Hyperpycnal Shelfal Lobes - Some Examples of the Lotena and Lajas Formations, Neuquen Basin, Argentina

M.Arcuri¹ and C.Zavala^1,2
¹Geología de Cuencas Sedimentarias (G.C.S.). Haiti 123. Bahía Blanca (8000), Argentina
²CONICET-UNS, Ushuaia, Argentina

Very thick massive sandstone bodies constitute new exploratory targets mainly in offshore mini basins. Although these bodies are relatively well known from subsurface studies, there are few detailed field descriptions. This contribution focuses on the description and analysis of field examples of very thick massive sandstone bodies from the middle to late Jurassic Lajas and Lotena formations, in the Neuquén basin, Argentina. The examples provided here are composed of up to 45 meters thick nearly homogeneous fine to medium grained sandstone beds and correspond to hyperpicnal shelfal lobes. These packages are disposed over a sharp or slightly erosive base and they commonly lack of internal bioturbation, mud deposition layers or other evidences of pauses in the sedimentation. The existence of very thick massive sandstones was usually related to an “in mass” deposition induced by a gravitational collapse associated with surge flows having high suspended load. However, more recent studies have proposed an origin related to the progressive aggradation from long-lived and quasi-steady turbulent flows. Massive deposits could be related to the absence of a sharp surface between the flow and the deposit.

The Neuquén basin is a back-arc basin located in the west-central Argentina. Its sedimentary infill took place during Jurassic and Cretaceous with a mainly clastic succession up to 7000 meters thick. The Lajas Formation (Early to Middle Jurassic) represent the first prograding unit after the initial marine flooding of the basin (Los Molles Formation). It is composed of conglomerates and sandstones accumulated in different deltaic and shallow marine environments. The Lotena Group (Middle to Late Jurassic) sharply overlies the Lajas Formation in the study area. This group constitutes a clastic to evaporitic unit widespread developed in the basin during middle Callovian to upper Oxfordian times. It is internally composed of conglomerates, sandstones, mudstones and limestones accumulated in a fluvial (Dellapé et al., 1979) to deep marine (Gulisano et al., 1984) environments.

The examples analyzed here correspond mainly to Lajas Formation at Lonopué area and Lotena Formation at the Sierra de la Vaca Muerta (Figures 1 and 2). Some characteristics of the study examples are numbered here: 1) The coexistence of erosion and deposition (aggradation) at the same time, associated with a facies family related to traction plus fallout suggest an accumulation related to long lived flows with high suspended load (hyperpycnal flows). 2) Internal erosion surfaces could not have been developed by different events since they disappear toward the laterals. 3) Individual thick beds could be related to a progressive aggradation of the depositional surface from a quasi-steady and long-lived sediment gravity flow. 4) The existence massive sandstone beds with anomalous thickness could be related to the presence of a basin topography which results in a flow confinement (McCaffrey & Kneller 2001). 5) Massive strata appears associated with confined areas related to an early tectonic-induced basin topography. 6) Field examples show that these sandstone packages compose elongate bodies ranging 0.8 - 15 kms long and 0.5 - 3 kms wide. 7) Internally discontinuous erosional surfaces with aligned clay clasts are common, and they suggest a lateral anisotropy in flow velocities. 8) Detailed correlation patterns also suggest that very thick sandstone bodies could have a convex-upward top. 9) Reservoir properties of these bodies are deeply controlled by burial and diagenetic history. Deep burial (> 3,000 meters) often result in tight sands.

References

Gulisano, C.A.; Gutiérrez Pleimling, A.R. y Digregorio, R.E., 1984. Esquema estratigráfico de la secuencia Jurásica del oeste de la provincia del Neuquén. 9º Congreso Geológico Argentino. Bariloche. Actas. t. 1. p. 236-259. Buenos Aires.

Dellapé, D.A.; Mombrú, C.; Pando, G.A.; Riccardi, A.C.; Uliana, M.A. y Westermann, G.E.G., 1979. Edad y correlación de la Formación Tábanos en Chacay Melehué y otras localidades de Neuquén y Mendoza. Obra del Centenario del Museo de La Plata. t. 5. p. 81-106. La Plata

McCaffrey, W. y Kneller, B., 2001. Process controls on the development of stratigraphic trap potential on the margins of confined turbidite systems and aids to reservoir evaluation.AAPG Bulletin, 85 (6): 971 –988.

Zavala, C. y González R., 2001. Estratigrafía del Grupo Cuyo (Jurásico inferior-medio) en la Sierra de la Vaca Muerta, Cuenca Neuquina. Boletín de Informaciones Petroleras. Tercera Época, año XVII, Nº65. p. 40-54.

Figure 1. An example of Sierra de la Vaca Muerta Sandstone lobes. Lotena Formation. A) Regional panel correlation of the Lotena Formation. Note the arrangement of confined shelfal lobes. Modified from Zavala & Gonzalez, 2001 B) Detailed correlation panel of an individual sandstone body.