SIDEBAR 1-6 A New Use of Cuttings: Analysis of Fluid Inclusions

Fluid inclusions are micron-sized liquid- or gasfilled cavities that occur in many rocks. They are formed when minerals and cements crystallize, trapping samples of interstitial fluids present at the time of their formation. The composition of fluid inclusions is not altered by removal from the subsurface, nor are their contents modified by storage over time.

Some fluid inclusions are large enough to be seen with a microscope. Commonly, however, inclusions are very small and cannot be resolved by optical methods. Energy-industry scientists have developed techniques to analyze these small amounts of trapped fluids. Down-hole profiles derived from fluid-inclusion analyses can be used to determine zones of hydrocarbon migration, proximity to potential reservoirs and, in some cases hydrocarbon contacts. In a representative example, cuttings from a well drilled in 1983, long before these techniques were developed, were analyzed in 1999. The hydrocarbon content of the fluid inclusions led to a completely different reappraisal of the source area. Drill cuttings, many of which contain fluid inclusions, are proving to hold a wealth of under-utilized information, underscoring the potential benefit of long-term preservation of such material.

SOURCE: ExxonMobil Upstream Research Company, personal communication, 2001.

Fluid inclusion (L) containing a bubble of gas (or vacuole, V), all within a single crystal from a cutting. New technologies now allow extraction of previously unimagined information from Earth’s library of rock. SOURCE: ExxonMobil Upstream Research Company.

Such evaluations are critical to urban planners and designers of construction projects in earthquake-prone regions. In 2001, core data were critical in averting a further disaster when natural gas and brine escaped from an underground storage facility in Kansas (see start of Executive Summary, and Figure 1-4).

Another example of unanticipated use comes from ice cores. Sampling techniques developed years after some cores were taken have allowed climate researchers to examine changes in the carbon dioxide (CO2) content of the atmosphere (see Sidebar 1-7) from as long as 420,000 years ago. In Virginia, maps of mine locations have been used to assist in emergency response planning should accidents happen in old mines. Law enforcement officials in North Carolina used similar maps to search for fugitives from justice. Geoscience data even have been used for genealogical research through old lease and mining data.

FIGURE 1-4 Flames from Hutchinson, Kansas, January 17, 2001. This fire and a fatal one in Hutchinson the next day originated when natural gas migrated through rocks from a storage facility 8 miles away (see the first page of the Executive Summary for a fuller account). SOURCE: Hutchinson, Kansas Fire Department.

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