Exploration Insights March 2020 - Page 14

Exploration Insights | 15
Evaluating Source Rock Maturity
Offshore Madagascar: Integrating the
Tectonostratigraphic History with Basin
Modeling
14 | Halliburton Landmark
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A
MOZAMBIQUE
Davie Ridge
C
Chesterfield
Heloise
D
Mozambique
B
E
F
Serinam
Ankamotra
Channel
MADAGASCAR
Morombe
by: Fouad Faraj, Alexander Braun,
Christine Yallup, and Colin Saunders
Morondava Basin
License Block Boundaries
Neftex ® Play Cross Section
Envoi Cross Section
Drilled well
200 km
Pseudo well
Baobabs at sunset in Madagascar. Source: Wikimedia
© 2020 Halliburton
OFFSHORE MORONDAVA
BASIN: A NEW FRONTIER FOR
HYDROCARBON EXPLORATION? offshore Morondava Basin, by integrating its
tectonostratigraphic history with 1D basin
modeling.
The Morondava Basin is located along the
western margin of Madagascar (Figure 1). Despite
two onshore discoveries made in the early
twentieth century (Tsimororo and Bemolanga),
the offshore remains largely unexplored. The
2018/2019 offshore Morondava Basin licensing
round was suspended, but the 44 blocks covering
63,296 km 2 (Figure 1) could eventually become
available for bidding again. TECTONOSTRATIGRAPHIC
HISTORY OF THE MORONDAVA
BASIN
Recent success along the conjugate East African
margin has increased interest in the Morondava
Basin, due to their similar tectonic histories,
stratigraphies, and petroleum systems elements
(e.g. Macgregor et al., 2018). Despite these
similarities, the western Madagascan margin
remains comparatively poorly understood in
terms of hydrocarbon exploration. Source rock
maturity within the potential license blocks is of
great interest because it affects the likely phase
of any hydrocarbons that might be discovered.
The East African margin is gas-dominated; an oil-
prone western Madagascan margin would be of
particular interest to the hydrocarbon industry.
This article investigates the hydrocarbon
maturity of key source rock intervals in the
The hydrocarbon story of the Morondava
Basin began with its formation due to the late
Carboniferous–Permian extension between
East Africa and Madagascar, caused by mantle
upwelling (Catuneanu et al., 2005). Two major
rifting episodes that had an impact on the
source rock maturity occurred along the western
Madagascan margin:
1. The Triassic rifting episode, peaking around
240 Ma — failed rift
2. The Jurassic rifting episode, peaking around
165 Ma — separation of Madagascar and
East Africa
The Triassic episode resulted in a failed rift (Figure
2). During this tectonic phase, Early Triassic
continental source rocks were deposited both
offshore East Africa and in the Morondava Basin
(Reeves et al., 2002; Wescott and Diggens, 1998).
The divergence in the tectonic histories of
Madagascar and East Africa occurred when
Figure 1> Map of the Morondava Basin. The locations of the Neftex Morondava Play Cross Section (based on Tari et al., 2017), and the
Envoi Cross Sections, are shown. These were used to construct the six pseudo wells, A to F. The locations of drilled wells are also shown.
®
they separated in the Middle Jurassic rifting
episode that separated West and East Gondwana
(Figure 2). Middle Jurassic marine source rocks
were deposited (Segev, 2002), which have
been postulated as the source of the majority of
the major gas discoveries in the offshore East
Africa region, including Mamba and Mzia fields
(Macgregor et al., 2018). In the Morondava Basin,
organic-rich rocks of this age have been reported by
Boote et al. (2019) from oil shows in several wells;
however, no known discoveries are linked to them.
In the Early Cretaceous, a major strike-slip fault,
known as the Davie fault, allowed Madagascar
to translate southwards, and the Davie Ridge
(Figure 1) formed along the western margin of the
Morondava Basin (Reeves et al., 2016). Deposition
of highly debated marine source rocks are
postulated to have occurred in the Late Cretaceous,
marking the final deposition of the primary source
rock groups in the basin (Tuck-Martin et al., 2018).
The Marion plume activated in the Late Cretaceous
between Madagascar and India, eventually
resulting in the separation (Tuck-Martin et al., 2018)
of these continental blocks.
By combining the key tectonic events and
their interaction with periods of likely source
rock deposition, a series of 1D models can be
generated to assess the hydrocarbon potential of
the basin.
INTEGRATING THE
TECTONOSTRATIGRAPHIC
HISTORY WITH 1D BASIN
MODELING
Tectonics had a regional and local impact on
the hydrocarbon source rock maturity of the
Morondava Basin. The plate movements shifted
Madagascar, changing its paleolatitude and,
thus, impacting the surface temperature of the
basin. The associated rifting episodes had a
more localized effect, influencing heat flow and
erosion. These factors affected the maturity of
source rocks in the basin, and their impact was
assessed during basin modeling. For this study,
volcanism was assumed not to have had a major
impact on the heat flow in this basin (Tari, 2017).
Source Rock
Group Input Age
(Ma)
Early Triassic Middle Jurassic
Cretaceous
Kerogen
Type Single Layer
Thickness (m)
247–250 I and III 50
162–165 II and III 50
90–93 II 50
Table 1> Primary source rock groups of the Morondava Basin,
and the parameters used as inputs for the basin models. Data
compiled from: Brownfield, 2016; Envoi, 2011; Cope, 2000; and
Ramanampisoa and Radke, 1992.