Narrowing down the most favorable spot for oil and gas production can be like finding a needle in a haystack. Potential reservoirs are huge, but chemical analysis of drill cuttings, outcrops, piston-core sediment, and oil and gas cores can be used to determine the composition of rock and infer mineralogical properties favorable to oil and gas exploration and production.
Core analysis techniques, including X-ray fluorescence (XRF) analysis, help geologists measure these properties in greater detail. Elemental chemistry gives clues to the rock properties that could affect oil & gas accumulation like porosity (Si and Ca content), permeability (Si/Al, Mg, Ca, and K as proxies for clays and dolomite), and the presence of special minerals (e.g., clays, pyrite, and carbonate cement from Si/Al, Fe/S, and Mg/Ca ratios). This information can be included in the well logs, adding valuable information to aid in the interpretation of petrophysical data and offering greater value to the exploration program
Recent studies have linked the abundance of redox sensitive trace metals – vanadium (V), chromium (Cr), uranium (U), thorium (Th), molybdenum (Mo), and rhenium (Re) – to strata that are enriched in organic material. This abundance serves as an indicator of gas potential in shale. Accurate stratigraphic correlations in these monotonous sequences of shale can be enhanced by chemostratigraphic techniques, employing the major, minor, and trace element abundances and ratios. Handheld XRF analyzers can be used to rapidly log the inorganic geochemistry of cuttings and cores in the field in minutes.
To learn how these core samples are obtained, read this excerpt from the Rigzone article, How Does Core Analysis Work? for a description of a typical core sampling process:
A core is a sample of rock in the shape of a cylinder. Taken from the side of a drilled oil or gas well, a core is then dissected into multiple core plugs, or small cylindrical samples measuring about 1 inch in diameter and 3 inches long. These core plugs are then dried and measured.
In order to complete a core sample, drilling must be halted at the top of the subsurface of the reservoir. The drillstring is removed from the wellbore, the drillbit removed and a rotary coring bit is attached in its place. Similar to a drillbit, the rotary coring bit consists of solid metal with diamonds or tungsten for cutting at the reservoir rock; but unlike a drillbit, a rotary coring bit has a hollow center.
On a rotary coring bit, the cutting apparatus surrounds a hollow center, called the core barrel, where the core sample is stored. This core barrel is made up of an inner and outer barrel separated by ball bearings, which allow the inner barrel to remain stationary and retain the core sample while the outer barrel is rotated by the drillstring and cuts the core.
The core catcher is located within the core barrel. The core catcher has finger-like apparatuses that move the core sample farther into the barrel and keep it from falling back into the well.
After the core sample has been cut from the well, the drillstring is raised, and the rotary coring bit, barrel and catcher are removed — and the core sample is retrieved. The drillbit is reattached, and drilling can commence again.
When performing coring operations, instead of having the pieces of broken rock removed from the well via drilling fluids, the rock is kept intact and raised to the surface for study. Because coring requires the suspension of drilling, the process is quite expensive and usually only performed at the reservoir interval.
The geochemical characteristics of the rocks analyzed with XRF can be used to infer rock properties that are favorable to oil & gas. There also are benefits in using portable XRF in the lab because the analyzer results have the ability to create detailed cm scale logs during the core logging process.