Porosity Prediction in Sandstones Using Erosional
Unconformities
G. Shanmugam
Erosional unconformities
of subaerial origin are created by tectonic uplifts
and eustatic sea level fall. Most erosional
unconformities
developed on
sandstones are planes of increased porosity because uplifted sandstones are
exposed to undersaturated CO2-charged meteoric waters that result in
dissolution of unstable framework grains and cements. The chemical weathering of
sandstones is intensified in humid regions by the heavy rainfall, soil zones,
lush vegetation, and accompanying voluminous production of organic and inorganic
acids. Erosional
unconformities
are considered hydrologically open systems
because of abundant supply of fresh meteoric water and relatively unrestricted
transport of dissolved constituents away from the site of dissolution, causing a
ne gain in porosity near
unconformities
. Thus, porosity in sandstones tends to
increase toward overlying
unconformities
. Such porosity trends have been
observed in hydrocarbon-bearing sandstone reservoirs in Alaska, Algeria,
Australia, China, Libya, Netherlands, Norwegian North Sea, Norwegian Sea, and
Texas. A common attribute of these reservoirs is that they were all subaerially
exposed under heavy rainfall conditions. An empirical model has been developed
for the Triassic and Jurassic sandstone reservoirs in the Norwegian North Sea on
the basis of the observed relationship that shows an increase in porosity in
these reservoirs with increasing proximity to the overlying base of Cretaceous
unconformity. An important practical attribute of this model is that it allows
for the prediction of orosity in the neighboring undrilled areas by recognizing
the base of Cretaceous unconformity in seismic reflection profiles and by
constructing subcrop maps.
AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.