Asking the right question

Conventional Reservoirs Hold Keys to the ‘Un’s

American Association of Petroleum Geologists (AAPG)
Figure 1: Essential Elements of a Quality Reservoir
Figure 1: Essential Elements of a Quality Reservoir

Last November, the AAPG Geosciences Technology Workshop (GTW) on Unconventional Reservoir Quality was held in Austin, Texas, and many of the presentations given during the session focused on porosity development, a key element of reservoir quality.

Given that reservoir quality (or storage) is one of the fundamentals of any reservoir – and since what we really want to do is get hydrocarbons out of the ground – we might consider turning the phrase around. Rather than discussing “reservoir quality,” we should consider what makes a “quality reservoir.”

Placed in a quality reservoir context, we need then consider all of thefi elements required to make a reservoir successful. All successful reservoirs, whether conventional or unconventional, must have the same fundamentals: storage, conductivity and drive. When these basic elements come together in appropriate combination, a rock unit then can be considered a quality reservoir (figure 1).

Storage in an unconventional reservoir often is associated with organic material – however, storage can commonly be associated with inorganic grains (primary and secondary) and with fractures. The primary difference between an unconventional and conventional reservoir is that the conductivity of an unconventional reservoir usually requires enhancement via hydraulic fracturing to be commercial.

Successful reservoir stimulation requires an additional subset of rock properties – the ability of the rock to fracture complexly (brittleness) and to maintain the induced fractures (stiffness). Natural internal reservoir fracturing can provide enhanced conductivity and some storage, although extensive tectonic fracturing may breach seals and rob a reservoir of the energy required to be successful.

The third fundamental, sufficient reservoir energy, also is required, and it must be retained over geologic time by sealing lithologies. Most mudstone successions contain clay-rich and clay-poor intervals. Both are needed to act as reservoir (mudstones) and seal (claystones) for pressure retention.

Each element of a quality reservoir has some forgiveness. For example, slightly lower conductivity can be compensated by increased storage or reservoir energy.

However, there are limits – beyond which the elements in a rock unit will fail to become a quality reservoir.

Figure 2: Composition of Quality Reservoirs
Figure 2: Composition of Quality Reservoirs

For source-rock reservoirs, the triangle in figure 2 represents a classification scheme that illustrates the primary properties for some of the major plays that control the enriched reservoir and conductivity elements important to make a quality reservoir.

Classification by elements may help describe some of the boundaries that make shale plays successful.

The horizontal axis is the hard component percentage, which is the volume percent of the hard/brittle elements (minerals) minus the soft/ductile elements (clay, TOC and porosity). Since clay is most often the element contributing to ductility, the triangle is divided vertically by whether a rock is primarily clay dominated or mud dominated.

Horizontally, the triangle is subdivided into segments of mature enrichment, when vitrinite reflectance (Ro) is greater than 1 percent.

Note that the organic content is displayed as a volume percent and not weight percent.

Most of the successful plays group together with similar properties in a class we call “Organically Rich Mudstones.” This area is where the hard elements exceed the soft ones and the enrichment of organic material is sufficient to provide storage and hydrocarbons, but not so much to soften the rock enough to diminish induced reservoir conductivity.

These characteristics of hardness and enrichment are primary elements of quality reservoirs.

For any given well or wells in a play, the range of the points on figure 2 could spread across the chart. However, those points that are quality reservoir will almost always plot as organically rich mudstones. The geologic conditions that come together to serve as the fundamental elements for quality reservoirs can be mapped to identify sweet spot areas:

  • Storage and conductivity through regional geology, sedimentology and sequence stratigraphy.
  • Drive through pressure data, burial history and basin modeling.

In summary, all successful reservoirs share the same fundamental attributes in appropriate combinations, and there are various ways to arrive at an appropriate combination.

Thus, we can use our understanding of conventional reservoirs – along with an appreciation of additional factors (organic-associated pores, ductile vs. brittle components, fractures, etc.) – for insights into what makes a quality unconventional reservoir.

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