The document describes the restoration of geological sequences in an area affected by salt tectonics and uplift. The restoration involves 5 steps: 1) restoring a sequence to a reference dip, 2) correcting a synthetic drop using a horizontal target, 3) recovering an eroded reference surface, 4) recovering eroded sequences, and 5) applying the overall restoration approach. A forward model of salt tectonics involving subsidence and diapirism provides context for interpreting a turtle structure in the area.

1. apparent inversion
reference target
geometry
The restoration has been carried on without using the decompaction module.
Anyway only the backstripping of the Oligocene to Recent sequences (A) would
significantly change the underneath geometries.
B
C
Reference depositional geometry
The partitioning of the sedimentary sequences utilized for the restoration
has been put forward on the “summary sketch”:
STEP 1: restoration of module C to the reference depositional dip
Aim of the restoration is to highlight the:
1. type of structural events
2. timing of structural events
affecting the area.
It is worth mentioning, in particular in such an area, that during the
restoration approach different hypothesis has been made, as deducted from
the regional interpretation and remarks, such as:
a. deep-rooted uplift involving above all the inner area (East);
b. presence of truncation and/or gliding surfaces, related to the
abovementioned uplift, splitting the section in a non-deposition area (East)
and a basinal area (West) from the Oligocene;
c. no salt withdrawal during the uplifting phase.
So, any deduction, during the restoration steps, is not relative only to the
interpreted sections, but is deriving from the analysis of the geometries
of the entire 3D volume through Geoprobe.
The used data for the interpretation come out from the 3D PSDM volume in which
4 seismic section has been chosen (see sketch map):
XLNs 2734 and 2512 for the restoration,
ILNs 632 and 475 for the cross-correlation.
In addition, the data of 3 wells (KTNS1, 4 and SUNDA 4) has been considered for
the interpretation
SUMMARY
No particular reference target is present in the area affected either by the
salt motion and by the uplift.
The only noticeable reference surface has been highlighted “summary sketch” :
the calculated dip (2°) is the reference target dip of the bottom salt surface.
a restoration proposal
CONGO OFFSHORE
To correct the “synthetic” drop an horizontal reference target has been supposed,
so through the flexural slip mode the pre-Senonian sequences have been stuck to it.
correction of the drop through an horizontal target
STEP 2: correction of the “synthetic” drop
Anyway, restoring to the reference target geometry the deformed section
incoherent geometries appear such as the apparent inversion and the eastward
regional drop.
The hazard is to consider the dip of a depositional surface as a target of a
regional monocline, anyway the inferred dip seems to be coherent with
the depositional environment (delta).
STEP 3: recovering the “reference” surface
STARTING SECTION
2°
10 km
INL 632 INL 475
0.0
5.0
km
EW
10 km
0.0
5.0
km
ORIGINAL INTERPRETATION
INL 632 INL 475
KITIN
A-04
KITIN
A-01
SO
UN
DA-4
2512
2734
632
475
5 km
SKETCH MAP
KTNS 4KTSM 4
Aptian
Cenomanian
Albian
SEQ.5Cenomanian
POINT INDIENNE Fm.
(lacustrine shales)
LIKOUALA Fm.
TCHALA
Fm
.
SENDJI Fm.
CHELA Fm. (gres)
LOEME Fm.
Basement
Syn-rift
Presalt
Salt
1B
1A
Seq.1
2A4
Seq.2
intraseq.3
SEQ.3
SEQ.4
Coniac.-Turon.
Olig.-Low.Senon.
Rec.-Oligocene
POINT NOIRE Fm.
DJENO Fm.
SIALIVAKOU Fm.
VANDJI Fm.
BASEMENT
Barremian
Neocomian
Precambrian
Albian
seq. d’ennoyement
upperlowertomiddle
Tight carbonates of external
platform with sandstone and
oolitic grainstone levels
shales
Siltitic
carbonates
(ext. platform)
Bioclastic
oncolitic
carbonates
(open shallow platform)
Siltitic
carbonates
(ext. plat.)
Dolomitic-silt
(plat./prodelta)
Carbonates and marls
(ext. plat.)
Dolomite
(lagoon)
Fuvial/deltas’ sds.
(shore/barrier)
Silt
dolomitic-silt
(plat./prodelta)
MADINGO Fm.
Coniac.-Turon.
Olig.-Low.Senon.
Rec.-Oligocene
PALOUKOU Fm.
CRONO-
STRATIGRAPHY
USED
STRATIGRAPHY
REFERENCE
STRATIGRAPHIC
CHART
(offshore Congo basin)
ENI-SPES, June 2004
GENERAL STRATIGRAPHY
SUMMARY SKETCH
erosionalandglidingsurfaces
A A. Oligocene to Recent sequences: backstripped without decompaction;
B. Low.Senonian to Olig. Sequences: backstripped without decompaction;
C. Aptian to Coniacian sequences: sequences undergone to the proposed
restoration steps;
D.Lower Aptian (post-rift?) to Neocomian (syn-rift?) sequences.
D
2°
Original dip of the base of salt
(deformed state section)
“synthetic” drop
Recovering the offset on seq.4
Used reference top
Uplift = 1500 m
Oligocene - Lower Senonian
Coniacian-Turonian
Cenomanian
Cenomanian-Albian
To recover the original depositional geometries of top of seq.5
the resored top of seq.4 has been chosen as a reference
Supposed top Coniacian paleomorphology
gliding surface
re-sedimented
Coniacian - Turonian
sequences?
eroded sequence(s)
inward migrating sequence(s) outward migrating sequence(s)
STEP 4: recovering the “eroded” sequences
STEP 5: the restoration approach
Recovering the offset
along the left-hand turtle side
(top of seq.4)
Total offset recover along
the highlighted fault
Former reference
Sticking the hgw. to the regional
through flex. slip
New reference
(.6° anticlockwise)
Eroded area
Stripping away the light-coloured)
portion of seq.4
upper (
(see the above sketch)
A
The new reference surface of the top of the intraseq.3 is the result of the merge
between the two previous top surfaces (w” and “z”).“
New top turtlereference.
Proposed westward dipping fault trajectory
(either according to coherent turtle geometries and 2512 geoseismic section indications)
The restore of the right-hand turtle side setting apart the left limb
from the post-kynematic collapse has been carried out isolating the turtle and splitting
the gliding surface.
(constant growing area)
Setting the whole section
Old reference boundary
Modifing the turtle decollement until reaching the best-fitting between the top of the restored intraseq.3
and the regional reference surface
The highlighted area (red dotted circle) shows a reverse offset that should be
considered ficticious because of:
-the scarcely focused seismic imaging on the left hand turtle side
-the lack of regional pins on the hgw.
(the interpretation should be revised according to the proposed model) ;
2,5 km
SOME REMARKS ABOUT KITINA SUD TURTLE
Verypoorseismicimaging
Poor seismic imaging
- Presence, on both turtle limbs, of areas of poor seismic imaging;
- constant presence of growing sequences on the right-hand side of the turtle
- outer migration of the depocentral zones (eastward turtle limb)
- complete salt withdrawal on both the turtle limbs
Depo-central areas
Area of post-kynematic collapse
Area of post-kynematic collapse
According to the forward model proposed by Hudec & al. (2003) the turtle is
bordered by rising diapirs on both its limbs.
So, even if completely withdrawed in the deformed state, it is wise to consider the
presence of such a diapirs supposing, as antithetical to the interpreted one (A), an
east-dipping listric fault (B) which interpretation is impossible because erased by
the later structuration (post-kynematic collapse)
B
Recovering the offset of intraseq.3 on fault B
w
z
w
z
Upper Albian
The right-hand side of the turtle has been stuck to the ftw. due to the constant
growing geometries on that limb.
As a consequence the salt diapirs should be topographically lower on the growing
side.
regional reference surface
Lower “Upper Albian”
Restoration of the top of seq.2
Stripping away seq.2
Overestimated salt diapir
Geoseismic section 2512 top salt reference surface
Correction of the top salt according to the regional top salt surface
Referring the top2A4 to the reference regional surface
Stripping away seq.2 (turtle area)
Middle Albian
Lower Albian
Lower to Middle Albian
From:
AGL Geodynamics Laboratory
Topographic relief keeps sedimentation focused in the basin
Central basin welds to base salt, causing subsequent
withdrawal to shift towards the diapirs on either side
Subsidence of flanks may not be synchronous;
here subsidence begins first on the right flank.
Subsidence begins on left flank
Volume of salt
decreases with time
due to dissolution
Central basin isopach thick
Right-hand basin isopach thick
Central basin isopach thick
Left-hand basin isopach thick Right-hand basin isopach thick
Salt is still left under both flanks, so the structure could continue to evolve
Annual Progress Report to Industrial Associates for 2003
Slide Set 22
M.R.Hudec, M.P.A.Jackson, B.C.Vendeville, D.D. Schultz-Ela.
Bureau of Economic Geology, Scott W.Tinker, Director
Jackson School of Geosciences
The University of Texas at Austin
TURTLE STRUCTURE BETWEEN TWO DIAPIRS:
an example based on forward modelling
Boundary conditions:
- subsidence of a central basin
- grounding of the central basin
- shifting of withdrawal to the two flanks
Michael Hudec with contribution by Kerza Prewitt and Xuejiao Liu
(AGL, Jackson Scool of Geosciences, University of Austin, Texas)
realized, through a forward modelling in Geosec sw. different
cases of structures related to salt withdrawal.
The reported model depicts the evolution
of a turtle related to a central sinking of a sequence (minibasin)
followed by continuous “jumping” subsidence due to salt withdrawal.
The derived geometries are amazingly similar to the interpreted
ones on the XLN 2734 particularly regarding the depocentre migration
during the structure evolution.
Taking into account that the seismic imaging is poor most of all on the
left-hand flank of the interpreted structure (westward side) it is worth
to consider the modelled geometries to improve the seismic interpretation.
(see right-hand sketch)
SUMMARY