Abstract

For most of a geoscientist’s career, shale is that guck rock that everyone knows gives shows but never amounts to much production. In our energy hungry society, we are looking at the “unconventional” parts of the hydrocarbon resource base in order to keep up with the demand for hydrocarbons. As a result, so-called unconventional reservoirs, such as ultra tight sands, shale, and coal have become a viable exploration target. Development of these reservoirs is known to be more costly and technically challenging. Wireline logs can make it effective.

During the decade of the 1990’s, there were numerous unconventional reservoir research projects funded by the Gas Research Institute and DOE designed to begin understanding the resource potential from unconventional reservoirs. This research provides the basis for identifying and quantifying unconventional reservoirs across the US. Many of the exploration and evaluation techniques identified by this research show that there is a similarity between shale and coal reservoirs. Although each of these reservoir types is unique, many of the wireline logging tool analysis techniques applicable to coalbed methane (CBM) analysis can also be readily applied to shale reservoirs.

Figure 1 is a cross section of commercial shale reservoirs from the east coast to the west coast of the USA. The wireline log characteristics in all these reservoirs have distinct similarities. They are, in general, elevated resistivity, “hot” gamma ray readings, lower bulk density measurements (see Figure 2) and some degree of natural fracturing (Figure 3). Combining a series of curve overlays is a simple technique that works to identify organic rich shales that could be potential reservoirs (Figure 4). Evaluating the gas content and gas in place of these shale reservoirs is accomplished in a similar manner to its distant cousin, CBM, (Figure 5). The magic bullet for any unconventional reservoir play is the bulk system permeability. The most reliable method to derive the permeability in the system is through the analysis of small volume diagnostic fracture injection testing (Figure 6).

Another type of shale reservoir is composed of thinly bedded sand and shale laminations. These reservoirs are difficult to recognize using conventional wireline logs. Resistivity Image logs can be of great assistance in identifying these types of reservoirs (Figure 7), by visually showing the laminations.

Unconventional reservoirs often require unconventional log analysis. One technique for locating clusters of thinly laminated sands uses a differential analysis technique (Figure 8). While porosity and water saturation are not readily measured due to the vertical resolution of logging tools, taking the first derivative of the basic logging tool measurements often highlights the clusters of thinly laminated sand sequences.

So where does one start looking for possible unconventional reservoirs such as shale? A good starting point is right amongst existing production. There is an abundance of wireline logs in the public domain that can be used for prospect exploration, shale maturation studies and pilot projects by applying the methods described in this presentation (Figure 9). Let’s do it!