Very Thick
Massive Sandstone Bodies: Origin and Internal Architecture
Arcuri, Mariano1,
Carlos Zavala2 (1) IADO-CONICET; Universidad Nacional
del Sur, Bahia Blanca,
Argentina (2) IADO-CONICET; Universidad Nacional del Sur, Bahía Blanca, Argentina
Very thick massive sandstones bodies
constitute new exploratory targets mainly in offshore mini basins. Although
these bodies are relatively well known from subsurface studies, there are very
few detailed field descriptions. This contribution focuses on the exhaustive
description and analysis of very thick massive sandstone bodies from core and
field examples from different Jurassic to Tertiary marine basins of Argentina. The examples provided
here are composed of nearly homogeneous fine to medium grained sandstones
bodies up to 45 meters thick. These packages develop over a sharp or slightly
erosive base and they commonly lack internal bioturbation,
mud deposition levels or other evidences of pauses in sedimentation. Very thick
massive sandstones were usually related to an “in mass” deposition induced by a
gravitational collapse associated with surge-like 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. In the examples here
discussed these massive strata appear to be associated with confined areas
related to contemporaneous basin topography. Field examples show that these
sandstone packages compose elongate bodies of 0.8 - 15 kms
length and 0.5 - 3 kms width. Internally
discontinuous erosional surfaces with aligned clay clasts are common, and they suggest a lateral anisotropy in
flow energy. Detailed correlation patterns also suggest that very thick
sandstone bodies could have a convex-upward top. Reservoir properties of these
bodies are deeply controlled by burial and diagenetic
history. Deep burial (> 3,000 meters) often result in tight sands.