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An inappropriate analog for Pure Lateral Depositional Pinch-out Traps
Since the discovery of the Jubilee Field in deepwater offshore Ghana in 2007, the industry has drilled numerous dry wells in the West Africa Transform Margin region with several non-commercial discoveries whilst spending hundreds of millions of dollars. Multiple references in published literature and within company presentations indicate that all of this effort has been predicated on finding another Jubilee and the reason for these failures is often ascribed to an over-emphasis on geophysical analogs. However, a comparison with other producing fields in the West Africa Transform Margin region indicates that the Jubilee Field represents a unique combination of critical elements.
Firstly, the Jubilee reservoir is Turonian in age (many of the subsequent dry/sub-commercial wells targeted reservoirs ranging in age from Albian to Campanian) and its deposition was controlled by the Dixcove and CDIG ridges which acted as barriers and resulted in the ponding of up to 19,000 ft of sediments to the north and west of the ridges (Fig.1). This ponding was important both for enhancing source rock maturation in the deeper areas and for stacking the source and turbidite systems, which act both as migration pathways and potential reservoirs. The Jubilee reservoir system is a slope system, that developed basinwards from an erosionally-confined channel system with crevasse splays and master levees, into less confined frontal splay systems of large areal extent.
Secondly, the source for the hydrocarbons are the organic-rich Turonian shales which accumulated under anoxic conditions in the deeper parts of the basin, especially associated with the Ocean Anoxic Event (OAE) 2. The southward plunge of the South Tano ridge into the western Tano Subbasin acted as a migration focus for the generated hydrocarbons and thus very efficient, relatively short distance, migration was possible (Fig.1).
Thirdly, the trap for the Jubilee Field is a combination structural-stratigraphic trap and is not a pure stratigraphic trap. The top and lateral seal for the Mahogany Sand is provided by Turonian deep marine shales that drape the fan complex. However, the critical aspect is that updip to the NE, the reservoirs are trapped by a series of faults which have prevented updip migration of the oil (Fig. 2).
The combination of all these critical factors (Fig. 3) created the Jubilee Field: the right age (Turonian) reservoir juxtapositioned next to mature Turonian source rocks with thick reservoirs (800 ft avg.) having a very good N:G and which are in communication for distances >6 km, an effective updip seal enabling a significant hydrocarbon column (>1700 ft) and a large productive area (>19,000 acres). To discover another Jubilee requires that all of these factors are present within the correct basin setting. Merely having one or two would lead to commercial failure.
All too often those using Jubilee as an analog assumed that Jubilee was a relatively simple lateral depositional pinchout trap supported by a brightening of amplitudes. However the single most important element in exploring for pure stratigraphic traps is the correct recognition of seals. Any leaky seal will change a very promising amplitude-based prospect into a dry well or a sub-commercial discovery.
Dolson et al (2018) state that most stratigraphic traps fail by leaky seals and low structural dip helps, as large traps with limited seal capacity can be developed over broad areas. However as the structural dip steepens in stratigraphic traps, unless outstanding seals are involved the trap size will become increasingly limited.
In benchmarking (Fig. 4) Jubilee against lateral depositional pinchout traps (i.e. pure stratigraphic traps) on passive margins, using DAKS, it can be seen that Jubilee lies outside the distribution. This is interpreted as being highly anomalous and would have been identified as a red flag in the benchmarking exercise.
Whereas when benchmarking (Fig. 5) Jubilee against normal fault traps on passive margins, using DAKS, it can be seen that Jubilee lies within the normal distribution.
A detailed understanding of an analog is required within a structured and rigorous classification system to ensure that valuable and real insights are captured. When applying analog learnings, it is important to bear in mind that only the relevant analogs can help. Whilst this sounds obvious, selecting the wrong population of analogs (Fig. 6) from which exploration play controls can be drawn is a common mistake. An apple to apples comparison is essential and this can only be achieved via a rigorous classification system, such as that in DAKS™.
A basic requirement in the DAKS Workflow (Fig. 7) in searching for applicable global analogs for Play concepts & Prospects, is the need for a detailed understanding on all aspects of the target play/prospect.
The misuse of Jubilee as an analog emphasizes the dangers of an oranges to apples comparison and along the West African Transform Margin the assumption that Jubilee was a pure stratigraphic trap combined with the lack of recognition of the definitive up dip fault seal led to many of the dry holes and sub-commercial discoveries such as Narina-1 (2012), Mesurado-1 (2016), Fatala-1 (2017) and Ayame-1X (2017).
Dolson et al., 2018, Advances and Perspectives on Stratigraphic Trap Exploration-Making the Subtle Trap Obvious, Search and Discovery Article #60054, 67 p.
C&C, 2018, Field Evaluation Report, Jubilee Field, Mahogany Sand Reservoir Ivorian-Tano Basin, Ghana, November 2018, 52 p.