Petroleum Detection

Four main issues control the occurrence and distribution of oil and gas: source, reservoir, seal, and trap.

A source is a fine-grained rock unit containing sufficient organic matter so that when it is heated and/or placed under pressure (maturation), hydrocarbons are generated. If the organic matter is of marine algal origin, then the source rock is most likely to generate oil under optimum maturation conditions, whereas rocks dominated by land plant matter will tend to create gaseous hydrocarbons. The hydrocarbons are of lower gravity than the surrounding groundwater and, therefore, move away from and generally upwards (migration) from the source rock until they are trapped in a reservoir.

A reservoir is a rock unit that acts as a storage device for the hydrocarbons that migrate from the source rock. Hydrocarbons are retained within the reservoir because these rocks contain numerous pores (essentially microscopic-sized holes) between the mineral grains making up the fabric of the reservoir. In good quality reservoirs the porosity is frequently over 20% of the rock volume. However, the pores need to be interlinked in such a manner that the fluids can move into (and out of, if we are to exploit the oil and gas) the reservoirs over geological time. This is known as permeability . There are two main rock types that make up the giant reservoirs around the world—sandstones that are made up of sand grains (quartz and feldspar in the majority) and carbonates that are made up of organically created calcium carbonate grains (corals, algae and shells) or mud. In order to stop the upward movement of hydrocarbons and constrain them to one zone of the subsurface (trap), there must be a barrier to prevent fluid migration. Generally this mechanism or seal consists of rocks that are impermeable to fluid flow. The most effective of these seals are mudstones or shales, very fine-grained rocks containing abundant clay minerals . Occasionally the impermeable layers are dense igneous rocks and in rare situations, there may be significant rock and fluid pressure differences in a region that prevents fluid flow and acts as a seal.

Equally important is the presence of a trapping mechanism. They are either of structural form where the reservoir rock unit is contorted to produce a zone where fluids naturally accumulate against a seal, or of stratigraphic nature where a reservoir rock unit changes laterally into an impermeable unit reflecting changes in depositional environment along one bed. Upwarping of rocks (anticlinal folds ) are particularly good at trapping hydrocarbons along with faults where permeable titled reservoir strata are moved up against impermeable strata.

Detection of hydrocarbons in the subsurface during exploration takes a number of forms: direct identification of hydrocarbons at the surface, direct hydrocarbon indicators (DHI) in the subsurface, and indirect indicators both at the surface and in the subsurface. Traditionally, oil exploration was primarily conducted by recognizing seeps of hydrocarbons at the surface. The Chinese, for example, used oil (mostly bitumen) obtained from seeps for use in medication, waterproofing, and warfare several thousand years ago. The ancient Chinese frequently dug shallow pits or horizontal tunnels at the seep locations in order to recover the oil. In Baku, Azerbaijan, there are still gas and oil seeps that are permanently alight and have been used to light caravanserai since the times of Marco Polo and the Silk route. With the dawning of the modern era in Oil Creek, Pennsylvania, Colonel Edwin Drake drilled the first well to intentionally look for oil in the subsurface in 1859. Again, this was based on direct identification of seeped hydrocarbons at the surface. Initially the oil was used to provide kerosene for lamps but the later invention of automobiles drove up demand and ushered in modern methods of oil exploration.

Around the turn of the century and up until the 1950s, the main exploration tool used for finding oil was the use of intensive and detailed geological mapping. This was frequently in terrain that was remote and inhospitable. The early pioneers working their way through the jungles of Burma, the deserts of Iraq, or the mountains of Iran, would conduct detailed evaluations of the nature and distribution of rock units that could represent potential reservoirs, seals, and source units as well as frequency, orientation, and geological history of folds or faults that could act as traps for the migrating hydrocarbons. If all four of the features required for oil or gas to be created and trapped can be recognized in a region, then a variety of play concepts can be generated. Detailed local study might identify a suitable target (prospect) and then a shallow well would be drilled to test the features.

One of the most important recent discoveries in petroleum studies has been plate tectonics . Not only has this revolutionized the earth sciences, but also it has provided a conceptual setting for oil exploration. The movement of plates around the surface of Earth creates large-scale depressions into which substantial quantities of sediments eroded from the surrounding high ground may accumulate. These accumulations can exceed thicknesses of several thousand kilometers and are referred to as sedimentary basins. By comparison of basins around the world and by analogy to existing producing hydrocarbon regions, an exploration team can say which basins are worth looking at in more detail. Then explorationists will spend time ensuring that within the basin there are present all the key elements that control the presence of hydrocarbons. Assuming that all the needed features are present, the team would agree that the basin contained a viable petroleum system and prospect generation can proceed.

In modern exploration programs, the mapping of gravity and magnetic anomalies would normally be the first two methods to be applied to a new basin or region being evaluated. These techniques would be used to identify large-scale changes in the structure of the basement and sedimentary basins together with major differences in rock density such as the influx of dense igneous rocks or light salt into a sedimentary sequence. These techniques are large scale, can be applied over both land and water and can even be collected remotely from plane or satellite .

At the same time, remote sensing of onshore areas initially based on large scale photogeological surveys and, after the 1970s, by satellite imaging, can identify areas with anticlinal and faulted structural features, seeps or salt domes frequently associated with oil occurrences. Offshore remote sensing of the sea surface can lead to the identification of slicks associated with the seepage of oil (both natural and man-made) into the water column. A coarse two-dimensional grid of seismic data is then collected to obtain a picture of the subsurface in the area to be targeted. Seismic data collection involves the generation of a seismic wave using an energy source such as an air-gun in water, dynamite in drill holes inland or a truck with a plate that is thumped down onto the road/soil surface (vibroseis). The wave travels through the earth's rock layers and reflects back off key surfaces. The time taken for the waves to be received back at the surface along with their strength is recorded via geophones and displayed on a seismic section. Processing the two-dimensional seismic sections using highly sophisticated software reveals the detailed structure of the subsurface and in certain circumstances shows the presence of direct hydrocarbon indicators such as bright spots associated with gas/water differences. Primarily, though, seismic is used to indicate the nature of folded and faulted structures that could prove to be suitable hydrocarbon traps. These are frequently referred to as leads.

The objective of seismic acquisition and processing is to acoustically image the subsurface in a geologically accurate manner with as high resolution as possible. For a detailed analysis of a small area representing a field or prospect, a high density and calibrated three-dimensional seismic is collected. Modern technology also allows scientists to accurately map changes in fluid movements through time (repeat multiple 3-D seismic surveys, known as 4-D seismic) and this technique is now particularly important in monitoring production performance of the reservoir.

Ultimately, however, the only way of confirming the presence or absence of hydrocarbons at depth is by drilling the prospect. In certain areas of the world where drilling is cheap and the subsurface has been explored extensively, such as certain onshore basins of the United States, drilling is commonly preferred to extensive and expensive seismic acquisition. Wells are then analyzed using electric, sonic, and radioactive logging techniques that measure characteristics of the rocks and fluids. These methods can identify the presence of oil and gas, which can then be tested to see if they occur at commercially viable production levels. On the other hand, at a cost of over ten million dollars per offshore exploration well, the oil companies are also likely to employ the sophisticated battery of direct and indirect detection techniques first before resorting to drilling in these areas.

See also Petroleum, economic uses of; Petroleum extraction; Petroleum, history of exploration