The Chihuido de La Salina field is located in the folded thrust belt of the Neuquén Basin in west-central Argentina, about 200 kilometers northwest of the city Neuquén (Figure 1). The field contains several fault blocks, which produce both oil and gas. Generally, production is controlled by a thrusted anticline, structurally high in the north and relatively low to the south. The most important portion of the field produces oil from the steeply dipping flanks. Efficiently drilling the swelling clay formations of the Neuquén group (Upper Cretaceous), in Chihuido de La Salina field (ChLS), has been a distinctive challenge and is the subject of this paper's analysis and multiple-well case studies. Clay Mineralogy To better understand the clay swelling issues in Chihuido de La Salina field, a basic review is provided of the Neuquén group shale formations. The group consists of terrestrial sediments (Figure 2) laid down during the Rio Grandico sedimentary cycle (Upper Cretaceous). The formations contain two main shale constituents including smectite (70%) and illite (20%). In the smectite shales, montmorillonite is the most critical mineral with regards to swelling. If an atom of aluminum (Al3+) is replaced by an atom of magnesium (Mg2+), it will cause an additional electron or negative charge (Figure 3). The net negative charge is compensated by cation adsorption on the surfaces of the external/internal structure and is likely to cause a phenomenon know as "exchangeable clay cations." These charged cations can be either an ion of sodium (Na+) or a double charged ion of calcium (Ca2+) or magnesium (Mg2+). This reaction produces either sodium montmorillonite, calcium montmorillonite and/or magnesium montmorillonite. The change in clay mineralogy reduces the strength of the repelling forces between layers allowing water to enter and occupy the intra-layer space. Smectites have an expandable structure which increases colloidal activity due to a significant increment of specific surface. All of the structure's surfaces, including intra-layers are available to hydration and cationic exchange. These characteristics give montmorillonite the capacity to swell between layers due to hydration (Figure 4). Although illite clays have the same basic structure as montmorillonites, they don't normally display the same swelling characteristics. In Chihuido de La Salina field, the 8–1/2" hole section must be drilled through formations that contain 70% of the problematic smectite shale constituent, negatively impacting drilling economics. Background - Drilling the Neuquén Group Initial field development began in 1995 using water base mud (WBM). Despite the presence of a potassium chloride inhibitor in the mud, the operator was experiencing severe lost-time incidents related to swelling clay formations in the 8- 1/2" hole section including excessive trips to ream, wash-over, free stuck pipe and in the worst case scenario, sidetracking due to BHA lost-in-hole. To help improve field economics, the operator began utilizing oil base mud (OBM) in 2003 significantly reducing the clay swelling issues. However, it became necessary to increase the mud weight to 10–11 ppg in order to stabilize the hole and limit salt intrusion into the wellbore slightly reducing drilling efficiency. In spite of the OBM success, environmental issues forced the operator to return to WBM in 2006, resulting in the same lost-time incidents previously associated with the swelling clay formations in the Neuquén group.