Origin of Oil and Gas
As the picture indicates, oil and gas are derived almost entirely from decayed plants and bacteria. Plant remains must first be accumulated, trapped and preserved in sediments. Geologists study and understand this realm of activities.
Sedimentary layers are commonly formed from large river deltas, like the Mississippi in the US or the Niger River in Africa, or from the fact that the sea levels have changed in some regions by over 300 feet, alternatively flooding the land with organically rich plankton and algae over the millennia.
By numerous geological forces, these sediments are buried deeply and slowly ‘cooked’ to yield oil or gas. Heat is available because the Earth’s temperature rises by 72 degrees Fahrenheit for every mile of subsurface depth.
Rocks containing sufficient organic substances to generate oil and gas in this way are known as source rocks. The most common and desirable hydrocarbon source rock is a black shale sedimentary rock.
Most oil and gas is found in and produced from what are called sedimentary basins, wide geographic areas of these sedimentary rocks, in some cases with layers 10,000 to 50,000 feet thick.
Importance of Temperature and Depth
Almost all oil forms from the buried remains of minute oceanic algae and bacteria.
Gas forms if these remains are deeply buried. The stems and leaves of buried land plants most often become coal. One exception is methane generated at the surface through decomposition of plants in swamps, forest, etc.
The processes of oil and gas formation resemble those of a kitchen where the sedimentary rocks are slowly cooked. Since temperatures within the Earth’s crust increase with depth, sediments and contained hydrocarbons warm up as they become buried under thick piles of new sediments. This is a very slow process taking hundreds of thousand to millions of years.
As the picture indicates, whether oil or gas is formed depends primarily on two things:
- The depth that the sediments are buried and
- The length of time, in millions of years, that they are under pressure.
As a source rock deposited under the sea, in a river delta or lake, becomes hot (typically 50oC, 150oF) and 7,000 feet deep, long chains of hydrogen and carbon atoms form waxy and viscous heavy oil.
At higher temperatures, (typically up to 150oC, 300oF) and 18,000 feet deep, shorter hydrocarbon chains break away to give light oil and
Above 150oC, 300oF and below 18,000 feet, gas is formed.
Most North Sea oil is the more valuable light oil. Gas from the southern North Sea is methane. US GOM oil quality depend on depth drilled below the seabed.
In the search for new oil and gas discoveries, the exploration function includes multiple disciplines: geologists, geophysicists, scientists and engineers of all types.
Orchestrating this variety of capabilities into a coherent and effective team requires a common and easily understood analysis and execution model, like the game plan of a sports team.
Historically the exploration game plan focused on defining a single exploratory well, called a prospect. During the first half of the 20th century, a broader perspective became prominent, called the basin – play model or framework. Important terms used by exploration departments today in analyzing where to search for oil and gas are described below:
- The basin is the term for an entire complex of petroleum source rocks, carrier beds and migration conduits, and traps that contain the reservoirs. A sedimentary basin often contains multiple plays with several different types of traps, all charged from a common source rock.
- A play is a group of prospects in a basin all having similar geologic origins— a family of geologically similar traps. Prospects and fields in a play have similar configurations and structural histories. Fields making up a play contain reservoirs that exhibit similar production patterns, and ultimately recoverable reserves.
- A prospect is defined as a location with a consistent set of geological criteria and conditions. When combined with related economics, a prospect can justify capital investment for an exploratory well.
- A field is a geographical area in which one or more oil or gas wells produce. A field may refer to surface area only or also to the underground productive formation(s). A single field may include several reservoirs separated either horizontally or vertically.
- The reservoir is a porous and permeable underground formation containing natural accumulations of producible hydrocarbons. The accumulation is confined in a trap by impermeable rock or water barriers and is characterized by a single, natural pressure system.
It is clear today that the most important decision in exploration is not which prospect to drill, especially with costly commitments and/or front-end bonuses. Instead, the key decision is which new play to enter.
The economic consequences of choosing a bad play can be serious to a medium-sized oil company or E & P operator, or financially disastrous to a smaller firm.
A basic rule of thumb in exploration is that the best place to find new oil or gas is near where it already has been found. That is precisely where the industry looks, for sound business reasons. The financial risk of doing so is far lower than that associated with drilling a rank wildcat in a prospective, but previously unproductive area.
Most successful E&P companies are now focused and limit exploration budgets to those basins and plays where they have a history or can come up with a technical competitive advantage.
Every seismic result, well, log or core sample improves an operator’s understanding of the geology and geophysics of the basin. The common term for this extensive collection of information is a basin study.
To quote one successful international explorationist,
“Basin studies are kept forever….regimes come and go but the rocks never change.”
Oil and Gas Migration
Another critical factor in understanding where to drill for oil and gas is the scope and extent of oil and gas migration.
Oil, gas and water migrate through permeable rocks in which the cracks and pore spaces between the rock particles are interconnected and are large enough to permit fluid movement. Fluids cannot flow through rocks where these spaces are very small or are blocked; such rocks are called impermeable.
Oil and gas are less dense than the water which also is present in the pore spaces, so they tend to migrate upwards. Oil and gas also migrate along large fractures and faults which may extend for great distances, or as a result of underground movement of the strata.
For example, in the Gulf of Mexico, vertical migration pathways have been shown to extend for more than 20,000-40,000 ft (5-8 miles). Lateral migration can extend for more than 15 miles (across at least five offshore blocks).
One of the main reasons for the acceptance of the basin-play model is the realization that the history of oil and gas (and especially its movements or migrations) is as important as the drilling history on an individual trap and reservoir. Any wildcat prospect inside or near a preferred migration pathway will carry a higher chance of success than one outside.
What is a Play?
Plays are defined by the structured information on the subsurface geology. Geologists describe five key attributes that control the occurrence of oil or gas in any play, including:
- source rock,
- reservoir, and
These attributes are thought to be generally independent of each other. While the millions of wells drilled around the world have yielded immense amounts of geological information, the data provided by any well bore is unique to that well. Seismic analysis and modeling techniques help the geologists infer from the visible well results the portion of the reservoir that is still invisible to them.
The probability that each key attribute is favorable can now be estimated by geologists from all the geological and geophysical information about (and across) the play. This set of consistent data and interpretation helps improve the probability of success of the next wildcat well.
In the US approximately 700 plays have been identified by the US Geological Survey (USGS), still providing a wide choice (and risk) in potential wildcat drill sites for the explorationist.
USGS National Oil and Gas Assessment
The U.S. Geological Survey (USGS), in collaboration with the Minerals Management Service (MMS), has developed the National Oil and Gas Assessment, a scientifically based, objective analysis of reserves.
USGS geologists develop geological and geophysical theories for occurrence and distribution of resources rather than just conducting inventories of discovered oil and gas resources. More than 50 geologists are responsible for analyzing relevant geologic information.
During the 1990’s, the USGS and MMS employed the play concept in carrying out several assessments of remaining onshore and offshore US oil and gas resources. The beauty of play analysis is that it ties statistics of oil and gas exploration and development to geological expertise.
Unconventional resources are also analyzed on a play basis using different analytic methods.
As the chart shows, about 700 US plays are grouped into 72 provinces which in turn are grouped into 8 regions (analogous to basins). Each play is described in narrative form in sufficient technical detail to allow a complete and consistent analysis, and to allow comparison among plays and provinces.
The databases are now available to the public, at no charge.
Understanding Surface Geology
Finding highly productive “sweet spots” in frontier areas depends on the proven principle that the surface geology can give important clues to the subsurface. Structures that hold oil and gas are inevitably located beneath non-productive rock, often below the seabed.
Shooting seismic is costly. So operators reserve seismography for areas that already show geological promise.
A stepwise approach has proven to be successful in delineating potential drill sites in new locations. As shown in the chart,
- The starting point is a thorough examination of the structure and geology of the basin, plays and the prospect area.
- Second, regional reconnaissance using Landsat images or other airborne radar data is used to identify surface geological indicators than can indicate subsurface faults and structures.
- Finally, the potential play areas are high-graded using data from a high-resolution aeromagnetic survey to identify targets and prospects.
Satellite images have been taken of the earth since 1972 by the USGS (US Geological Service). Real time data is available on their website.
Today, explorationists make use of detailed Landsat data as a very cost-effective starting point for examination of frontier basins, to determine accessibility and high level surface geological details.
In April 2008, Landsat data became available to the public for free.
Landsat scenes can be requested and downloaded from
- Glovis (http://glovis.usgs.gov) or
- Earth Explorer (http://earthexplorer.usgs.gov).
An aeromagnetic survey is a fast and cheap type of geophysical survey carried out using a magnetometer aboard or towed by a helicopter or an aircraft. Use of aircraft allows large areas of the Earth’s surface to be covered quickly for regional reconnaissance. Aeromagnetic surveys can also be conducted in shallow coastal waters.
The aircraft typically flies in a grid like pattern with height and line spacing determining the resolution of the data (and cost of the survey per unit area).
As the aircraft flies, the magnetometer records tiny variations in the intensity of the earth’s magnetic field. Different rock types differ in their content of magnetic minerals. For example, the most common sedimentary rocks are sandstone and limestone, and they have low density and are not very magnetic.
The aeromagnetic survey data, once processed, allows a three dimensional visualization of the subsurface geological structure of the Earth’s upper crust. Salt domes, folds and faults can also be identified.
Understanding Subsurface Geology
Once the aerial survey work finds a play with geological promise, the next step in an exploration program is to develop a more detailed understanding of the subsurface geology. Vast improvements in understanding geology can be attributed to developments in the science of seismology.
Seismology began with the desire to understand the destructive nature of large earthquakes. Seismologists soon learned, that the seismic waves produced by an earthquake also contained valuable information about the large-scale structure of the earth’s interior.
Today, much of our understanding of the earth’s mantle, crust, and core is based on the analysis of the seismic waves produced by earthquakes. Thus, seismology became an important branch of geophysics, the physics of the earth.
Onshore Seismic Surveys
The underlying concept of seismology applied to oil and gas exploration is simple. Man-made seismic waves are just sound waves (also called acoustic waves).
In onshore seismic surveying, the sound waves are generated either by an explosion or use of a large piece of equipment called a thumper – Vibroseis truck.
As the chart for an onshore seismic survey shows, these sound waves leave the seismic source and travel downward into the earth. As they encounter changes in the earth’s geological layering, echoes (or reflections) travel upward to the surface.
Electromechanical transducers (called geophones or hydrophones) detect the echoes arriving at the surface and convert them into electrical signals, which are then amplified, filtered, digitized, and recorded.
Where there is no reflecting stratum, seismic is useless. The sound waves keep on going into the earth until they deteriorate.
The recorded seismic data then undergo elaborate computer processing to produce images of the earth’s shallow structures, similar to the ultra-sound used to create pictures of unborn babies.
An experienced geologist or geophysicist can interpret these images to determine what type of rocks are represented and whether or not the rocks might contain valuable resources.
Today, approximately 30% of seismic surveys use explosives and the rest use “thumper” trucks, trademarked as Vibroseis vibrator trucks, when they were invented by the Continental Oil Company in the mid 50’s.
These 30,000 pound vehicles generate vibrations under the ground by elevating themselves above the ground on a short pole, thus concentrating their entire weight on a platter and “shaking” for several seconds per location.
The data gathering process is very similar to the explosive process. This process is the most precise process as it uses controlled vibrations that are spread over period of time, as oppose to the explosion vibration that is just a giant burst of energy.
Vibrator trucks are safer than explosives and are able to operate inside major cities because the short burst of vibration has little impact on the surroundings.
Seismic Shot Point Base Map
To conduct a onshore survey, a seismic crew first sets out a number of shot holes and a carefully patterned array of geophones. The geophones detect the reflected sound waves and transmit the data to recorders.
Raw seismic data can be reused many times and over a long period. Thanks to modern reprocessing with computer interpretation software that “sharpens” the data, seismic data recorded in the 1960s is still being used for exploration.
Marine crews must constantly pay attention to their latitude and longitude locations to ensure accurate data readings.
Land seismic crews must contend with rough, mountainous terrain, jungles and desert conditions, population centers and permafrost when “shooting seismic.”Because of these difficulties, land seismic acquisition costs are generally higher per kilometer than the cost of acquiring marine data.
Offshore Seismic Survey
In offshore seismic surveying, the choice of equipment depends upon the accuracy required and distance off shore.
Explosives are not used offshore as an energy pulse. Various types of air guns are used and the signature of the source should be known before a survey begins.
Offshore surveys, costs and data quality are all affected by the number of cables or streamers towed. For both 2-D and 3-D lines, the length and group interval of the streamer(s) are important. Long streamers at wide intervals improves interpretation accuracy by reducing the amount of data/location distortion due to the ship itself and any wave action.
Satellite navigation or onshore and offshore range equipment must be used to carefully position the ship. Round-the-clock Global Positioning System (GPS) satellites are most common today.
The cost of both marine and land acquisition is influenced by the mobilization/demobilization costs of the seismic crew to the survey area and the cost of the capital equipment. For large surveys, marine acquisition costs-per-kilometer tend to be lower because, once the vessel is in the survey area, a larger amount of data can be recorded per day.
Types of Seismic Surveys
Seismic cannot change the underlying geology, but sophisticated computer imaging of subsurface structures can enhance the likelihood of a successful wildcat well.
A seismic program is expensive and time consuming. Collecting and processing the data can take 12-18 months. So it is used after a play has shown some promise by the basin analysis and aerial surveys.
There are three basic types of seismic technologies used to help explore for oil and gas:
- 2D seismic is recorded using straight lines of receivers crossing the surface of the earth.
- In 3D seismic surveys the data imaged is an entire volume of earth, not just a vertical cross-section.
- 4D seismic uses three-dimensional seismic data acquired at different times, over the same area.
Advanced imaging, also widely used in medicine, not only helps find and produce oil and gas more efficiently, but has advanced the identification and recovery of important subsalt reservoirs in offshore locations in the GOM and Brazil.
Two-D seismic is recorded using straight lines of receivers crossing the surface of the earth. A 2-D seismic survey works well for imaging major structures.
Two-D surveying is still popular because data gathering and analysis of 2D seismic information is much quicker and cheaper than 3D or 4D. Two-D data requires much less permitting, surveying, and processing time than even small 3D surveys. Large 3D seismic shoots may take one to two years to acquire, and three to four months to process the information.
The late 1970s saw the development of the 3D seismic survey in which the data imaged was not just a vertical cross-section but an entire volume of earth.
One of the most obvious differences between 2D and 3D seismic is that 3D imaging provides information continuously through the subsurface (bounded by the survey) whereas 2D seismic reveals only strips of information.
3D seismic data collection improves exploration performance by:
- fewer dry holes,
- more optimized well locations,
- guidance for horizontal drilling projects,
- more complete evaluation of mineral rights and
- better understanding of the nature of prospects.
Nevertheless, 3D seismic may not be cost-effective in many onshore provinces, especially in the early stages of exploration. In onshore 3D seismic, many lines of receivers are used and recorded across the earth’s surface. The area of receivers recorded is known as a “patch”.
In offshore, the main difference between 2D and 3D seismic is that 2D seismic is acquired using a single listening cable towed behind the seismic vessel, whereas 3D seismic is acquired using six parallel listening cables, and the cables can be up to six kilometers long.
Three-D operations are considerably more elaborate than 2D and the daily cost of the crew is substantially increased. However, the rewards include fewer dry holes, more optimized well locations, guidance for horizontal drilling projects, more complete evaluation of mineral rights and better understanding the nature of prospects.
Time-lapse or 4D seismic is the process of using three-dimensional (3D) seismic data acquired at different times, over the same area.
It is used to assess changes in a producing hydrocarbon reservoir over time (the fourth dimension). Changes may be observed in fluid location and saturation, pressure and temperature.
To maximize the value of a 4D seismic project, exploration and production assets are carefully screened because of the expense to acquire and analyze the data.