Unconventional plays in the Permian Basin are nothing new to the shale scene, as the basin has been horizontally drilled and hydraulically fractured for years.
Still, a new approach for evaluating sweet spots, particularly in the Avalon Shale of the play's Delaware Basin, is slowly changing the way unconventional resources are explored - and perhaps one day, developed.
It all started in 2011, when the University of Kansas Interdisciplinary Carbonates Consortium (KICC) was approached by ConocoPhillips and Devon Energy to further study the Avalon Shale with the hope of making the play more economical, explained AAPG member Robert Goldstein, Haas Distinguished Professor in the department of geology at the University of Kansas.
"It's a hot play," Goldstein said. "We wanted to see what we could do to improve the success in that play by applying concepts you would normally apply in conventional reservoirs."
So Goldstein, along with AAPG members Evan Franseen, professor and Senior Scientific Fellow in the University of Kansas geology department, and Dustin Stolz, a geologist at Samson Resources and a graduate student at the University of Kansas at the time, put their heads together and began looking at the Permian Basin in unconventional ways for an unconventional play.
For decades, shale primarily has been looked at as a source rock during the course of exploring conventional reservoirs.
Having worked on the Mississippi Lime Shale play for years, Franseen saw firsthand how source rocks became plays of their own during the shale oil and gas revolution.
"The devil is in the details for these source rocks, and it is important to look closely and understand the details in order to produce from these rocks," Franseen said.
Wanting to better understand the Avalon Shale, Franseen, Goldstein and Stolz - who eventually would take on the project for Stolz's master's thesis - began an evaluation process of the Delaware Basin, with the ultimate goal of mapping and understanding the sweet and not-so-sweet spots.
"I was fortunate to be able to work the Avalon Shale during an internship with ConocoPhillips, which allowed the study to be aimed at specific questions the industry faced in this play," Stolz said.
The project was based largely on evaluating the distribution of sediment gravity flow (SGF) deposits and how they shaped the reservoir properties of the Avalon Shale.
Where It All Began
The Avalon Shale is a mix of organic-rich siliciclastic mudstones interbedded with carbonate-rich deposits - so to call it strictly a "shale" play is misleading, Goldstein said.
"This is a great example of carbonate-rich lithology treated as a shale play in that there is horizontal drilling and hydraulic fracturing of the system," he noted. "It also has low permeability overall, but it follows the sedimentary rules of a carbonate system."
To better understand the play, the three started with the properties of the rock itself, with a focus on which rock properties led to good reservoir and which led to poor reservoir, Goldstein said.
After understanding the primary controls on carbonate-rich versus non-carbonate-rich strata, they would then need to determine how siliciclastic mudstones and carbonate-rich deposits got into the basin - and where they ultimately settled. The answers to those questions would be key to pinpointing the location of hydrocarbons, Franseen explained.
Using hundreds of well logs and data from cores, the team learned that many carbonates likely were deposited via SGFs, when carbonate platforms or ramps that surrounded the basin gave rise to apron, sheet, fan and linear geobodies in the basin center.
Storms, sea-level changes, climate change and bathymetry all could have played a role in transporting carbonates from shallow water, where they are typically formed, into the deep areas of the Delaware Basin and stretching for miles. Many of the SGFs in the basin's center were found to be rich in carbonate and low in porosity - not an ideal unconventional reservoir.
Yet, areas near the margins of those flow systems contain mixtures of carbonates and siliciclastic mudstones, making them more porous with a higher silica content and a lower carbonate content, and containing more organic matter. In other words, the margins of the SGF flow systems were better for drilling.
Franseen explained that while identifying rock type is important, so is dividing the basin into "time slices" so that one can see how a basin was formed over time. That is what helped guide them to the margins of these systems and to particular locations, rather than the basin center.
"This is where the better reservoirs are," Goldstein said of these areas. "The focus of flow of the SGFs into the basin will have a huge impact on where the most productive wells are found."
The systems and processes that form the SGF deposits are complex, and the areas that focus flow can vary through time, Stolz explained.
"Ultimately, these changes affect where the system margins, and better reservoir, are deposited and can create a complex distribution of reservoir rock within the basin," he said.
In addition to understanding the basin's sedimentology, a thorough evaluation of the basin's stratigraphy and diagenesis - just as would be done in a conventional reservoir - is equally as important to mapping an unconventional reservoir, Goldstein said.
"We study the rock and try to understand the factors that control where the best unconventional reservoir is. If you understand those three factors - sedimentology, stratigraphy and diagenesis - you have a much better chance of identifying the sweet spots," he said.
The Final Product
Having combed the Avalon Shale in the Delaware Basin from top to bottom, the team eventually was able to produce a map identifying areas likely to contain the most economically viable deposits.
"From the base of the basin to the top, we could identify the sweet spots, which were the thickest accumulations of these muddy rocks," Franseen said. "The study results give a first-level prediction as to where you should concentrate in the basin for finding oil - and also where to stay away."
Unlike in conventional reservoirs, the properties of rocks in unconventional reservoirs don't necessarily "hit you in the face," Franseen said.
"In the past, they have been looked at more as source rocks," he said, "but now the details that are coming out of them are really important for actually trying to produce from them."
The team expects that these details will help in the hydraulic fracturing of these reservoirs, and that such information could be applied to the development process.
"We have not gotten all of the rock property details yet in this study, but getting those details is important to predict where to frac," Franseen said.
A New Trend?
Using conventional applications in unconventional reservoirs is a practice not widely implemented today, but Goldstein said he could see it gaining momentum.
"In the early days, it wasn't done to this extent in unconventional systems. But you eventually come to realize that an awful lot of stratigraphic, diagenetic and sedimentologic variables are associated with unconventional systems," he said. "To make these plays profitable, you must find the best unconventional reservoir rock."
While the teams' study applied solely to the Avalon Shale, their applications can be applied to similar unconventional systems, Goldstein said.
"Not many people are doing this yet," Goldstein added. "But it's definitely on the upswing."