How is natural gas found?

Initially oil and natural gas exploration was as simple as locating surface seeps or places where oil and gas had been discovered accidentally while digging or drilling for water.

Today, exploration begins, in a previously undeveloped sedimentary basin, with aerial surveys to identify potential petroleum basins. Through satellite or airborne surveys, data regarding magnetic fields, gravity and radiation are collected and analyzed. As well, aerial photography and outcrop surveys are conducted. All of this data is used to confirm the presence of potential source, reservoir and cap rocks.

If available, a review of existing information is conducted, which may include academic and government studies in addition to well data.

Once the prospectivity of an area has been established, the next step is to run a seismic survey. Seismic is a relatively accurate and cost-effective way of modeling the earth’s subsurface. The seismic method involves transmitting acoustic energy into the earth and recording the energy reflected back from subsurface geological boundaries. The source of the acoustic energy can be dynamite detonated in a shallow drill hole, or vibrations generated by vibroseis trucks or, in the case of offshore seismic, by air guns towed behind a ship. The returning energy is collected by a series of geophones, or listening devices. By measuring the two-way travel time of the acoustic energy, a reasonable model of the subsurface can be defined. There are two primary types of seismic surveys – two-dimensional (2-D) and three-dimensional (3-D).

• 2-D seismic surveys
  With 2-D seismic, the geophones are arranged linearly at regular intervals with the energy source points arranged along the same line at greater intervals. The resulting data is displayed as a two-dimensional vertical cross-section of the earth directly beneath the line.

• 3-D seismic surveys
  With 3-D seismic, the survey is laid out as a grid, often with receiver lines running perpendicular to the energy source lines. The resulting data is displayed as a three-dimensional cube from which can be derived planes or cross-sections at almost any angle. 3-D surveys over the same area shot at different times form a 4-D survey that measures changes in reservoir fluids and is used in development activities.

If structures are identified that fit economic criteria established by the exploration company, wildcat wells are drilled. Information needed to evaluate the structure is obtained from cuttings or chip samples brought to surface by drilling mud, cores cut through prospective layers, and from wire line logs run after the well is drilled which measure permeability, porosity and fluid composition. Logging can also be conducted continuously while drilling using mud pulse and measurement-while-drilling technology. Drillstem tests actually sample the fluids from selected prospective strata. The final series of tests conducted on a well are flow tests, which measure the flow rate in barrels per day (bbls/d) for oil and thousands of cubic feet per day (Mcf/d) for natural gas. If all tests indicate an economical well, the next stage is to complete or ready the well for production.

Where is natural gas found?

Natural gas reserves are found in sedimentary basins throughout the world. They are classified as recoverable, probable, proved and established reserves.

• Recoverable: both economical and non-economical reserves that can be produced with existing technology
• Probable: reserves believed to exist with reasonable certainty based on geological information
• Proved: reserves that can be economically produced with a large degree of certainty from known reservoirs using existing technology
• Established: generally defined as proved reserves plus one-half probable reserves