Petroleum Geology
Several geologic elements are necessary for oil and gas to accumulate in sufficient quantities to create a pool large enough to be worth producing. These elements include an organic-rich source rock to generate the oil or gas, a porous reservoir rock to store the petroleum in, and some sort of trap to prevent the oil and gas from leaking away. Traps generally exist in predictable places - such as at the tops of anticlines, next to faults, in the updip pinchouts of sandstone beds, or beneath unconformities.
To find a convergence in the subsurface of the geologic elements necessary to form an oil or gas pool requires a careful blend of science and art. To discover what geometries and compositions the rocks might possess deep underground, geologists examine the rocks where they are exposed in surface outcrops, or they examine aerial photographs and satellite images when surface access is limited. Geologists also work closely with geophysicists to integrate seismic lines and other types of geophysical data into their interpretations.
| The collection of seismic data involves sending shock waves into the ground and measuring how long it takes for the subsurface rocks to reflect these waves back to the surface. The shock waves that are used today are usually generated by pounding the earth with giant vibrator trucks, but in the past gephysicists preferred to explode small dynamite charges in shallow holes. However, environmental restrictions in most places today prevent using explosives to collect seismic data. |
When shock waves created by the vibrator trucks travel into the earth, boundaries between the rocks reflect the waves back, and the arrival times of the waves back at the surface are detected by listening devices called geophones. Computers then process the geophone data and convert it into seismic lines, which are nothing more than two-dimensional displays that resemble cross-sections.
Seismic lines in the old days were just that . . . two-dimensional lines created by laying the geophones out in single line. But today, the data is commonly collected as an intersecting grid of seismic lines referred to as 3-D seismic volume. Data collected in this fashion may even be used to help create 3-D computer models of the underground geometries of the rocks.
Geologic and geophysical clues are enticing, but drilling is the only way to learn if an oil or gas field really exists. Once a well is drilled, well logs yield data on the types of rock present and, most important, what fluids these rocks contain. The information interpreted from the logs is used to decision whether a well should be completed and used to produce oil and gas, or filled with cement and abandoned. The logs are also used to update the geologic models originally used to locate the well.
Today, the average wildcat well has only one chance in ten of finding an economic accumulation of hydrocarbons. A rank wildcat, if drilled in a frontier area, stands only one chance in forty of success. The odds are much better for a development or extension well, but nothing is a sure bet in the oil business. Thus, even though explorationists (oil and gas prospectors) of today have better tools than their ancient predecessors, luck remains a significant factor in the search for oil and gas. The reality is that most wildcats turn out to be dry holes and not every development well becomes a producer.
History of the Oil Industry
How to find Oil and Gas
How to make Gasoline
McKittrick Tar Pits
What is Oil?
Oil Facts