Offshore Argentina: Lessons from Previous Exploration

SPECIAL TOPIC: PETROLEUM GEOLOGY Offshore Argentina: Lessons from previous exploration Jonathan Castell1*, Andrew Lavender1 and James Scotchman1 provide new insight into basin evolution, play types, and hydrocarbon prospectivity. Introduction of the South Atlantic. Of these, only two (Austral and San Jorge) Argentina’s Energy and Mining Ministry is in advanced prepara- have been extensively explored, and five (Salado, Colorado, tions to stage a new offshore licensing round in 2018 — the first Rawson, San Julian, and Malvinas) have been drilled with lim- in almost a decade. Despite its size (approximately 1.5 million ited success. The final basin — the Argentina Basin (sometimes km2, equivalent in scale to the US Gulf of Mexico) and its referred to as the East Patagonia Basin) — is the only deepwater moderate water depths, there have only been approximately basin and remains undrilled. 150 exploration wells drilled to date, making it one of the least Exploration frontiers with proven petroleum systems and explored Atlantic provinces. strong analogues such as the Argentine offshore are rare, driving The Argentine margin is underlain by a series of eight strong industry interest in the area. This is further stimulated by diachronously rifted basins (Figure 1) associated with the opening recent success around the Atlantic margins to the north in Brazil and Guyana, in West Africa, and in the UK-administered North Falkland Basin. Despite being an underexplored province, there is a vari- ety of publicly available legacy data for the Argentine margin (Figure 1). Available data for offshore Argentina include gravity and magnetics, seismic, well data, rock property information, and geochemistry data — all of which can be augmented by additional datasets from local and Atlantic margin analogues. As demonstrated in this article, these data, when interpreted in the context of a robust sequence stratigraphic and geodynamic framework, can provide new insight into basin evolution, play types, and hydrocarbon prospectivity. Exploration context Hydrocarbon exploration offshore Argentina began in 1969, focusing initially in the Salado, Colorado, and San Jorge basins. Approximately 30 wells were drilled by 1971, although results were disappointing and despite some shows, no commercial discoveries were made (Figure 2). A second phase of exploration began in the early 1980s when a consortium led by Exxon drilled a series of wells testing offshore extensions of plays from the onshore Austral Basin. The discovery of multiple large gas fields (Aries, Vega Pléyade, Carina) during this campaign provided renewed optimism for the province. Exploration continued at a steady rate until the late-1990s when a plateau in the discovery rate, compounded by acute economic instability, resulted in a cessation of drilling activity. Only a few wells have been drilled and only two relatively minor discoveries have been made since the turn of the century. The first Figure 1 Location map of the major sedimentary basins of the Argentine margin and only deepwater well (Malvinas x-1) was spudded in 2011 overlain by the distribution of public domain seismic and exploration well data in the Malvinas Basin in water depths just greater than 500 m. available for this study. Wells mentioned in the text are labelled. Unfortunately, it failed to prove a petroleum system in the area. 1 Halliburton Landmark * Corresponding author, E-mail: [email protected] FIRST BREAK I VOLUME 36 I MARCH 2018 47 SPECIAL TOPIC: PETROLEUM GEOLOGY Figure 2 Cumulative offshore-discovered hydrocarbon volume (red) and wells drilled by year (black line) illustrating the recently static state of exploration in offshore Argentina. (Figueroa, 2010; Rystad Energy, 2017). Pre-rift Documented pre-rift stratigraphy is Permian in age and is present in the north of the study area. The supercontinent of Gondwana dominated global plate configuration during the Permian, and the Argentine margin was influenced by a series of collisions resulting in the formation of the Hesperides Basin that represented a foredeep for the Gonwanides orogeny. The pre-rift stratigraphy here consists of organic-rich shales deposited during sea-level highstands and deep-marine sands shed from the Gonwanide mountains to the south. The southern part of the study area was uplifted as part of the Gonwanides orogenic belt (Pangaro et al., 2016), resulting in no age-equivalent pre-rift stratigraphy being deposited and/or preserved. Syn-rift The disaggregation of Gondwana began in the latest Triassic and was the precursor to the opening of the South Atlantic. Continental extension during the Late Triassic–Middle Jurassic is believed to have initiated and reactivated basin formation Figure 3 Paleo Digital Elevation Model (PaleoDEM) showing the global plate across the Argentine continental shelf, resulting in the earliest configuration during in the Early Cretaceous period and the opening of the South syn-rift deposits being represented by volcanics, volcaniclastics, Atlantic. and continental sandstones (Loegering et al., 2013). In the Late Jurassic period a renewed phase of rifting began, leading to the However, between 1998 and 2017 three drilling campaigns were opening of the South Atlantic. Initial rifting began in the south of undertaken in the UK-administered North and South Falkland the study area and progressed northward, reaching the north of Basins, resulting in multiple discoveries of both oil and gas and the study area by the Early Cretaceous period (Figure 3). During igniting a keen interest in the province in general. this time, continental and lacustrine sediments were deposited The upcoming offshore licensing round, announced by Argen- in graben and half-graben structures. Seafloor spreading in the tina’s Energy and Mining Ministry, will occur in three annual Early Cretaceous period was accompanied by transient volcanic rounds and is expected to begin with the Austral, Malvinas, and activity, resulting in the emplacement of extensive seaward Argentina Basins. 2D multi-client seismic data is currently being dipping reflectors that are readily identifiable in seismic profiles acquired and processed across these fundamental areas. (Hinz et al., 1999; Franke et al., 2007). Tectonostratigraphic evolution Post-rift The geological history of the margin is dominated by the opening The initial stage of post-rift subsidence resulted in marine of the South Atlantic from the Middle-to-Late Jurassic until the transgressions across the basins ranging in age from Berriasian mid-Cretaceous. As most of the basins along this margin devel- in the south to Barremian in the north of the study area. Many oped during this Gondwanan break-up phase, they exhibit similar of these initial marine deposits are organic-rich. A period of (though sometimes diachronous) depositional patterns and related tectonic quiescence and thermal subsidence from the Aptian petroleum systems elements. period resulted in widespread marine transgressions during the 48 FIRST BREAK I VOLUME 36 I MARCH 2018 SPECIAL TOPIC: PETROLEUM GEOLOGY mid-Late Cretaceous and Tertiary (Autin et al., 2013; Loegering Farther north, in the Colorado Basin, strong continuous reflec- et al., 2013). tors identified on seismic profiles might represent organic-rich lacustrine facies (Bushnell et al., 2000). Additionally, Fryklund Petroleum system elements et al. (1996) described a mature source rock of syn-rift origin Throughout the geological history of the Argentine offshore penetrated by the Cruz del Sur x-1 well. Average TOC content is margin, multiple periods of source rock, reservoir, and seal dep- reported to be 2.4% with a hydrogen index (HI) of 500. osition have occurred. Figure 4 summarizes this information in Because of the strong stratigraphic similarities of the syn-rift a regional petroleum systems events chart. Knowledge from the basins, it is expected that continental and lacustrine, organic-rich individual basins was collated to build a regional understanding deposits might be relatively widespread within basin depocentres. of petroleum system element occurrence. Similarities can be This is important for prospectivity of the Rawson and San Julian observed among the basins, indicating that regional events are a basins where a younger Aptian source rock is predicted to be absent. controlling factor in the distribution of source rocks, reservoirs, and seals across the area. Early Sag (Aptian) Aptian source rocks are the primary contributor for discoveries in Source rocks the Austral Basin (Rodriguez and Cagnolatti, 2008) and are likely Pre-rift (Permian) to be important along the margin. Deposition of organic-rich The oldest possible source rocks occur in the north of the study mudstone was widespread and is contemporaneous with a global area where Permian kerogen-rich shales have been penetrated in Ocean Anoxic Event (OAE1a) (Naafs et al., 2016). Marine organ- the Cruz del Sur x-1 and Puelche x-1 wells in the Colorado Basin ic enrichment is identified in multiple DSDP wells in the South (Fryklund et al., 1996). These organic-rich strata in the Puelche Atlantic (Tissot et al., 1980) and described in the Bredasdorp and x-1 well have been correlated to the Whitehill, Prince Albert, and Pletmos basins in South Africa (Van Der Spuy, 2003), illustrating Collingham formations (known organic-rich units on the African its widespread occurrence. margin) with an inferred Cisuralian age. Remnant deposits of the In the Austral and Malvinas basins, the Lower Inoceramus Permian age

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