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An introduction to the porosity the best known physical characteristic of an oil reservoir

Compare the results and suggest reasons for the marked difference.

  • Oil fills the smaller pores and coats the grains;
  • When they are fractured, they can make good reservoirs;
  • There is now an increasing effort to make commercial use of excess gas, as in the Clair field west of Shetland;
  • It is in the nature of exploration that more often than not geoscientists are wrong with their predictions, but this approach at least helps to reduce their uncertainty;
  • Reserve calculations These calculations are usually done by a reservoir engineer if there is one in the company you work for;
  • When the well is drilled the lowered pressure above means that the oil expands.

The simplest explanation for this difference is that younger reservoirs tend to have higher porosities because they usually occur at shallower depths and are less compacted than their older counterparts.

Seals are fine-grained or crystalline, low-permeability rocks such as mudstone, anhydrite and salt. Rock salt is by far the most effective seal, because it is crystalline and therefore impermeable.

Seals are also enhanced if they are ductile ductile deformation prevents the formation of open fractures and jointssubstantially thick and laterally continuous; little surprise then that the largest oil fields in the Middle East are sealed by evaporites Argles, 2005 with these characteristics.

However, seals are rarely, if ever, perfect. Hydrocarbons can migrate through almost all rock types, but at different rates that depend upon any fracturing and microscale fluid flow, and whether liquids adhere to or are repelled by the surfaces of mineral grains. Many oil and gas fields have active surface seeps of petroleum overlying them that provide a direct indication as to their location.

In marine settings seeps may be detected as bubbles of gas rising from the sea bed, or as an oily sheen on the water. On land, plant communities are stunted, surface layers of rock and soil may be altered, tarry residues may encrust the surface, and sometimes there may be active oil seeps.

The first oil fields to be developed in the 19th century were located beneath such obvious features. It is thought that ignition by lightning strikes of petroleum escaping above the huge oilfields of Iran gave rise to the fire-worshipping Zoroastrian religion.

Even odder, the Ancient Greek Oracle at Delphi is thought to have made her prognostications while hallucinating under the influence of escaping natural petroleum gas. That is because many wells, each with only a small rate of production and lifetime, would be needed to extract the petroleum.

To be worth working, a sealed petroleum-bearing 'container' or trap must be shaped naturally to retain and focus petroleum, rather as the curved upper surface of a balloon traps buoyant hot air.

Related terms:

The lower surface of a trap is defined either by a petroleum-water contact or sometimes by another seal. There are many different styles of trap see Figure 4 but the most common are structural traps in the form of anticlines produced by tectonic processes, by differential compaction of soft rocks above hard, irregular surfaces and by evaporitic salt masses that rise gravitationally.

The low density of salt, combined with its ductility, enables it to rise to form domes and intrusive masses. Because they produce distinctive geological and geophysical features, structural traps are the easiest to find. View larger image Figure 4 Types of traps. Types A to E are explained in the text. A-C are structural traps, D is a stratigraphic trap and E is a combination trap. They include simple anticlines Afaulted structures that juxtapose reservoirs against seals Band traps created at the flank of a salt dome or in the compaction anticline above it C.

Most fields in the North Sea occur in structural traps.

  • Pressure and oil production decrease steadily during production, but the GOR increases;
  • They may be zones of mixed waters that allow carbonate precipitation, or oil water contacts;
  • The gas will often migrate to the crest of the structure;
  • These are the kind of calculations you would go through if you wished to "join" in the drilling or a well, or if you were planning to bid on acreage;
  • The favorable texture is depicted by packaging of similar sized grains, not a combination of coarse and fine grained composition.

Stratigraphic traps result from lateral changes in rock type and typically consist of discontinuous sandstone bodies encased in mudstone D. In practice, traps often form through a sequence of different processes over the course of tens of millions of years. For example, in E the reservoir was first deposited, then folded, uplifted and eroded, before being overlain by a much younger impermeable mudstone. The resulting configuration is appropriately called a combination trap.

Provided it was intact before the reservoir received a petroleum charge, it forms a valid trap regardless of how long it took to form. As petroleum accumulation continues it is possible for traps to fill beyond their natural spill-point, when petroleum can escape sideways to re-migrate to other traps Figure 4, upper C or to the Earth's surface where it emerges as oil or gas seeps. SAQ 5 Suggest another type of trap on Figure 4 that might experience such leakage. Answer The lowest trap associated with the fault might leak along fractures produced by the faulting, to help charge higher traps.

Combining the ingredients Having examined the essential ingredients for a petroleum accumulation, this section discusses how knowledge about them is combined to create a petroleum play.

This is a particularly useful concept, since it consolidates what is known or not known about the petroleum potential of a particular level within a basin and forms the basic strategy for oil and gas exploration. A play is defined as a perception or model of how a petroleum charge system, reservoir, seal and trap may combine to produce petroleum accumulations at a specific stratigraphic level. By examining whether each of the play ingredients is both present and effective, it is possible to define parts of a basin where petroleum accumulations can reasonably be expected to exist.

This process can be conducted systematically in a given area to generate a play fairway map that depicts where the ingredients coexist, even though the precise details of trap location and size may not be known.

As more data become available the play becomes better defined, but even when the play is proven by a discovery it does not imply that every trap within the same fairway will contain a petroleum accumulation.

It is in the nature of exploration that more often than not geoscientists are wrong with their predictions, but this approach at least helps to reduce their uncertainty. To illustrate the petroleum play approach an example is provided from the Upper Jurassic of the North Sea Table 4. Table 4 Upper Jurassic petroleum plays of the North Sea. View larger image There are two play types: Whilst the depositional settings for the two reservoir types are quite different, both plays share the same petroleum charge, seal and trap ingredients.

Reservoir Evaluation

Importantly, notice that the onset of petroleum generation comfortably post-dates trap formation. For simplicity the two plays can be combined and their distribution plotted to create an Upper Jurassic play fairway map Figure 5. This illustrates the close correspondence between the limit of mature Upper Jurassic Kimmeridge Clay source rocks and the fairway, such that the migration pathways between the two are short less than 15 km and highly permeable.

More than 60 Upper Jurassic fields have been discovered to date beneath the North Sea. They have combined oil reserves of about 2. The Upper Jurassic play fairway in the North Sea. Beyond the mapped fairway limits one or more of the key ingredients of the play, for instance suitable traps, seals or reservoir rocks, are missing and therefore it is unlikely that Upper Jurassic petroleum discoveries will be made there. The golfing analogy of staying within the fairway in order to be successful seems particularly appropriate in the context of exploration.

When geological knowledge was far more limited than it is today, most of the discoveries were beneath quite obvious signs of petroleum seepage at the surface. Eventually, such easy targets ran out, although some are still being discovered. The key ingredients for petroleum accumulation which we have discussed in this course were gleaned from the knowledge gained by drilling such targets and examining the geology around them.

  1. It is salutary to remember that more than half of the world's countries produce no oil. The clean-up remediation process cost more than 2 billion dollars and took several years.
  2. This paper emphasizes the critical role of organic matter in the formation and evaluation of shale oil reservoirs, and its critical control on the oil generation potential and storage capacity of shale, which eventually determine the oil content and productivity of shale oil reservoirs. Despite the increasing number of large tankers carrying crude oil around the world, the number of large spills shows a significant decrease over the last several decades Figure 13.
  3. The most useful geological data are derived from pieces of rock recovered from specific depth intervals. Figure 8 An example of a seismic section.
  4. The most useful geological data are derived from pieces of rock recovered from specific depth intervals.

As more has been learned, increasingly sophisticated methods have been developed that increase the odds of making a discovery in less obvious situations. This section covers some of those methods an introduction to the porosity the best known physical characteristic of an oil reservoir describes how an exploration well is drilled and evaluated.

In the pioneering era of onshore exploration the search for anticlinal structures at shallow, drillable depths usually began with the recognition of an overlying part of such a structure at the surface. These days, with ready access to various forms of subsurface data, field mapping is more commonly used to assess the structural style of a basin and to provide analogues for concealed reservoirs or source rocks. Far from being an outdated technique, modern fieldwork is becoming increasingly sophisticated as digital data collection is underpinned by Global Positioning Systems GPSsatellite imagery and digital terrain models.

In Norway's Lofoten Islands a detailed analysis of onshore fault and other structural patterns is being extrapolated offshore in order to calibrate three-dimensional 3-D seismic data in unexplored portions of the Norwegian continental shelf.

Satellite, gravity and magnetic methods see below are commonly used during the early phase of exploration when a sedimentary basin, or at least a substantial part of it, is not known in sufficient detail to deploy more expensive methods. Their interpretation is simple and can be done relatively cheaply in the office.

Satellite images take the form of spectral data over a wide range of wavelengths, from the visible through infrared to microwave radar. They can sometimes detect unknown petroleum seepages.

On land, the presence of a seep is often associated with a change in vegetation or soil colour, especially if the seep is of crude oil, whilst in the offshore setting rising gas bubbles may draw deep water to the surface, giving a cool thermal image.

Alternatively, satellites can provide photographic imagery with an extraordinarily good resolution, sufficient to map rock exposures, analyse topography, and to locate roads, habitations and so on. Gravity surveys are often used to analyse sedimentary basins at the regional scale.

Because sedimentary rocks usually have a lower density than crystalline rocks, thick sequences of relatively low-density sediments effectively reduce the Earth's gravitational force and they are characterised by regional gravity lows. Gravity data may be collected on land, at sea or by air and they are particularly useful in areas of difficult terrain, such as jungles and deserts, where access is difficult.

Regional airborne magnetic surveys can also be used to define the shape and gross structure of a basin and they are often acquired in tandem with gravity surveys. Magnetic rocks cause perturbations in the Earth's magnetic field, whereas non-magnetic rocks have little effect.

Petroleum reservoir

Sediments are typically poorly magnetic because they do not contain large amounts of iron-rich minerals, whereas igneous rocks such as volcanic lavas often do. So sedimentary basins characteristically have a low, uniform magnetic signature that contrasts markedly with the highly variable magnetic anomalies associated with metamorphic basement rocks and near-surface volcanic intrusions.

Where faults juxtapose rocks with different magnetic properties at depth, the faults show up as distinctive linear features. Seismic surveys can be acquired at sea as well as on land. The marine method is the most common in petroleum exploration and is shown schematically in Figure 6, although the same principles apply to any seismic reflection survey. Figure 6 Marine seismic acquisition - pulses of sound energy penetrate the subsurface and are reflected back towards the hydrophones from rock interfaces.

Compressed air guns towed behind a boat discharge a high-pressure pulse of air just beneath the water surface. The place of detonation is called the shot point and each shot point is given a unique number so that it can be located on the processed seismic survey. The sound waves effectively the same as seismic P-waves produced by earthquakes pass through the water column and into the underlying rock layers.

Some waves travel down until they reach a layer with distinctively different seismic properties, from which they may be reflected in roughly the same way that light reflects off a mirror.

For this reason such layers are called seismic reflectors. The reflected waves rebound and travel back to the surface receivers or hydrophonesreaching them at a different time from any waves that have travelled there directly. Their exact time of travel will depend on the speed that sound travels through the rock: Other waves may pass through the first layer and travel deeper to a second or third prominent reflector.

If these are eventually reflected back to the hydrophones they will arrive later than waves reflected from upper horizons. The hydrophones therefore detect 'bundles' of seismic waves arriving at different times because they have travelled by different routes through the rock sequence.

Computer processing allows the amalgamation of recordings from all the shot points, filtering out unwanted signals of various sorts. The final result is a two-dimensional 2-D seismic section.

By using closely spaced survey lines or hydrophones arranged in a grid it is possible to produce 3-D seismic datasets.

  1. Formation[ edit ] Crude oil is found in all oil reservoirs formed in the Earth's crust from the remains of once-living things. Magnetic rocks cause perturbations in the Earth's magnetic field, whereas non-magnetic rocks have little effect.
  2. Basically shale oil experiences no migration or only undergoes some primary, short-distance migration within the source rocks.
  3. Geologists and engineers focus on the reservoir in particular, by attempting to provide an improved definition of the trap geometry and considering whether or not the reservoir is segmented by barriers to lateral flow, such as faults or impermeable layers that will require wells to be drilled into each segment.
  4. The key issues now appear to be cost and legislation, rather than feasibility.
  5. Water, as with all liquids, is compressible to a small degree.

These are usually interpreted on a PC workstation and colours are normally used to enhance the image and aid interpretation. The data can be viewed in any orientation in order to create a 3-D visualisation of selected horizons Figure 7.

The image is derived from a 'cube' of closely spaced 3-D seismic data, onto which the paths of the production wells are superimposed. Bright colours in this perspective view relate to depths to a particular reflecting boundary. Reds and greens are structurally highest, where petroleum may be trapped. Seismic data of all forms 2-D or 3-D are displayed with the horizontal axis indicating geographic orientation and distance, whereas the vertical axis is calibrated in time.

The time, measured in seconds, records how long it took the seismic wave to travel from shot to reflector and then back to the hydrophone, so it is described as two-way travel time TWT. Further processing and the incorporation of seismic velocity data allows TWT to be converted into depth.

Depth-converted seismic data is the mainstay of exploration since it provides a meaningful basis for all subsequent interpretation.