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Technical summary

Late Jurassic plate reconstruction (click image to expand)

West to East ED seismic section highlighting the Aptian shelf, shelf edge and slope (click image to expand)

Technical summary

A breakthrough in the
South Falkland Basin

Borders & Southern’s acreage is located in the South Falkland Basin, a frontier basin extending to the south and east of the Falkland Islands. The South Falkland Basin developed as a passive margin to the developing Weddell Sea as southern Gondwanaland began to break up in the Late Jurassic.

The area to the South and East of the present day Falkland Islands remained as a passive margin throughout the rest of the Cretaceous period and a series of prograding clastic wedges were developed bordering the Weddell Sea. During the early development of the basin restricted oceanic circulation led to the development of think source rocks during the Early Cretaceous. These source intervals are found regionally within the Magallanes and Malvinas basin and more widely throughout the South Atlantic conjugate margins.

In the vicinity of the Falkland Islands, quartz rich metamorphic Devonian rocks were eroded, and river systems debouched their contents onto a broad shallow marine shelf boarding the developing ocean. Relative drops in base level created periodic erosion of the shelf feeding coarse sediments out into deep water in a series of feeder systems and submarine fans. During the later part of the Cretaceous period rising global sea levels drowned the shallow shelf resulting in a more quiescent depositional environment. The sediments deposited at this time formed excellent seals to the Early Cretaceous sandstone reservoirs.

Technical summary

Scotia plate formation
& play fairways

During the early Tertiary differential plate movements between the South American and Antarctic plates caused the initiation of the Scotia Sea oceanic plate. The developing plate caused the eventual separation of South America from the Antarctic Peninsula creating the Drake Passage. As the oceanic plate developed the former Weddell Sea crust was subducted and continental fragments were pushed onto the surrounding plates. In the vicinity of the South Falkland Basin the Burdwood Bank was pushed onto the southern margin of the South American crust, loading and warping the plate downwards, creating a foreland basin that filled with sediments.

As the Burdwood Bank encroached further onto the down warped plate the sediments deposited on the South American crust were inverted forming an elongate fold and thrust belt ahead of the advancing landmass further loading the plate. The fold belt is similar to many of the fold and thrust belts found along the western edge of the South American continental landmass. As the South American plate was warped, erosion took place and clastic sediments were fed down into the developing foreland basin. The eroded sediments form potential reservoir intervals within the foreland basin sequences. The evolution of the Scotia plate was the driving mechanism that formed the large anticlinal folds forming the exploration structures targeted by the Stebbing well. The same tectonic forces also caused major fractures in the down warped plate. These basement faults offset the Early Cretaceous passive margin sediments creating tilted fault block structures like the Darwin field.

Seafloor spreading within the Western Scotia Sea ceased approximately 7 million years ago. Since this time differential movements between the South American and Antarctic plates have taken place through left lateral strike slip displacement mainly focussed along the South Scotia Ridge.

The main reservoirs within the licence area are the Early Cretaceous passive margin sandstone targets and the late Cretaceous/Early Tertiary sediments eroded into the developing foreland basin. Structural traps were created by the compressional events during the mid to late Tertiary forming both a series of tilted fault blocks and large thrust cored anticlines. In addition the Lower Cretaceous slope fan targets are stratigraphic traps with up dip seal. Both Late Cretaceous and Tertiary pelagic sediments form excellent seal rocks. All targets within the licence area are reliant on charge from the mature Lower cretaceous source rocks. The southern portion of the licence is dominated by the large thrust cored anticlines whereas the Early Cretaceous shelf and slope fan plays occupy the north.  

Early cretaceous aptian palaeogeography (click image to expand)

North to South schematic cross section through the South Falkland Basin (click image to expand)

Well location map (click image to expand)

falkland islands

South and East Falkland
exploration activity

In 2005 Borders & Southern acquired 2,862 km of 2D seismic. Prior to 2005 only 4 legacy seismic lines partially covered the licence area. The primary objective of the initial 2D dataset was to identify the principal targets to help located a targeted 3D survey. In 2008 the company shot it’s first 3D survey acquiring 1,492 sq km of high-quality data. This dataset enabled a robust risk reduction process and identified several key exploration targets. 

Six wells have been drilled in the basin. Toroa-1 (61/05-1) was mapped on 2D data and drilled by BHP Billiton in 2010. The well targeted the early Cretaceous Springhill Sandstone Play as a stratigraphic onlap trap. The prospect  was supported by a class 4 AVO amplitude anomaly and a positive CSEM response. Reasonable quality water wet sands were encountered at the primary target. Very good quality sands were encountered in the Mid Cretaceous interval although there were no identified trap at this level. The reservoir information obtained at Toroa-1 helped de-risk the pre-drill interpretation of the Darwin target interval. Additionally, the well also encountered a thick immature source rock interval in the Lower Cretaceous. The well is located on the margins of the South Falkland Basin and therefore the source rocks have not attained enough burial to reach maturity. To the south and east of the well location the greater burial of the source interval will result in hydrocarbon generation and expulsion as demonstrated by the hydrocarbons encountered at Darwin East. 

Darwin East-1 (61/17-1) was the next well to be drilled in the basin in 2012. The Darwin field comprises of two adjacent tilted fault blocks with down dip closure. Pre-drill, the target was supported by a class 3 AVO response, flat spots and excellent amplitude conformance to structure identified on both fault blocks. The well targeted the Darwin East fault block. The well was a success and penetrated an 84.4m gross hydrocarbon bearing reservoir interval with high net to gross (80%) with an average porosity and permeability of 21% and 335md.The reservoir interval is Aptian in age and was deposited in a shallow marine environment as an offshore/barrier bar. Reservoirs of this nature offer excellent poroperm characteristics as demonstrated by the well data. The reservoir facies was deposited during a late highstand to falling stage system tract. Reservoirs deposited during this stage of the relative sea level cycle have high vertical and horizontal communication. The vertical communication within the reservoir was confirmed via wireline pressure tests. The textbook amplitude conformance to structure across the whole field demonstrates the quality of the horizontal communication. The hydrocarbon encountered are a gas condensate. At reservoir pressure and temperature, the liquid hydrocarbons present are in a gas phase. As the pressure and temperature drops the liquids drop out, or condense, similar to how water droplets entrained in air will ‘condense’ out on a cold window pain. A gas has a higher mobility in a reservoir than a liquid meaning that it will travel through the pores of that reservoir with relative ease compared to a liquid. Therefore, our expected hydrocarbon recovery rate is high due to the mobility of the hydrocarbon in the reservoir and the excellent reservoir quality.

Source rock intervals were also encountered by the well in the lower Cretaceous. Like the Toroa-1 well the source rocks were immature but would be mature with only slightly greater burial depth. It is most likely that the same organic rich interval down dip from the well location is the source to the hydrocarbons trapped within the Darwin field.

The Stebbing-1 (61/25-1) well was drilled immediately following the completion of the Darwin East-1 well. This well targeted a large thrust cored anticlinal structure. The stratigraphic interval targeted by this well was completely different from the Darwin East penetration. The objective in the initial drilling campaign was to test separate play types and to maximise on stratigraphic knowledge to help better understand the evolution of the basin. The well had two objective intervals, an Oligocene deep water channel play and an Early Tertiary/Late Cretaceous shelf sand interval that has been encountered in the Malvinas basin. Although wet gas was encountered at the Oligocene target the interval was siltier that expected and not regarded as an effective reservoir. Below the Oligocene target pore pressure began to increase and additional casing was run to mitigate the rising pressure. To avoid a large gas hydrate located across the crest of the structure a deviated well was drilled to intersect both targets. As the well progressed the well trajectory brought it closer to the core thrust within the structure. Several strings of casing were run to ensure a safe drilling margin but eventually the well could not proceed to the deeper targets. So, the deeper early Tertiary/Late Cretaceous interval remains untested. The fold and thrust belt plays are located to the south of the licence. The Darwin field and surrounding shelf plays are all normally pressured.

Following the completion of the Stebbing-1 well the Leiv Eirikson rig was then employed to Falkland Oil and Gas to drill the Loligo-1 (42/07-1) well. The target was a late Tertiary deep water distributary fan system feeding from the vicinity of the present-day Falkland Islands. The well was reported to have encountered wet gas within a poorly developed reservoir interval. Upon completion of the operations at Loligo-1 the rig moved north to drill the Scotia-1 (31/12-1)  well. The Scotia prospect was a mid-Cretaceous deep-water fan play that exhibited a strong Class 2 AVO response and was located directly above the Aptian source interval. Although gas was encountered the reservoir was of poor quality due to extensive calcite cementation. The geothermal gradient in the vicinity of the well was reported to be higher than expected which would have contributed to the reservoir degradation. Higher temperatures also meant that the source rocks that would have charged the Scotia prospect were now in the gas window. Scotia was eventually classified as a tight gas discovery.

Following interpretation and integration of the Darwin East-1 well data the company decided to acquire additional 3D seismic data to target the successful lower Cretaceous shelf play to the north of the Darwin discovery. The survey was acquired in 2013 and totalled 1,025 sq km. During the seismic processing stage the original 2008 3D survey was re-processed to enhance the quality and to ensure a seamless join between the two surveys.  

Noble Energy farmed into the FOGL acreage in 2013 and after acquiring two 3D seismic surveys drilled the Humpback-1 (53/02-1) well in 2015. The Humpback prospect comprises of a series of stacked deep water slope fans of Early Cretaceous age forming part of the larger Diomedea fan complex. The well encountered several sandstone reservoir intervals at the target depth. In total the well encountered 65m of sandstone reservoir. It was reported that the reservoir units were hydrocarbon bearing but because of poor reservoir quality no samples were acquired. The well reached a total depth of 5,136m. It is likely that the depth of burial and reservoir temperatures may have contributed to the poor reservoir quality given the higher temperatures encountered in the Scotia well to the north. The Diomedia fan complex is at the same stratigraphic level to the slope fans identified on the Borders & Southern acreage. Reduced burial depth and a more favourable geothermal gradient will reduce the risk of reservoir degradation for these targets in the South Falkland Basin.

Darwin East-1 reservoir interval (click image to expand)

Leiv Eirikson rig drilling Darwin East-1

Darwin East-1 condensate sample (49° API)

Darwin discovery

Darwin comprises two adjacent tilted fault blocks, Darwin East and Darwin West. The discovery well is situated on Darwin East. A clear flat spot and amplitude conformance to structure help define the hydrocarbon charged reservoir.
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Falkland islands

The Falkland Islands are a United Kingdom Overseas Territory. Supreme authority is vested in His Majesty The King and exercised by a Governor on his behalf, with the advice and assistance of the Executive Council and Legislative Assembly.
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Technical summary

Borders & Southern’s acreage is located in the South Falkland Basin, a frontier basin extending to the south and east of the Falkland Islands. Six wells have been drilled in the basin. In 2012 Borders & Southern made the first significant hydrocarbon discovery with exploration well 61/17-1.
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