The exploration processes for oil and gas are the same. Both oil and gas reservoirs are buried deep underground, at depths of a few hundred meters to many thousands of meters. In addition, these reservoirs are often found under the sea. Interestingly, other than the obvious usage of boats versus land surface vehicles and the presence of surface features, there is relatively little difference between exploration on land or water. Recent advances in exploration and production methods have allowed production of reservoirs located thousands of meters below the seabed, which itself could be thousands of meters below the surface of the sea.
The study of geophysics uses physical properties measured either on the surface or inside wells to determine the property and structure of rocks below the surface. Geophysics has dramatically changed the way reservoirs are discovered. The first geophysical methods were simple gravity and magnetic surveys on the surface, progressing to subsurface measurements of seismic energy waves, radioactivity, and sonic properties.
Gravity surveys measure the slight variations in gravity readings to locate subsurface rocks of different densities. Magnetic surveys measure the changes in the magnetic field over an area to locate sedimentary rocks, which have a lower magnetic field than igneous and metamorphic rocks. Mapping the variations in gravity and magnetic readings over a large area produces subsurface maps showing the lateral extent of potential reservoir rocks. By drilling at the high point of the sedimentary rock formations, exploration professionals (“explorationists”) hoped to locate the peak of the anticlinal trap and find a hydrocarbon reservoir.
The development of seismic technology may be the most profound development in the hydrocarbon industry since the discovery of the first oil wells. Explosives, air guns, or vibrating pads directly on the surface generate low-frequency energy waves, which reflect and refract as they pass through the different rock layers. The figure below shows a simplified schematic of this process. The speed at which the waves travel is related to the density of the individual rock layers. Each rock layer has its own density, which determines the time it takes for the waves to pass through the layer (refracted into the next deeper layer) or to be reflected back to the surface. Sensitive microphones (known as geophones on land and hydrophones on the surface of the sea) record the time taken for the waves to return to the surface after they have been refracted and reflected in the earth. These sensors are similar to seismographs used to measure naturally occurring earthquakes.
For a single seismic source, hundreds of microphones are placed at precise locations on the surface of the earth or floating on the surface of the water. Combining the hundreds of measurements for each source location and repeating the measurements after moving the source hundreds of times can produce a fairly accurate representation of the subsurface geometry. A two-dimensional seismic survey, involving a simple array of surface geophones or hydrophones, will show large subsurface features. A more expensive three-dimensional survey, with multiple lines of geophones or hydrophones, as shown in below, can show subtler reservoir characteristics and smaller structural features. This data can be further processed to allow visualization of subsurface geology, hydrocarbons, and even potential well locations.
Once a promising feature has been identified, either by surface observation or by gravity, magnetic, or seismic interpretation, an exploration well must be drilled to confirm the discovery—or more likely, a duster, or nondiscovery. Better technology and wider data coverage have increased chances of discovery from 10% a few decades ago to 30% or more today.
The process of drilling an exploration well is deceptively simple. Drilling a producing well (or developmental well) is similar to drilling an exploration well. The section on Gas Production describes this process in detail.
Wireline logging surveys follow the drilling of exploration wells. A spool and data-transmission cable lower sophisticated sensors into the well bore (see below). The sensors measure various physical and chemical properties of the rock layers and the fluids present in the pores of the rock. Common measurements include resistivity, sonic porosity, and nuclear radiation and density. Oil and gas are more resistant to electricity conduction than water, so measuring electrical resistance of rock over constant intervals can identify the fluids in the rock. Sound waves travel faster in dense materials. Thus, rocks with large amounts of pore space will have a slower acoustic speed. The sensor measures this relationship, and sophisticated computation can estimate the porosity of various rock layers. Because shales contain a higher concentration of radioactive elements, measuring natural radiation of rocks can identify shales versus other sedimentary zones. This differentiates potential source and trap rocks from reservoir rocks. Nuclear logging surveys can determine rock density, which can help identify the rock type.