Jurassic sandstones are often situated very deeply in the Central Graben in the
North Sea. Later they are very often faulted during several deformation phases, thus
fragmentizing the reservoir sandstone. The reservoir sandstones may have been
deposited during active faulting and differential subsidence which may result in
different thickness’ and facies developments in the area. It is important to restore
the structural development so it is possible to reconstruct the basin configuration
at the time when the reservoir sandstones were deposited. Where was the sand source
areas located? Where were the highs, plateau areas and the basin areas situated? In
which different sedimentary environments were the sandstones deposited?
By examining cores from the reservoir sandstone it is possible to determine
the depositional environments. Extrapolating these to petrophysical logs it is
possible to expand the interpretation of depositional environments above and below
the cored sections in each well. Depositional environments may be correlated from
well to well by logs. This way palaeogeographic maps may be reconstructed at the
time when the reservoir sandstones were deposited. Some depositional environments,
such as beach sands, may develop very porous and permeable reservoirs. The thickness
of these beach sandstones may be determined knowing the basin configuration when
these sandstones were deposited. Thus thick porous and permeable sandstones may be
mapped and volumes of hydrocarbons can be estimated. It may turn out that the
reservoir contains heterogeneities where impermeable sediments may separate the
reservoir sandstone in different units which are not in communication. Later
faulting activities may separate porous and permeable sandstone units. Faults may be
tight so there is no communication between the sandstone units. The reservoir
sandstones may undergo a diagenetic evolution where the porosity and permeability
decreases and perhaps later increases.
A case study of the development of deeply buried Jurassic reservoir
sandstones in the Central Graben will be presented.
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