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:
- Geophysical surveys
- Sub surface mapping
Sub Surface Mapping
The search for hydrocarbons frequently begins with the 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 production intervals or residual oil and gas left improve formations. Many oil and gas fields were abandoned at times where oil and gas prices were substantially lower than today. When wells cost more to operate than they generate they are plugged and abandoned. The volatility of oil and gas demand and prices may cause a field that was once non-profitable, to be a viable option at a later date. The most recent market condition has been by way of technological advances in drilling, completion and stimulation techniques. Some very well mapped areas, such as the Eagle Ford Shale have never been economical viable, but are now among the most profitable formations in the United States. This market condition has brought about tremendous potential for developmental oil and gas programs. The data from previous wells can be used to help pinpoint the optimum location for new wells to extract the reserves. These programs generally offer a lessor degree of risk.
Geophysical techniques used for exploration utilize equipment to measure the following: electrical currents, gravitational and magnetic anomalies, heat flow, geo-chemical relationships, and density variations from deep within the earth. Each technique records a different set of characteristics, which can be used to try and 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 of 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 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 can also determine what type of rock is present because different rocks transmit different sound waves.
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.