Zooming In on Shale to See the Big Picture

By Stephen Rassenfoss 28 Aug 2014

The search for unconventional oil and gas reserves has kindled increased interest in studying the structure of the fine-grained rocks where oil and gas is trapped in spaces so tiny they can only be seen with the most powerful scanning electron microscopes.

Some companies have begun using these tools when making decisions on what prospects to drill and how best to develop them. Among those, Whiting Petroleum stands out.

Sampling based on images measured in microns raised a tough question: Is that location representative of the rock around it? This led to the development of hardware and software able to create larger 2D images of the rock fabric. The images below were created by a partnership between the University of Oklahoma and FEI, which developed the automated system used to create and stitch together thousands of images. Each map is made from 2,000-25,000 individual images.

Using the Map

Click to zoom in. Scroll to zoom in or out. Use the links to load a new map (e.g., "North American Shale") or a specific part of that map (e.g., "Porosity filled organic material.") You can also use the icons in the upper left corner to zoom or return to the original size.

North American Shale

The nature of organic matter observed in shale is highly heterogeneous in this gas-producing shale. There is non-porous, highly fractured organic matter mixed with organic matter that preserves a highly porous structure. The former is likely to be non-productive because of its limited ability to transport hydrocarbons, while the latter is typical of a productive gas shale. Although the organic matter appears disconnected in 2D, when a 3D model is created using a focused ion beam and scanning electron microscopy (FIBSEM), it becomes clear that much of the organic matter is interconnected.

Porosity-filled Organic Material

The many pores present suggest the presence of gas because they are thought to be related to catagenesis, the natural process that converts organic matter into hydrocarbons. Such porous organic matter is also likely to be good for transporting hydrocarbons.

Understanding the Line of Death

There is an area on the northeastern edge of the Marcellus, known as the Line of Death, where the risk of unproductive wells rises. The rock fabric observed in these images offers visual evidence of geologic differences between productive and non-productive Marcellus rock.

  • Marcellus Shale—Producing Side: This productive rock from a Marcellus well near the line displays a high total organic content compared with a non-productive well. The pores of organic matter are well-connected in this 2D view. These point to a natural transport network and a strong gas production. This sample also contains the fractured and non-porous organics common on the non-productive wells along the line.
     
  • Marcellus Shale—Non-Producing Side: The sample from the unproductive well is dominated by highly fractured, non-porous organic matter. The organic content is far lower than seen on the producing side. The non-porous, fractured nature of this organic matter offers poor transport properties, which all mean little production.

Stephen Rassenfoss is the Emerging Technology Senior Editor for the Journal of Petroleum Technology.