and the faults which runs along it that forms the trap.
| See the line up above that is labeled "A" on one side and "B" on another? If we look at this structure in cross-section (from the side), we can see that from point A to point B, the package of rocks goes from low to high, is cut by a fault (a reverse/thrust fault) near its highest point, then falls away. After the oil matured in the deep source rock below, it moved towards the surface until it reached this structure and became trapped. |
Cross-sectional diagrams are very important in oil exploration. For us, they provide a very simple way of summarizing some very important features about not only the structure, but stratigraphy of key zones. [What do you think cross-section C-D looks like? Give it a try using a pencil and paper (you don't have to use the computer for EVERYTHING, you know!) and click here to see how close you got.]
...continue learning about the geology of Signal Hill
click here to learn about the history of oil at Signal Hill
