SpeakingOilGas

For instance, if the presence of hydrocarbons has been detected, sampled and traced back to source in an area, the model can be used to predict migration paths (sometimes referred to as the basin’s plumbing) and point to parts in the basin where it would be worthwhile to look for traps with larger holding capacity. If there are no pre-existing wells or seeps to sample, analogues from another basin with similar or linked geological systems are often used to indicate where the same set of circumstances may be repeated. Most software for this work is available for PCs and the computer programs are off-the-shelf products. Geochemical modelling is an increasingly accessible tool for petroleum explorers.

Geochemistry can also be used directly in an area where a number of prospects have been delineated and there is little to choose between them in size or potential. The exploration company may decide to begin by drilling the ones that have a corresponding geochemical anomaly, or ones that modelling predicts have a higher chance of containing hydrocarbons. The investigation used is basically one of hydrocarbon gas detection. Despite the fact that an oil or gas reservoir is sealed in a trap, there is still some minute leakage of gas through the rocks to the surface.

The usual method of detection on land is to drive a hollow probe into the ground to a depth of a metre or so and draw off gases present at that depth through the seal at the top of the probe using a syringe. The sample is then analysed in a gas chromatograph. Any hydrocarbon values above normal background levels constitute an anomaly. Airborne methods are also used (albeit less commonly in recent years) both on and offshore where a chromatograph in a low-flying aircraft is employed to detect gas ‘halos’ emitted from subsurface structures.

Satellite imagery

The use of satellites in petroleum exploration has mushroomed in recent years, particularly in connection with geographic positioning, navigation and communications. But explorers have also found a niche in terms of satellite imagery where it is now possible to obtain very high resolution through the light spectrum. This can be used to present very detailed and

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accurate pictures showing bathymetry and topography. Landsat pictures can provide resolutions up to 30 metres on the ground incorporating eight spectral bands. The high resolution is useful in the study of modern depositional processes such as the formation, shape and extent of deltas to use as analogues for planning programs to explore the potential of subsurface deltaic sand reservoirs within buried coastal sediments.

Satellite imagery can also be used to detect offshore oil seeps. This is a visual technique and is based on light reflected back off the water surface. If there is cloud cover, radar can be used. This detects patches of smooth water that may be caused by oil on the sea in an otherwise rough water surface (rough water scatters the rays and presents a ‘foggy’ image). Using several satellites, explorers can home in on the possible source of the seeps. This technique has been tried with some success in the Gulf of Mexico. The method can also be used for environmental monitoring, although care needs to be taken to filter out effects from false positives like algal blooms.

Radiometric surveys

Gamma ray spectrometer surveys are used to detect radiation emanating from concentrations of uranium and thorium which may be associated with hydrocarbons. Detection of subtle radiation patterns and anomalies may indicate surface hydrocarbons which, in turn, may point to subsurface accumulations.

The earth’s crust contains uranium, thorium and potassium randomly laid down during the formation of the planet. These elements emit gamma rays in the course of radioactive decay and contribute to the earth’s natural radiation background. Of the three elements, uranium is the most mobile, being water soluble and easily transported by groundwater. However when the uranium encounters organic matter, such as subsurface hydrocarbons, the ion becomes insoluble and immobile. Hence higher- than-background readings of gamma rays may indicate the presence of a hydrocarbon trap.

The anomalies can be identified by airborne radiometric measurements using a spectrometer.

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Controlled electro-magnetic (EM) surveys

Controlled electro-magnetic (EM) surveys, recently developed for marine petroleum exploration, involve sending low frequency electro-magnetic signals into the sea bed and measuring the response to determine the resistivity of the subsurface.

The Controlled Source Electro-Magnetic measurements (CSEM) method uses an electric dipole source to transmit low frequency electro-magnetic signals to a series of receivers on the sea bed that measure the electromagnetic field at the sea floor. As the source is towed over the receivers the variation and phase of the received signal indicates the resistivity of the subsurface structure down to depths of several kilometres.

The Controlled Source Audio-frequency Magneto Telluric method (CSAMT) uses an artificial signal source (usually in the range of 0.1Hz–10kHz) in addition to naturally occurring electro-magnetic source fields to determine the resistivity of the subsurface. This provides a stronger, more reliable signal and enables imaging of shallower targets than is possible with low frequency natural signals alone.

Both these technologies are increasingly being employed during the later, more detailed phase of exploration work to provide complementary information to conventional surveys and attempt to identify the fluid content in defined reservoirs. They use the fact that there is a significant contrast between resistive hydrocarbon-saturated reservoirs and the surrounding more conductive layers saturated with saline water.

Electro-magnetic techniques can also be used to define the lower boundaries of salt bodies. Conventional seismic surveys generally outline the top of such structures, but the diffuse scattering of signals in the lower layers due to the presence of carbonates or basalts or even other thin salt ‘blankets’ often clouds the picture. The resistivity contrast between such layers and the sediments below can be identified using the EM methods. By studying the variation in response as a function of frequency, the variation in resistivity as a function of depth can be determined.

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Overall, the improvements to all non-seismic survey techniques in recent years include better instrumentation, the advent of extremely accurate global positioning systems, improved computing power and processing algorithms, advanced visualisation techniques such as image enhancement and 3D visualisation and the ability to combine different data sets. All these techniques can high-grade areas for a more detailed examination through seismic survey work.

Seismic techniques

The principal method geologists use to explore the subsurface, besides the direct and expensive process of drilling, is through the use of sound waves. Sound waves travelling through the earth are called seismic waves, a term originally used in reference to earthquakes. Just as ultrasound is used to investigate the shapes of organs within the human body, seismic waves are used to map out the geologic structures of the earth. While ultrasound penetrates a few centimetres into the body using very high frequency (short wavelength) sound waves, seismic surveys use lower frequency, longer wavelengths to look many kilometres into the earth.

There are two types of seismic survey: refraction and reflection. Refraction surveys were common early last century for reconnaissance and salt dome exploration. They are seldom acquired nowadays, except for deep crustal studies, because seismic reflection surveys provide far greater information and accuracy for hydrocarbon exploration.

A controlled pulse of sound is sent into the ground and a range of detectors are used to pick up the reflected waves as they come back to the surface. In marine environments the main source of sound energy is an airgun array — a group of pistons which let out a pulse of compressed air. Typically this will be two or three litres in volume at a pressure of 2000 pounds per square inch (psi).

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Seismic survey — diagram showing typical reflection and refraction wave patterns

Typical marine seismic reflection survey

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On land, vibrating trucks send out a controlled sweep of sound between six and 16 seconds long. This technique, which sends the vibrations through a large metal plate pressed onto the ground, has largely replaced the older method of a dynamite charge set off a few metres down a specially drilled shothole. By spreading the energy over a longer period of time (many seconds as opposed to a fraction of a second) the same amount of energy can be used without damage to the local environment. Seismic surveys have even been conducted on the Champs Elysées in Paris using a fleet of vibroseis trucks without danger to life, limb or architectural heritage.

When planning a survey, the geophysicist carefully sets the geophone/ hydrophone spacing to provide the required subsurface information. For instance, the maximum offset (the distance from the energy source to the furthermost group of phones along the grid line or the streamer) should be comparable to the depth of the deepest zone of interest. Conversely, the minimum offset should be comparable to the depth of the shallowest zone of interest.

Vertical section of 4-fold common depth point shooting

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Average P-Wave Velocities

Material	Velocity
	feet/second	metres/second
Weathered surface material	1,000–2,000	305–610
Sea water	4,800–5,000	1,460–1,530
Sandstone	6,000–13,000	1,830–3,970
Shale	9,000–14,000	2,750–4,270
Limestone	7,000–20,000	2,140–6,100
Salt	14,000–17,000	4,270–5,190
Granite	15,000–19,000	4,580–5,800
Metamorphic rocks	10,000–23,000	3,050–7,020

There are three distinct stages in the seismic technique — acquisition of data, processing, and interpretation.

Data acquisition

2D surveys

When exploring a new area where little is known of the subsurface geology, a 2D survey is usually performed. This consists of survey lines spaced one, two, five or more kilometres apart.

Offshore the seismic vessel will sail along with seismic guns deployed close off the stern (so they are near the on-board air compression source), letting out pulses every 25 metres or so, with a 12 second gap between ‘shots’. Behind the guns is the recording cable (or cables) whose length is measured in kilometres. The cable contains groups of pressure sensitive hydrophones which record the sound waves as they are reflected back from the geological layers. The recordings are a few seconds long and sampled every 1–2 milliseconds (thousandths of a second). Sound waves travel at about 1.5 kilometres per second through water and increase to 2–6 kilometres per second when they pass through rock layers.

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Aeromagnetics — Sulu Sea case history

A 2D seismic survey was to be acquired in an area of known buried volcanic ridges.

Reprocessed aeromagnetic data was used to optimise the layout of the new 2D data and avoid wasted lines over these ridges.

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Onshore the recording devices are called geophones and they are placed at measured distances along a pre-surveyed seismic line from the shot or vibration point. Geophones (usually grouped together in arrays, with three or more connected electrically so that the array acts as a single sound detector) transform the returned seismic energy into electrical voltage which is then transmitted by cable to recording equipment housed in a vehicle accompanying the survey team. Typical seismic records will image 10 or more kilometres down into the earth.

Each time the seismic pulse meets a change in rock properties, for example going from a shale layer to a sand layer, part of the pulse will be reflected back to the surface. This is called an event. By measuring precisely the difference in arrival time of a given event from the nearer and further hydrophone groups, the velocity of the rock material can be measured. The seismic measurements are made in time, so if the velocity and time are known, geophysicists can work out the depth of the event.

A seismic line looks like a cross section through the earth. Initially these are used to map structural traps where hydrocarbons may accumulate — at its simplest at high points of domes known as anticlines, but also places where faults or erosion cut off a reservoir. Stratigraphic traps, where the geology changes laterally from one rock type to another, such as a buried sandy channel, sand bar, or carbonate reef can also be mapped using seismic data.

3D surveys

In areas where the larger and more obvious traps are mapped, petroleum explorers are increasingly using 3D surveys to obtain greater definition. By placing survey lines much closer together, a more detailed three dimensional picture can be built.

To economically survey a given area offshore, increasing numbers of recording cables are being towed behind seismic vessels, with between eight and 16 cables now the norm. A pair of submerged towed wings pull the trailing array of cables 50–100 metres apart from each other, resulting in a typically rectangular acquisition system that is one kilometre or more wide and six kilometres long.

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The demands of deploying tens of kilometres of cables, along with the ability to repair and replace defective sections of the cables and the running of large compressors to generate the compressed air for the seismic sources, mean that seismic vessels have become extremely specialised and expensive-to-run ‘information factories’. A typical 3D survey covering 1000 km2 of ocean will take one or two months to acquire.

Marine operations

The cables stretching back six kilometres or more from the boat are affected by waves and currents. Depth controllers keep the cable within a metre or so of the desired depth, typically seven metres below the water surface. Towing the cable shallower than this makes it more susceptible to wave-induced noise, while towing deeper results in the loss of higher frequencies due to ‘ghosting’. This latter effect is where the water surface acts like a reversing mirror to the sound waves. The surface reflection (or ghost) will cancel out the frequencies of later arriving reflections. Thus, there is always a compromise between this and using results from a shallow cable capable of recording higher frequencies (and hence better resolution) but incurring more swell noise.

Currents displace the cables laterally and while a limited amount of cable steering is possible, any reasonable current may displace the ends of the cables by a kilometre or more. As the vessel sails up and down the survey area, there must be very accurate positioning of the boat and of each recording hydrophone along the cable. A buoy with a geographic positioning system is attached to the end of each cable and their position can be determined to within a metre or two.

When the vessel turns to make a second sweep across an area, there is always the danger of tangling the cables. Tangles can be difficult and time consuming to unravel. In bad weather it may not be possible to deploy a work boat to fix the tangle and the equipment must be pulled back on board.

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