According to a report issued by the U.S. Energy Information Administration, there are an estimated 345 billion barrels of technically recoverable world shale oil resources and 7,299 trillion cubic feet of world shale gas resources. The report distinguishes between technically recoverable resources, meaning oil and natural gas that could be produced with current technology, and economically recoverable resources, meaning there is a worthwhile return on the cost of extracting the oil and gas. So the main challenge in oil and gas exploration is no different than in other mining exploration efforts—where are the most productive and profitable deposits?
To answer this question, geologists must be able to quickly evaluate the geological indicators of a high quality shale gas reservoir, or as it’s known in the business, find the “sweet spot.”
Shale gas deposits, or “plays,” are contained in fine-grained sedimentary rocks that contain clay and other fine-grained minerals. Shale gas plays are unconventional resources that are difficult to produce because their low permeability and high clay content keeps the gas trapped in the rock. This is why techniques such as hydraulic fracturing are used to release the gas.
Other unconventional resources include tight gas, coal bed methane, oil sands, and heavy oil. “Conventional” gas is derived from various naturally occurring rock formations such as carbonates, sandstones, and siltstones. Gas producing shales also have a high organic content originating from kerogen, a fossilized organic material. Kerogen content, or total organic carbon content (TOC), is an important indicator of overall reservoir quality. Over time, pressure and temperature changes convert kerogen into hydrocarbon, which remains trapped in the rock. Production potential and strategy is determined by the volume of hydrocarbon and how easy or difficult it is to recover. To figure this out, geologists carefully analyze geochemical and geological information: the mechanical properties of the rock (porosity and permeability); the presence of faults and fractures; thermal maturity; depth and thickness; and environment.
Techniques that can help to gain this data include inferring mineralogy, inferring lithology, and chemostratigraphy.
Mineralogy: Shale plays have complex mineralogy containing many major and trace elements. Among the minerals evaluated are quartz, feldspar, carbonate, clay minerals, and Fe-oxides. Mineralogy can be used to infer chemical and physical properties of the shale. High quartz or carbonate content, for example, indicates the shale is more brittle, which makes it easier to stimulate (frac).
Lithology: Lithology (commonly used during mud-logging) is the practice of examining rock chips that are brought to the surface by circulating drilling media (most commonly mud). Evaluating the fracability and permeability of the rock-types that are brought up plays a large role in shale play characterization.
Chemostratigraphy: Chemostratigraphy tells you the variation of chemistry within sedimentary sequences. Chemostratigraphic markers can be used to steer the well bore in real time or confirm the well bore path.
Field-portable x-ray fluorescence (FPXRF) is emerging as an important enabling tool in E&P operations because it provides valuable information about the mineral composition of the rock and whether or not it has properties favorable to oil and gas production. Although FPXRF cannot analyze hydrocarbons, they can be used to characterize reservoir properties that influence porosity (cements), permeability (clays, cement type), fracture population (Si content), and productivity (e.g. Si, Mg content). New FPXRF analyzers are able to detect light elements (Mg, Al, Si, P, S) to more accurately locate oil-bearing strata, improve mud-logging, and support geo-steering. By providing fast and reliable geochemical data at the drill, in the field, and in the core lab, FPXRF analyzers allow the geologist to predict:
- Where the oil and gas is in the rock formation
- What factors affect the porosity and predict the volume of oil and gas present
- How the permeability of the rock can affect the flow of oil and gas from the rock to the well bore
- How a rock formation can be engineered to produce more by fracturing and well treatments.
Read an application note about the use of XRF technology to analyze a variety of sample types common in the upstream E&P industry.