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Tectosedimentary Evolution of Early-Middle Jurassic (Lajas-Molles–Challacó Fms.) Southeastern, Neuquén Basin, Argentina. A Conventional and Tight Basin Center Gas System


Molles (MOF), Lajas (LJF) and Challacó (CHF) Formations consist of 1000 meters thick, second order transgressive-regressive mainly clastic sequences going from distal marine (MOF) to coastal (LJF) and continental environment (CHF). These units were deposited during Pleinsbachian to middle Callovian times and represent the first sedimentary record of Neuquén Basin. Huincul High is an east-west tectonic high, composed of several inverted Late Triassic half grabens as a result of several stages of oblique compressive tectonism (σ1~30W°) during the Jurassic-Cretaceous. Historically, these inverted half grabens have been producing oil fields at this latitude since 30s. However, downdip into the basin these sequences are almost completely unexplored because of its depth (+4000mts) and the lack of good reservoir properties. In this study, we present a tectosedimentary evolution of early-middle Jurassic units based on multisurvey 3D seismic interpretation and more than 50 wells information over an area of 3000km2. Seismically, older sequences are composed of ~300m thick package, evolving from E-W to SE-NW prograding sigmoidal facies. Distal portions of these units, characterized by high amplitude and very continuous events, corresponds to TOC-riched distal marine facies (MOF). Slope and proximal positions are more chaotic and show lateral amplitude variations (LJF). Above these units, lies a 500 m thick aggrading, parallel seismic facies, composed of sand-shale alternations interpreted as coastal environment deposits (LJF). These sands intervals represent: a tight gas deep reservoir system in relative distal positions, and a shallow conventional reservoir system in more proximal locations. The Early Middle Jurassic unit ends with a 200 m-thick unit of continental deposits (CHF) interpreted as transparent seismic facies bounded at its top by the IntraCallovian angular unconformity. In the vecinity of the Huincul High, these sequences are completed eroded as a result of several tectonic compressive pulses, developing different types of structural and stratigraphic traps. Understanding the relationship between sedimentation and timing of the different tectonic events during this period is key in both exploration and development of these conventional and unconventional plays.