Walther’s Law states that rock facies in a vertical succession were deposited in laterally
adjacent depositional environments. The condition for this rule to work is that such
facies must be in a conformable succession within the same depositional cycle.
Therefore, across unconformities, Walther’s Law can not work.
1. Construct a depositional profile for the vertical succession shown on the next
page. Please note that Bryozoas require open marine circulation and relatively
cooler water, whereas Meliolid Forams and Gastropods are more tolerant to
higher salinity. Also note that pin-stripe cross-bedding in quartz peloidal
grainstone is characteristic of carbonate eolianites (wind-blown dunes)
See diagram below
2. Please mark on the stratigraphic section on the next page, the shallowing and
deepening trends up stratigraphic section
See diagram below
3. Comment on the distribution of dolomite relative to facies, knowing that the
dolomite in this setting was all formed early
Dolomite is 100% on tidal flats and decreases seaward where there is some
dolomite in the lagoons and even less on the shoals, distal open marine mudstone
is undolomitized. This indicates that such early dolomite is forming on the tidal
flats possibly due to hypersaline water and is seeping/refluxing seaward where
primary depositional permeability might have allowed it
4. Comment on the climate during the deposition of the package
Well developed sabkhah on the supratidal, with dolomitized, mud-cracked,
bacterial laminated, tidal flats, abundant ooids all are indications for Arid climate
5. Comment on the distribution of porosity in the succession
Porosity is tied to ooid grainstone formed in high energy shoal settings, where
these had to have the highest primary interparticla (between particle) porosity. It
is not possible to tell the type of porosity in these rocks, no information was given
on that, but we can see some dolomitization influencing those shoals and that
might suggest that primary porosity might have been altered, to what degree and
in what way can not be determined. But we can see a direct correlation between
the grainstone and the porosity development
The set of stratigraphic columns in the following page are cores from subsurface wells,
and are arranged from north (column # 1) to south (column # 5), where the distance that
this traverse covers is 200 km and the vertical scale is shown on the left side of the
diagram. The sections are displayed on the page at equal spacing, but in reality they are
not equidistant, the real spacing between columns is not important for the purpose of this
exercise. Using the facies on the previous problem and your understanding of their
depositional environments, you are now to display the distribution of facies in space and
time on a cross section by answering the following questions:
1. Please construct the facies cross section for the conformable package in the
columns on the following page
See diagram below
2. Label the landward direction on one end of the cross section and the seaward
direction on the other end of the cross section, then denote the geographic
direction of the depositional dip, independent of any thickness variability in the
succession
Landward is northward direction of section 1; seaward is southward, direction of
section 5. Depositional dip is southward. See labels on the diagram below.
3. Label the sequence boundaries, systems tracts, the maximum flooding surfaces,
and progradational and retrogradational directions
See diagram below
Note that the best way to define the MFS is by using it as a downlap surface
4. Based on your understanding from the previous problem, identify the facies that
are most likely to have porosity and relate the porosity baring facies to their
position in the sequence stratigraphic framework, i.e. which systems tract or
when/where in that particular systems tract did they occur
Best reservoir development is in the grainstone belt, which is backstepping (or
retrograding) during the mid-to-late TST. In this case, the facies thickens
landward, toward section 1 and thins significantly by section 4 and further south
of that, it is non existent at section 5.
This belt also exists as in the prograding HST, where it is thickest updip in the
north toward section 1 and gets shingled southward, where it will be thinner and
interbedded with non-reservoir mudstone
5. Which areas are best to drill vertical wells and which areas are candidates for
horizontal drilling. Take into account the following facts:
a. Horizontal wells are more expensive and are to be used in the difficult
areas only where the targeted horizons are thin
b. The spacing between sections is more than 10 kms whereas horizontal
wells are only a few kms at best
c. Take into account the vertical scale to help you out
d. Accuracy in placing the wells, their directions, and their trajectories are
not important as this is a crude plan to help you and your engineer
colleagues zoom in on areas of interest, so keep your answers general
Vertical wells would be best placed northward where the grainstones are thickes
and where the TST grainstone grade directly upward with those of the HST. This
would be applicable down to section two where we can get maximum coverage of
the porosity in a vertical section.
As for horizontal wells, they would be best placed south of section two, targeting
the mid TST above the tidal flat and lagoon horizons as well as in the HST
grainstone where they will be relatively flat within the reach of the horizontal well
as they probably exhibit this shingling only at the regional level, but that is to be
tested during development