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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. Conclusions: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.