In the Neuquen basin, center west of Argentina, a tight gas field was developed in submarine volcanic rocks. The field called Cupen Mahuida is a faulted anticline, which produced oil and gas from the upper formations. This deeper reservoir was discovered in 2001. The average depth is 3500 m and there are 16 wells producing from these rocks. The production is dry gas with a low CO2 content.
Because the low permeability hydraulic fractures are needed to produce the wells and the productivity of them is highly variable. The pressure behavior during the fracture pumping showed the presence of natural fractures.
These highly variable results in the wells pushed to get a more predictable reservoir model, and then 99 m of cores, 58 sidewall cores and 8500 m of cuttings were described in detail. Based on the information obtained the geological model was adjusted. Was found that these rocks were originated in volcanic eruptions both sub aereal and submarine and the eruptive products were deposited in a water body. There were several volcanoes feeding the area and were located about 20 km away from the deposits, and then the deposits have a distal position from the volcanoes. A probable volcanic center in the NE of the field was identified. Only one event has had sub areal exposition and could be use as a local correlation level.
Micro fractures due to cooling, gas escapes, etc that are common in this kind of rocks when they are deposited sub aerealy are not present. The primary porosity was completely occluded and the current porosity is due to mineral dissolution. Open natural fractures are present but the density is low as well as the fracture width, some tectonic micro fractures were identified.
The detailed rock analysis allowed to define the depositional environment, this knowledge changed the approach to analyze the porosity distribution.
There are micro fractures, although this is not the largest source of porosity, primary porosity was destroyed because the rock alteration, the main porosity source came from the mineral dissolution.
The good results in the first wells drilled in the volcanic reservoir encouraged to decide a more aggressive drilling campaign, although all the new wells tested gas, the production obtained from them did not fulfill the expectative because the highly variable gas rates obtained.
At a very beginning, the well logs did not show evident differences among the good producers and the poor ones. The log analysis did not provide a clear answer. The first approach was to treat the reservoir like a naturally fractured reservoir and to award the differences in productivity due to the amount of fractures connected with the wellbore.
Nevertheless the images logs did not show many open fractures to justify the differences in production related with the fracture network development. Then the problem was that the geological model couldn't explain the wells behavior, for this reason it was taken the decision of review all the rock samples under a unified vision to adjust the geological and the petrophysical model.
After a detailed rocks revision (including thin sections, electronic microscopy, X ray, etc) a complete new vision of the reservoir was made. The main conclusion was that nevertheless all the rocks have had a volcanic origin all of them were deposited under the water, changing the original vision that have considered a subaereal deposition environment. This conclusion has a high impact in the porosity origin a distribution, because under the submarine environment the microfracturing of the rock due to cooling, gas escapes, etc (Very common in subaereal volcanic deposits) didn't happen.
Other important conclusion was that all the primary porosity was occluded due to mineral deposition and the existent porosity was originated for glass and mineral dissolution during the diagenetic process.
A little porosity fraction is related with microfractures and those microfractures are related with the overburden.
Finally the presence of minerals deposited during the digenetic process has played an important role in the porosity variation and distribution.
The field is located in the Neuquen basin, center west of Argentina (see fig 1). Is a faulted anticline (see fig 2). The field is a deeper reservoir of an old oil field that produces from the formations deposited immediately below Vaca Muerta fm. (see geological column, fig 1).
The reservoir was discovered in 2001 when the exploration department decided to explore the deepest portion of the Neuquen basin. Many gas accumulations were found in subaereal volcanic deposits of upper Triassic- lower Jurassic age at that time. Those volcanic deposits were found immediately bellow the Molles fm (Middle to lower Jurassic age). Because in Cupen Mahuida volcanic rocks were found below Molles Fm. an upper Triassic-lower Jurassic age was suggested and the environment was interpreted as subaereal volcanic type (Pangaro et al, 2002) based on several regional studies (Francese et al 2006, 2007;.Llambias et al 2007; Vergani et al 1995).
These rocks were subdivided in two sections, the upper one was called "Precuyano Superior" and the lower one was called "Precuyano Inferior". None of the wells have drilled the whole section; from the seismic the thickness of these deposits was estimated in more than 1000 m in the thicker portion of the depocenter (see Fig 3). Towards the edges of the depocenter the Precuyano Superior disappears. But in the field only the Precuyano Superior have had gas production, in a neighbor structure out of the thickest depocenter zone the lower section has produced gas.
All the zones in all the wells were fractured to get a commercial production. The fractures were conventional fractures and some of them have showed the presence of natural fractures during the process of pumping the fracture.
From the very beginning the rocks descriptions was challenging because the geologist had to deal with non common rocks for the oil industry as the volcanic rocks are. During the field development, different people have made rocks descriptions, but most of them only have described the rocks from a petrological point of view but not many assumptions related with the environment or with the origin of porosity/permeability were done.
Because the disappointing result, especially in the last wells, the decision of redescribed all the rock samples under a unified vision was taken. All the cuttings, sidewall cores and cores were sent to a lab and a group of professionals have made a detailed description. When a considerable amount of information was available a group of volcanologists was hired to discuss the possible volcanic environment and the origin of porosity.
All of the volcanologist concur with the laboratory descriptions and fundamentally with the submarine environment of these rocks. Was described a single deposit, which coincides with the top of the "Precuyano Inferior", present evidence of subaereal exposure and has an extension that allows his use as a marker level in the field.
Other important conclusions related with the environment was that there were several volcanoes feeding the area and it were located about 20 km away from the deposits, because it the deposits have a distal position from the volcanoes. Besides samples of organic matter and remains of the marine fauna were found into de volcanic deposits, for these reason a submarine environment was preferred instead of a lacustrine one that was the other possibility managed. This fact is a key point and based on these facts we can conclude that these rocks were deposited in a sea floor and are part of the base of the Molles Fm.
The origin of the porosity determined that most of the porosity is due to dissolution of different constituents of the rock and a little porosity fraction is related with microfractures related with the overburden as is shown in the figures 4 a, b and c.
Based on the rock descriptions and the well logs and image logs, six litofacies were described. In figure 5 is shown the maximum and minimum porosity and permeability that were found in each of them.
With the litofacies described in all of the wells we have a powerful tool to make a good correlation of the events.
Now it has set a vision of the deposition environment and age of these rocks.
It was established that the porosity is related to diagenetic processes, rather than the original terms of deposicion.
The distribution of porosity and permeability is related to the dissolution of various components of the rock and is therefore more difficult to follow that if any factor was related to depositional environment The way to fracture these rocks has to be analyzed again in order to determine if there is a more effective way to fracture it.
The possiblity of making of hybrid fractures is under analysis.
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The authors are grateful to YPF SA by authorizing the presntacion of this paper.