Exploration Insights March 2020 | Page 16

16 | Halliburton Landmark
Figure 2> Global plate reconstruction centered on Madagascar (in green). The first two tectonic events (bold headings) had a significant
impact on the hydrocarbon maturity. Reconstruction data based on Muller et al. (2019).
Three primary source rock groups are present
in the Morondava Basin (Table 1). Permedia ®
petroleum systems modeling software was
used to run 1D basin models to assess these
source rocks. We modeled five drilled wells, and
calibrated them using publicly available vitrinite
reflectance data from the Neftex ® Organic
Geochemistry offering. These wells were used
to inform the creation of six pseudo wells. In
each pseudo well, nine different scenarios were
tested, in which heat flow and erosion were
varied using the minimum, best, and maximum
estimates. By testing many scenarios, we were
able to quantify the uncertainty in the effect of
the tectonics on the basin.
In order to create the five calibration 1D models,
wells and gross depositional environment maps
from the Neftex portfolio were used to inform
interpretations of the geology of the region. The
upper and lower thermal boundary conditions
for the 1D basin models were derived using
modified McKenzie (1978) calculations (heat
flow) and an understanding of the variation
of surface temperature with paleolatitude
(mudline temperature). The lithology, source rock
parameters, and mudline temperature were fixed
in each pseudo well for each modeling scenario;
the only inputs allowed to vary were erosion and
heat flow.
Constraining Erosion
The Morondava Basin underwent several erosive
events from the Permian to the present day,
which were identified as unconformities in a
series of tied well logs (Tari et al., 2017). The
amount of erosion in different parts of the basin
was estimated using the Neftex Morondava
Basin Play Cross Section, in conjunction with
well logs and seismic data. Drilled wells with
vitrinite reflectance data were used to calibrate
levels of erosion within the offshore.
Constraining Heat Flow
The heat flow was one of the most uncertain
inputs to the basin models. The heat flow history
of the basin was derived using a modified
McKenzie (1978) simple shear model and
constrained using available vitrinite reflectance
data from drilled wells. In the deep offshore, the
crust is generally thinner, so present-day heat flow
is lower; in the shallow offshore, the converse
is true. However, during rifting, the heat flow in
the deep offshore is likely to have been higher.
Minimum, best, and maximum estimates were
made to test several scenarios for all six pseudo
wells. The range between the minimum and
maximum estimate increases deeper offshore,
reflecting the increased uncertainty (Figure 3).
SOURCE ROCK MATURITY
EVALUATION FROM 1D MODELS
The results of the modeling study suggest a
decrease in source rock maturity towards the
deeper offshore (Table 2). Despite the similar
lithologies throughout the offshore, the vitrinite
reflectance output from the pseudo wells
suggests that the source rocks in the deep
offshore are significantly less mature than those
in the shallow offshore (Figure 4). The decreased
maturity in the deep offshore is due to the
lower heat flow, and lower erosion related to
the Triassic and Jurassic tectonic rifting events;
however, uncertainty in these predictions is high.
Early Triassic
The Early Triassic source rock is risky. If pursued,
potential should be targeted in the deeper
offshore, where it is less mature; but not beyond
the Davie Ridge, where oceanic crust is present
and, thus, there was no Triassic source rock
deposited.
The Middle Jurassic source rock is the most
prospective in the Morondava Basin. The
30
25
20
15
10
5
0
300
250
200
150
Time [Ma]
Regional
100
Shallow Offshore
50
0
Deep Offshore
120
Deep Offshore
100
80
60
40
Shallow Offshore
100
80
60
40
300
The Early Triassic source rock is mature in the
Morondava Basin, but much more likely to
produce gas than oil. The chance of oil being
produced from this source rock increases towards
The range in hydrocarbon expulsion timing for the
Middle Jurassic source rock ranges from 160 Ma
to the present day, from the deep to the shallow
offshore (Figure 5). Traps for the Middle Jurassic
source rock would have had to form as early as
the Late Jurassic in the shallow offshore, while
the deep offshore traps could have formed in
the Late Cretaceous, nearly 100 My later. The
Middle Jurassic
© 2020 Halliburton
Marion Plume
interpreted seismic line from Tari et al. (2017)
was modified to show the predicted oil and
gas windows from the onshore to the offshore
(Figure 6, location on Figure 1). This shows
that the key Middle Jurassic source rock is
hydrocarbon mature over much of the study area,
but maturity levels decrease as you go further
offshore.
the deeper offshore. Expulsion from the Early
Triassic source rock is consistent across all wells
and scenarios tested. Our modeling indicates that
expulsion occurred in the Middle to Late Jurassic,
175–138 Ma, meaning that trapping structures
would have needed to have formed prior to this.
Strike Slip Fault
India
Antarctica
Seafloor Spreading
India
India
Antarctica
Rifting Event
India
Late Cretaceous
(75 Ma)
Late Cretaceous
(100 Ma)
India
Antarctica
Failed Karoo Rift
Early Cretaceous
(130 Ma)
Antarctica
East
Africa
India
East
Africa
India
Late Jurassic
(150 Ma)
East
Africa
East
Africa
Mid Jurassic
(165 Ma)
Mid Triassic
(240 Ma)
Exploration Insights | 17
250
Upper Bound
200
150
Time [Ma]
Best Estimate
100
Lower Bound
50
Potential Range
0
© 2020 Halliburton
Figure 3> Comparison of the thermal boundary conditions in the shallow (Pseudo Well F) and deep (Pseudo Well D) offshore. The top graph
shows the best estimate for the mudline temperature. The bottom graphs show the minimum, maximum, and best estimates for heat flow,
along with the total potential range based on the minimum and maximum estimates.