Carbonate facies in Los Molles Formation: record of depositational conditions in unconventional gas shale reservoirs

RAINOLDI ANA LAURA; FORTUNATTI NATALIA BEATRIZ; ITURAIN VERÓNICA ROCÍO

Buenos Aires

Simposio; VII Simposio Argentino Jurásico; 2019

Los Molles Formation (Cuyo Group-Jurassic) is a well-known hydrocarbon source rock of the Neuquén Basin, which has been focus as unconventional shale reservoir for the last years. Shale reservoirs look homogeneous, but when studied in detail, show significant differences in geochemical and reservoir properties. Identification of sedimentary facies are useful for depositational environment record, including both energy and geochemical conditions. Los Molles Formation is mostly composed of siliciclastic fine-grained mudrocks with subordinate carbonatic facies. This study reveals petrographic and diagenetic data of minor carbonate-rich lithologies present in Los Molles Formation, in order to characterize environmental and geochemical conditions of deposition, which strongly influence reservoir properties.Carbonate lithologies analyzed arise by dilution of extrabasinal siliciclastic sediment with intrabasinal calcite and carbonatic particles. Two main groups of carbonates facies were identified: host rocks with carbonate components primarily dominated by skeletal debris, and carbonate concretions mostly composed by authigenic calcite. Lithologies corresponding to host rocks include bioclastic wackestone, peloidal-bioclastic wackestone and peloidal mudstone. In all cases, bioclastic constituents are calciespheres, bivals and foraminifera. Calcite-bearing concretions contain dominantly authigenic carbonate together with bioclasts and peloids defining bioclastic wackestones and peloidal wackestone; bioclasts are the same than the observed in the host rock lithologies, but they differentiate because of the presence of authigenic calcite and higher peloids content.Identified skeletal components are characteristic of impoverished marine faunas consistent with low environmental oxygenation. This is also supported by the presence of thin-walled bivals, which result as an adaptation to low-oxygen conditions and soft substrates (Milliken et al., 2012). Presence and formation of carbonate concretions suggest that siliciclastic input was reduced, probably related to sea-level major flooding events where sediments remains for a long time in the eodiagenetic zone promoting cementation (Machent et al., 2007). The general lack of dolomite indicates that sedimentation did not occur under strongly reducing conditions (Milliken et al., 2012).Intermittent anoxia, minor participation of carbonate facies and high carbonate cementation has been determined in the Barnett Shale (Milliken et al., 2012), the most famous shale gas reservoir worldwide. This new data show similar characteristics of Los Molles Formation with the Barnett Shale that may have used as useful clues for reservoir characterization. Further studies focused in facies associations for host rock and concretions may help to identify better lithologic relationships, possible stacking patterns and support core/well log data to locate sweet spots into Los Molles unit.