Techniques
Deciding where to drill is the most in-depth part of exploring for oil and gas. The modern exploration geologist (a person who explores for petroleum) must rely on many techniques to find profitable oil and gas reserves. There are three primary methods used to find hydrocarbons in the subsurface:
- Sub-surface mapping
- Geophysical surveys
- Wildcatting
Sub-Surface Mapping
The search for hydrocarbons frequently begins with analysis of sub-surface terrain, well production, and previous well test data. These types of programs normally focus on finding undeveloped reserves in older fields. These reserves may stem from overlooked producing intervals or residual oil and gas left in proven formations. Many oil and gas fields were abandoned in times where oil or gas prices were substantially lower than today. When wells cost more to operate than they generate in sales they are plugged and abandoned. The recent demand increase for natural gas and oil has caused market prices to rise to an all time high.
This market condition has brought about tremendous potential for developmental oil and gas programs. Some older fields that were abandoned at low oil and gas prices can now be reactivated and profitable. The data from the previous wells can be used to help pinpoint the optimum location for new wells to extract the remaining reserves. These programs generally offer a lesser degree of risk, but also suffer smaller production numbers due to depletion within the reservior. Although field production may be greatly diminished, the dramatic increase in the price of the commodity can make these types of programs extremely viable.
Geophysical Surveys
Geophysical techniques used for exploration utilize equipment to measure the following: electrical currents, gravitational and magnetic anomalies, heat flow, geochemical relationships, and density variations from deep within the earth. Each technique records a different set of characteristics, which can be used to locate hydrocarbons beneath the surface of the earth.
Seismic surveys use vibration (induced by an explosive charge or sound generating equipment) to provide a picture of subterranean rock formations at depth, often as deep as 30,000 feet below ground level (BGL). This is accomplished by generating sound waves downward, which reflect off various boundaries between different rock strata. The sound waves are generated by small explosive charges embedded in the ground or by vibrator trucks, sometimes referred to as thumpers, which shake the ground with hydraulically driven metal pads.
The human ear can barely hear the thump, but the frequency generated penetrates the earth's crust. The echoes are detected by electronic devices called geophones which receive the reflected sound waves. The data is recorded on magnetic tape which is printed to produce a two-dimensional graphic illustrating the subsurface geology.
In this type of survey, sound waves are sent into the earth where they are reflected by the different layers of rock. The time taken for them to return to the surface is measured as a function of time. This measurement reveals how deep the reflecting layers are; the greater the time interval, the deeper the rock layer. Moreover, this technique also can determine what type of rock is present because different rocks transmit different sound waves.
Wildcat
A true wildcat well is one that is drilled in a new area where no other wells exist and generally with little information. It is drilled in an effort to locate undiscovered hydrocarbons. About 1 in 10 wildcat wells strike oil or gas, but reserves can be extremely profitable when these programs are successful. Many wildcat wells are drilled on a hunch, intuition, or a small amount of speculative geology. Many times they are based on surface trends, photography, and experience in a particular area.
