Offshore Madagascar Part II: The Golden Zone

The final part of this 2-part article reviews the validity of the potential plays in the area in terms of the Golden Zone concept.
This article appeared in February, 2018


What is The Golden Zone Concept?

The Golden Zone (GZ) concept, developed in the 1990s and early 2000s by Nadeau, Bjorkum, Walderhaug and colleagues from Statoil and from other institutions, is based on the findings that, contrary to conventional wisdom at the time, the controlling force behind the reduction in porosity and permeability at depth was temperature (i.e. thermo-chemical reactions) rather than mechanical compaction (i.e. effective stress).

Other items to note with regard to the Golden Zone are:

  1. Porosity reduction does not stop in the presence of hydrocarbons.
  2. Above 70°C the rates of porosity loss due to quartz cementation, and therefore fluid expulsion, increase exponentially; above 120°C porosity loss is sufficiently rapid to generate hard overpressures when drainage is restricted.
  3. The generation of hydraulic fractures and their role in hydrocarbon migration is governed by the ‘thermally controlled chemical pump’ which continues to work irrespective of the pore pressures generated, creating open fractures. These typically remain open for several million years, so long as the amounts of fluid to be expelled are greater than the amounts that can be expelled via the pore network.
  4. About 5% of the hydrocarbons in the GZ migrate upwards via traditional fill/spill mechanisms.
  5. As hydrocarbon traps subside and become subject to temperatures over 120°C, fractures will be created and hydrocarbons will escape to traps in a lower temperature zone, so prospects become separated from their ‘parent’ source rocks. This remigration is vertical and may occur long after a source rock has ceased expelling significant volumes. This means that we can dispense with notions that present-day traps have to be directly charged from active source rocks; or that there is one ‘critical moment’ for hydrocarbon migration and entrapment; or that oil prone basins will convert to gas prone (since oil may have been simply transferred to higher levels) or that oil and gas cannot co-exist at the same levels. This is part of the paradigm shift in thinking which the Golden Zone concept introduces.

There are circumstances where High Pressure High Temperature (HPHT) and Normal Pressure High Temperature (NPHT) reservoirs can exist at temperatures above 20°C, but these are not discussed in this article.

85% of The World's Oil in The Golden Zone

Composite thermal zonation model for siliclastic basins. We need to note the differences between the Golden Zone concept and the ‘Goldilocks Zone’ concept, which focuses on a more conventional view of oil and gas generation zones and temperature: ‘oil only’ generation between 60° and 120°C (the ‘Goldilocks Zone); ‘oil and gas’ generation between 120-200° and ‘gas only’ generation above 200°C. Image courtesy of Statoil. Under the Golden Zone concept hydrocarbons in siliclastic basins will generally occur in a predictable manner, controlled by temperature. Basins can therefore be modelled using a thermal zonation scheme where the Golden Zone is defined as the accumulation zone for hydrocarbons, which exists between the 60° and 120°C isotherms. Below this we have the ‘expulsion zone’, whilst above it is the ‘sealing zone’ (which we prefer to call the ‘cold’ zone). The zones are shown in the adjacent figure.

A study of over 120,000 proven accumulations revealed that 85% of the world’s conventional oil reserves occur in the Golden Zone.

Distribution of Global Conventional Reserves. Courtesy of Statoil. The majority of the high percentage of dry gas found in the upper ‘cold’ zone (shown in the table above) is believed to be biogenic in origin or to be the result of exsolution gas from rising formation waters entering a lower pressure regime. Under the GZ concept, oil in this zone would be expected to have migrated through carrier beds or via vertical percolation through relatively permeable sediments and leaky seals, with hydrocarbon reservoirs expected to be ‘quite dynamic with an equilibrium of leakage in and out of the reservoir’.

The concept has also been extended to cover carbonate reservoirs, suggesting that around 75% of globally discovered oil and gas reserves reservoired in carbonates occur within an 80-120° interval. 

Geological History: Uplift, Subsidence and Thermal Gradient

Map showing the location of the BGP/TGS MAD-13* survey (blue), the basis for the analysis in this article, with wells mentioned in the text. Critical to the definition of the GZ in a particular basin is an understanding of its geological history (e.g uplift and/or subsidence) and a knowledge of its present geothermal gradient.

Subsidence is a key element in the (vertical) re-migration of hydrocarbons and their preservation in ever-younger Golden Zones through time. Uplift, on the other hand, can lift the GZ to shallower levels and destroy or alter the hydrocarbons. In the context of Madagascar, this is probably what happened with the onshore heavy oil fields of Tsimiroro and Bemolanga. Alternatively, lateral fill-spill migration can be invoked. Regarding the preservation of hydrocarbons onshore Madagascar, we can also note that hydrocarbons have been preserved at deeper Permo-Triassic levels, as evidenced by light oil in the Manadaza-1 well, where reservoir porosity was reported as only 6%, with comments made about the effects of faults on the reservoir.

  • Subsidence and the Golden Zone: During continuous deposition and basin subsidence, sedimentary intervals progressively pass downwards through the various temperature windows (zones) as they become more deeply buried (dashed arrow). The greater majority of hydrocarbons, however, will always be concentrated in the GZ, because subsiding hydrocarbon traps entering the expulsion zone will continually release their charge upwards due to hydraulic fracturing and re-migration (thin, vertical arrows). Image courtesy of Statoil.

  • Uplift and the Golden Zone: Uplift and erosion will lift a former Golden Zone to shallower depths and lower temperature intervals. As seen on the right, a former GZ may be partly or completely removed by severe erosion. Image courtesy of Statoil.

The key to estimating the thermal regime in an area is to use estimates from nearby wells, although since the offshore Morondava Basin covers a number of different tectonic domains, as well as includes areas of volcanism, both intrusive and extrusive, these are expected to show considerable variation over the region.

Madagascan Hydrocarbon Plays and the Golden Zone

A number of petroleum systems are understood to be present in the offshore Morondava Basin (link to previous article) leading to the identification of a number of plays, ranging from the Karoo Rift to Tertiary Clastics, which are summarised in the table below.

  • Play fairways in the Offshore Morondava Basin. (KG: Kerimbas Graben; DR: Davie Ridge; MB: Morondava Basin; CP: Coastal Platform).

Let’s now examine these plays further, focusing on some key events which have implications with respect to the application of the Golden Zone concept in this basin.

  • Composite line (PSTM) 220km wide from west-east/south-north/west-east showing the location of some of the major plays as numbered and outlined in the table above. Turonian volcanic level indicated by white Vs.

  • PSTM section illustrating rotated fault blocks (Play Fairway 4) on the Coastal Platform. The blocks are expected to be of Karoo age with syn-rift Jurassic sediments. Image courtesy of TGS. Section width ~ 40 km.

  • PSTM section illustrating the Upper Cretaceous reef play on the Coastal Platform (Play Fairway 2B). Image courtesy of TGS.

The Davie Ridge has undergone deep burial and metamorphism, so there is no suggestion of plays in the Permo-Triassic and older rocks which make up the core of the Ridge itself (unless these are basement type plays sourced from younger sediments); however, the nature of the rocks within this core (gneiss and metasediments) will influence the thermal gradient and the temperature of the overlying Cretaceous and younger sediments.

By comparison, Permo-Triassic/Jurassic sediments are expected to have undergone a more benign environment outside the Davie Ridge area. Underlying the offshore Morondava Basin there is a deep graben (>3,000m of presumably Jurassic and older sediments), which covers an area of approximately 100 x 300 km), split in places by horst blocks. The faulted nature of the sediments within the graben are highlighted by igneous sills, presumably associated with Turonian extrusive activity. Although igneous intrusions can have a negative effect on a petroleum system, overall it can be said that this graben is ideally situated to have supplied hydrocarbons to the reefs and sand bodies in the overlying Golden Zone. On the Coastal Platform are we also see rotated fault blocks and the potential for syn-rift plays.

The Cretaceous in the offshore Morondava Basin is characterised by volcanism in the Turonian, coinciding with the break-up of Madagascar and India, with high amplitude events on seismic indicating extrusives. These volcanics account for plays associated with compaction/drape and as sites for reef build-up. It is also characterised by carbonate sedimentation on the basin margins and by a deepwater anoxic basin formed by the presence of the Davie Ridge, where organic-rich shales are overlain by basin floor fans and turbidites – good potential reservoirs.

In the Tertiary, studies have indicated uplift of between 850 and 2,000m from the Miocene onwards for the onshore, presumably leading to a greater influx of sediments into the Morondava Basin and to the subsidence of the basin, thus bringing some of the Cretaceous sediments into the oil generation/preservation window.

Regarding the present-day thermal gradients, a spatial variation based on the different heat flow of sandstones, shales, carbonates, volcanics and metasediments can be expected. In addition it should be noted that heat from the mantle is an important contribution to present-day heat flow and that differences in crustal thickness would influence the thermal regime. 

The Golden Zone on Seismic

Thermal Gradient and the Golden Zone. (Depth and thickness in metres from zero, gradient in C/km from zero.) With the above in mind, let’s now look at the Golden Zone with respect to the plays in the survey area. The table shows depth and thickness of the GZ under three different theoretical scenarios, although in our actual calculations, we have used sea floor temperatures from a global database as a starting point for thermal gradient.

The seismic lines below show the Golden Zone plotted on selected depth sections using a thermal gradient of 35°C/km (typical of nearby onshore and shallow water wells) and 25°C/km as an alternative value (DSDP 242 in the Mozambique Channel shows 27°/km). Play fairways are indicated using the abbreviations previously outlined.

  • PSDM section across the Davie Ridge and into the Morondava Basin showing the Golden Zone, using a thermal gradient of 35°C/km. The red dashes show an alternative using a thermal gradient of 25°C/km.

  • 110km PSDM section within the Morondava Basin showing the Golden Zone, using a thermal gradient of 35°C/km. The red dashes show an alternative using a thermal gradient of 25°C/km.

These images are in depth, but they can also be shown in time, using as input the PSTM data plus their stacking velocities. This method uses the assumed regional temperature gradient and seafloor temperature to position the Golden Zone in depth and a data dependent correlation factor with the velocities is used for the lateral interpolation of temperatures. Although there is a significant error margin associated with the methodology, there is a benefit in looking at the approximate location of the Golden Zone in time, as can be seen in the following examples.

  • A 180 km PSTM line from the Kerimbas Graben, through the Davie Ridge and into the Morondava Basin, over-plotted with a ‘Golden Zone in time’ overlay. The inlay shows a PSDM version of the line in depth.

  • 280 km PSTM section through the Morondava Basin, overplotted with a ‘Golden Zone in time’ overlay. The inlay is a zoom of the same section showing the large isolated carbonate reef.

Useful Screening Technique

Analysing the plays with respect to the Golden Zone in these various seismic examples, we can now make a number of observations regarding the identified play fairways and also comment on the value of the Golden Zone technique:

  • Play Fairway 1: Tertiary Clastics: These mainly fall into the ‘Cold zone’ (< 60°), although a case could be made for lateral migration into onlap plays, with preservation due to thick mudstone seals.
  • Play Fairway 2: Late Cretaceous: Clastics and reefs – the majority of these plays fall into the Golden Zone, especially so with the higher gradient. In the case of onlap onto the shallower parts of the Davie Ridge and onto the Coastal Platform, then lateral migration could also be invoked.
  • Play Fairway 3: Late Jurassic/Early-Middle Cretaceous: interestingly, some of the upper levels of the Morondava Graben fall into the Golden Zone, so possibly not all of the hydrocarbons have migrated into Play Fairway 2.
  • Play Fairway 4: Karoo Rift: with the Upper Jurassic penetrated in the Chesterfield-1 well at around 2,600m and in Heloise-1 at around 3,600m, the Karoo Rift could be seen as a possible viable play on the Coastal Platform, although further work needs to be done to confirm this.

From this analysis it would appear that the Golden Zone concept is a useful screening technique when analysing a frontier area and it has indicated that the likelihood of finding hydrocarbons in the offshore Morondava Basin could be high.


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