Geoscience Reference
In-Depth Information
For a fuller discussion of the sedimentology
and stratigraphy of these systems refer to the
literature, including Van Wagoner et al. (
1990
),
Van Wagoner (
1995
), Reading (
1996
), and
Howell et al. (
2008
).
Alongside a fairly wide range of depositional
processes, including waves and storms, fluvial
delta dynamics, and re-adjustments to base level
change, shallow marine systems are characterised
by active benthic fauna, 'worms and critters',
which churn up and digest significant quantities
of the sandstone deposits. The trace fossils from
these creatures (the ichnofacies) provide an
important stratigraphic correlation tool, and give
vital clues about the depositional setting (e.g.
Bromley
1996
; McIlroy
2004
). They can also
modify the rock properties.
Fig. 6.19
Example model of hummocky cross stratifica-
tion (HCS) from a shallow marine shoreface system
(model is 2.5
2.5
0.5 m)
unidirectional currents (Allen and Underhill
1989
) and are visible in core as bedsets with
low-angle intersections (typically
5
). The
laminations are sub-log scale and may be poorly
sampled at the core-plug scale too, but make a
significant contribution to flow heterogeneity.
Such heterogeneity lends itself well to effective
property modelling using small-scale models
(such as that shown in Fig.
6.19
).
<
6.4.2 Stacking and Laminations
Many reservoir characterisation and modelling
studies of these systems have been published,
(e.g. Weber
1986
; Weber and van Geuns
1990
;
Corbett et al.
1992
; Kjønsvik et al.
1994
;
Jacobsen et al.
2000
, and Howell et al.
2008
).
The last of these was part of a very comprehen-
sive analysis of the geological factors which
most affect oil production in faulted shallow
marine reservoir systems (Manzocchi et al.
2008a
,
b
). They concluded the most important
factors were:
1. The large-scale sedimentary stacking archi-
tecture (determined by the aggradation angle
and progradation direction), and
2. The small-scale effects of lamination on two-
phase flow (determined by the shape of the
capillary pressure function).
That is, both the large-scale architecture and
the small-scale laminations are important in
these systems (as also concluded by Kjønsvik
et al.
1994
).
In wave-dominated shallow-marine settings,
fine scale laminations are common in the form
of swaley or hummocky cross-stratified
lithofacies (Fig.
6.19
). These represent bedforms
produced as the result of either wave-related
oscillatory
6.4.3 Large-Scale Impact of
Small-Scale Heterogeneities
To illustrate the dynamic interplay of geological
factors with flow processes in shallow marine
reservoirs, we use the case study presented by
Ciammetti et al. (
1995
). They used a detailed
outcrop model of a shallow marine parasequence
(1,370 m long and 45 m high) to study the effects
of geological architecture on a simulated water-
flood (Fig.
6.20
).
Of the many cases run, the three cases shown
in Fig.
6.21
illustrate the main effects. Water
override generally occurs due to the coarsening-
up (permeability increasing upwards) nature of
the prograding shallow-marine parasequence.
This is generally positive, as it is in opposition
to gravity which drives the water downwards,
thus giving a balance between gravity slumping
and viscous override of the water front.
currents
at
the
seabed
or
