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

RAINOLDI, ANA LAURA; FORTUNATTI, NATALIA; ITURAÍN, VERÓNICA ROCÍO

Buenos Aires

Simposio; VII Simposio Argentino del Jurásico; 2019

Universidad de Buenos Aires

Los Molles Formation (Cuyo Group-Jurassic) is a well-known hydrocarbon source rock of the Neuquén Basin, which has been regarded as an 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 depositional environment interpretation, including 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 result from dilution of extrabasinal siliciclastic sediment with intrabasinal calcite and carbonatic particles. Two main groups of carbonate facies were identified: host rocks with carbonate components dominated by skeletal debris, and carbonate concretions mostly composed of authigenic calcite. 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 content is similar than in the host rock, but they are richer in authigenic calcite and peloid content.Skeletal components are characteristic of impoverished marine faunas, consistent with low environmental oxygenation. Thin-walled bivalves are the result of an adaptation to low-oxygen conditions and soft substrates. Formation of carbonate concretions suggests reduced siliciclastic input, probably related to major flooding events with sediments remaining for a long time in the eodiagenetic zone, promoting cementation. The general lack of dolomite indicates that sedimentation did not occur under strongly reducing conditions.Intermittent anoxia, minor participation of carbonate facies and high carbonate cementation has been determined in the Barnett Shale, the most famous shale gas reservoir worldwide. Similar characteristics of Los Molles Formation and Barnett Shale may have used as useful clues for reservoir characterization. Further studies focused in facies associations for host rock and concretions may help to better identify lithologic relationships, possible stacking patterns and sweet spot intervals into Los Molles Formation.