A paleoenvironmental model of the upper section of the source rock Pozo D-129 formation was generated in the Cañadón Seco field, located in the Golfo San Jorge basin southern flank. This model allows us to identify development opportunities and book reserves.The information of well log analysis, production tests, cutting lab analysis (thin sections - SEM) and seismic interpretation at the top & base of the sequence by identifying structures and geological bodies, allowed us to build the geological model for Pozo D-129 formation which could correspond to a transitional lacustrine shelf zone (proximal to distal) mostly composed of fine material from carbonate shore facies of low energy (oolitic limestones and fine grained sandstones) to mudstones with dissimilar carbonate content and abundant pyroclastic interbedding. Lab studies showed an important diagenesis with the formation of secondary porosity, mainly due to carbonate dissolution and dolomitization. These reservoirs are very good gas traps and have a strong structural dependence with a closure against faults. According to lab analysis there are regions with more than 15 % of secondary porosity, with very good connection between pores.Only three wells drilled Pozo D-129 formation upper section in the area, and they were hydraulically fractured. Several methodologies and treatment designs were used to minimize the formation damage caused by the fluids in order to contact as much reservoir as possible. The maximum gas production measured was 100 Mm3/d, with an estimated of 50-100 MMm3 gas reserves per well.Based on the good results, a development plan for the block located in the south-east of the field was proposed, it includes future deepening of old wells and new well drilling, with 750-1500 MMm3 gas in place for this block. In addition, stimulation of lower new zones into the section in the same wells are proposed, that could even more increase gas reserves in the block. The challenge for the future is to define if this reservoir is a mixture between Conventional, Tight Gas or Unconventional, due to lithological characteristics and the production behavior. For that reason, with the development of the block is also proposed a data acquisition plan including lab analysis and other studies to better understanding of the reservoir.Also, this study was the kick-off to evaluate the Pozo D-129 fm. as a new target in other fields of the southern flank of the basin, for instance in El Huemul and Meseta Espinosa fields.