Since the discovery of the Vaca Muerta shale as a commercial play in 2010, one characteristic that has stood out is the formation’s incredible thickness – measuring roughly 1,000 feet in its northern reaches.
As geophysical companies work to characterize the shale using seismic data, some are finding that, in addition to its thickness, the shale is unique in terms of anisotropy.
“With the interest in developing and exploiting shale resources in Argentina, many companies have undertaken characterization of the Vaca Muerta formation in terms of the elements of shale plays,” said AAPG member and award-winning geophysicist Satinder Chopra, chief geophysicist, Reservoir, with Arcis Seismic Solutions, TGS, in Calgary, Canada, and editor of the EXPLORER’s Geophysical Corner. “But we are finding that the characterization workflows that have been applied to some of the shales in the United States and in the North Sea do not seem to be applicable to Vaca Muerta.”
As a result, geophysicists such as Chopra are developing new ways to help identify the sweet spots of Argentina’s emerging shale play.
Not only were Chopra and his team able to successfully characterize Vaca Muerta and subsequently identify ideal locations for exploiting its shale, they have applied their new characterization method to similar shales in Canada, namely the Montney and the Duvernay formations in Alberta.
Hesitant to call the new method of characterization a “breakthrough,” Chopra instead said, “This workflow has the potential for good application to other shale plays around the world.”
A Successful Shale
For a shale play to be successful, it must have the following key elements:
- High total organic content (TOC).
- Oil/gas in place.
- Pore pressure.
Vaca Muerta, a formation of the Late Jurassic-Early Cretaceous periods in the Neuquén Basin, exhibits all the above elements in a favorable way, Chopra said, explaining that it has served as an important source rock for many conventional oil and gas fields in Argentina for decades.
“An optimum combination of these elements leads to favorable productivity,” Chopra said. “The geology of the Neuquén Basin is favorable.”
Yet, the workflow implementation for the characterization of Vaca Muerta and subsequently turning it into a successful unconventional play has required a trial-and-error exercise using well and seismic data.
Approached by Pan American Energy to help characterize the formation that the company operates in two blocks, Chopra and his colleagues set out to find the sweet spots that could be exploited for oil production through horizontal drilling and hydraulic fracturing operations.
“Vaca Muerta is a world-class shale play that has unique characteristics that are different from others of its kind,” said Luis Vernengo, Pan American Energy’s head of geophysics. Vernengo worked with Eduardo Trinchero, a senior staff geophysicist, also of Pan American Energy, on the project.
“We knew it was necessary to find other ways of analysis to take into account its specific characteristics and anisotropy,” Vernengo said.
As with most shale plays, the key to successful development and production is finding a high TOC.
“Better TOC leads to better production,” Chopra said.
However, TOC cannot be directly measured using seismic data. It must be estimated indirectly.
Because TOC affects compression velocity (P velocity), shear velocity (S velocity) and density, geoscientists have – in the past – attempted to compute TOC using a linear or non-linear relationship it might have with P-impedance, Chopra said. However, using that approach in Vaca Muerta leaves an element of uncertainty that is too great for an accurate characterization to be made, he said.
“We know that we needed to characterize the Vaca Muerta shale from a different and more efficient point of view,” Vernengo said. “The difficulty came when it was time to find a tool or workflow that combines the interpretation of TOC and special seismic processes.”
“There is evidence of a linear relationship between the uranium content in shale and its organic content. As a result, a large gamma ray (GR) response is expected for organic-rich shale formations,” Chopra explained.
Because a linear relationship appeared to exist between GR response and TOC, as well as with P-impedance, GR was seen as another parameter of interest for characterizing the Vaca Muerta formation, Chopra said.
“This is a much better relationship,” Chopra said. “High GR and TOC are characteristics of a better quality shale play.”
Using P-impedance and GR volumes, Chopra explained that a Bayesian classification approach was followed to obtain a reservoir model with different facies based on TOC and the associated uncertainty with it.
The first step included defining different facies based on cut-off values for GR and P-impedance computed from well log data, Chopra said. The second step required Gaussian ellipses to capture the distribution of data in a crossplot of GR versus P-impedance.
Then, 2-D probability density functions (PDFs) were created from the ellipses for each of the facies. By combining the PDFs with GR and P-impedance volumes, different facies were identified on the 3-D volume. Post-stack, model-based inversion was used to compute the P-impedance volume, and a probabilistic neural-network approach was used to compute the GR volume. (Only stacked seismic data was available for this case study.)
“Derived P-impedance and GR volumes correlated nicely at blind wells on the 3-D volume, and that gave us confidence in the characterization of the Vaca Muerta formation,” Chopra said. “An overlay of the discontinuity detail in terms of curvature lineaments on the determined TOC content at the level of interest helped in obtaining a more complete picture, which is useful for the planning of horizontal wells.”
“It can be concluded that the quality of the Vaca Muerta formation increases with depth, which is trustworthy based on the known geological history,” Chopra added.
Believing their chances of a successful discovery have increased as a result of this new approach, Vernengo said Pan American is in good standing in the Vaca Muerta shale patch.
“This tool allows us to efficiently characterize the Vaca Muerta formation in our area, and in any other area of the same basin, through the knowledge and special distribution within the shale associated facies,” Vernengo said. “Because of this, we believe that Pan American has advanced knowledge that will help tackle the challenges of shale reservoirs.”
Chopra and his team published a paper, entitled “Reducing uncertainty in characterization of Vaca Muerta Formation Shale with post-stack seismic data,” on their approach, which can be found at the TGS website (TGS.com).
Determining TOC in shale-resource reservoirs is a “desirable” goal in most projects that aim to characterize unconventional reservoirs, Chopra said.
“This opens the door for more detailed work that should be done to characterize the Vaca Muerta formation,” Chopra said. “This workflow has demonstrated that it has the potential for application to other shale plays around the world.”
Saying he is “quite happy” with the results, Chopra said that going forward, he and his team will focus on determining the brittleness of the formation using pre-stack data. “We will be working on this in more detail,” he said.
“You might think the name Vaca Muerta, the Dead Cow, is irrelevant, but is it?” Chopra asked. “They should call it the Vaca Viva, which is the Living Cow, which Argentina can look forward to for decades.”