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 N 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.