GEO ExPro

The Pre-Salt Hydrocarbon Reservoirs of the South Atlantic

There are wide lessons to be learned from the controversy over the origins of the enigmatic Cretaceous lacustrine carbonate reservoirs of offshore Brazil.
This article appeared in Vol. 16, No. 4 - 2019

Advertisement

The Pre-Salt Hydrocarbon Reservoirs of the South Atlantic

  • Figure 1: Location of Santos and Campos Basins, offshore Brazil.

Much of the interest in Brazil’s autumn bid rounds will again be on the Santos and Campos Basins with their lacustrine carbonate reservoirs (Figure 1). In the Santos Basin over 30 discoveries have been made in such reservoirs since the Tupi (now Lula) discovery in 2006, with combined recoverable reserves estimated at >30 Bboe in the Cretaceous (Aptian) Barra Velha Formation (known as the ‘Microbialite’). Discoveries have also been made in the adjacent Campos Basin, where the same unit is known as the Macabu Formation, and also in the Kwanza Basin, West Africa. The Barra Velha Formation can reach thicknesses of over 550m, with some wells producing in excess of 28,000 bopd. It occurs beneath a cover of marine salt (the Ariri) and represents a later rift-to-sag interval of South Atlantic opening. Deeper in the section are the mainly Barremian coquina (shelly limestone) reservoirs of the Itapema Formation, well documented from the Campos Basin where they constitute the Coqueiros. The Barra Velha and its correlatives represent a lake system that covered an area of at least 335,000 km2: a similar size to the present-day Caspian Sea.

Carbonate Controversies: The Barra Velha & Itapema Carbonates Offshore Brazil

Both the Barra Velha and the Itapema carbonates are highly problematic. The Itapema reservoirs are coarse shell deposits 10s to 100s of meters thick, clearly seen as clinoformal features on seismic. The overall settings for the Itapema were relatively deep, stratified lakes similar to those of present-day East Africa. The shells, mainly bivalve mollusks, produce both inter-shell and mouldic porosity through the dissolution of the original shell material. There is evidence that some of these units represent deposition in the shallower parts of the lakes, whereas others were deposited or redeposited in deeper water. What is especially challenging about these hydraulically concentrated shell masses is that we see nothing like them anywhere else in the geological record, in either marine or lacustrine settings. The lakes were periodically highly alkaline at times and the remarkable concentration of shells raises questions regarding the ecology of these mollusks. Such high concentrations might imply unusual feeding strategies such as the bivalves having symbiotic algae (utilizing light like some species of bivalves, corals and foraminifera, for example) or even chemo-symbionts using CO2.

While debate continues over the origins of the coquina reservoirs, the main controversy relates to the Barra Velha and its equivalents. The implications of the different interpretations for this unit are quite fundamental in terms of both exploration strategies and reservoir modelling. Unlike the Itapema, which was clearly associated with relatively deep lakes during active rifting, the Barra Velha was deposited during the latter stages of rifting but shows clear evidence of syn-depositional control on stratal patterns. Localized deformation continued through the later ‘sag’ phase.

  • Figure 2a: Main components of the Barra Velha reservoirs: Calcite shrubs with inter-shrub porosity (blue) and some mouldic porosity within the shrubs.

  • Figure 2b: Main components of the Barra Velha reservoirs: Same image under cross-polarized light with distinctive sweeping extinction patterns reflecting the fibrous microstructure.

  • Figure 2c: Main components of the Barra Velha reservoirs: Spherulites set in a talc-stevensite matrix within which are dolomite crystals and elongate bridge-like dolomites.

  • Figure 2d: Main components of the Barra Velha reservoirs: Spherulites with pore spaces, with dolomite crystals and bridges, after the dissolution of the stevensite; the internal structure of the spherulites is clearly seen in cross-polarized light.

The Barra Velha carbonates are very simple in their composition. They consist of just two main primary components (Figure 2): millimeter to centimeter-sized crystalline, fibrous calcitic shrub-like structures resembling features found in present-day abiotic travertines (thermal spring deposits); and millimeter-sized spherulites made of fibrous calcite. Both types are commonly reworked, resulting in much reduced reservoir quality, but in their original growth positions, they can be porous and constitute the most widespread reservoir facies. However, the in-situ examples can also be non-reservoir if the carbonate features occur within magnesium-silicate matrices (talc-stevensite).

What is unique about these reservoirs is that some of the clays, which initially formed as gels precipitated out of the lake waters and were deposited on the lake floor, later dissolved to produce a previously unrecorded type of mouldic porosity. In addition, some microbial carbonates occur but represent very little of the formation in terms of thickness. Although many academic researchers have interpreted the spherulites as probably microbial in origin, to date no actual evidence has been presented to justify that conclusion. 

Also present, especially on structural highs, are very well sorted carbonate sands, formed on wave-influenced shorelines, spits and fan deltas. Finely laminated carbonate muds formed as thin units during deepening events in the lakes caused by increased run-off, bringing in fresher waters as indicated by fish and invertebrate remains. In deeper parts of the lakes somewhat different laminites, locally microporous, were also deposited, associated with turbiditic carbonates.

There are meter-scale cyclic packages with thin fish-bearing laminites, overlain by in situ spherulites, overlain by in situ shrub units (Figure 3). These have been interpreted as reflecting freshening and deepening of the shallow lakes by run-off to produce the laminites, followed by evaporation, a view supported by geochemical studies using C and O stable isotope analyses as well as thermodynamic modeling. The occurrence of what was originally stevensite suggests the pH of these alkaline lakes likely exceeded 10, and the surprising rarity of microbial carbonates implies that the pH may have been even higher during the evaporation phases.

Two Geological Models Hypothesized for the Barra Velha & Itapema Carbonates

Devising a geological model has proved difficult because there are no known modern or ancient analogs. Figure 3 is an attempt to show how these unusual reservoir rock types might relate to each other due to the interaction of active faults, wave action and evaporation of the highly alkaline lakes. To add to the complexity, the porosity system, effectively controlled by clay dissolution, is unique in the geological record.

  • Figure 3: Shallow lake geological model for the Barra Velha Formation based on a tilt-block setting. On the gentler dip slopes localized wave-dominated shorefaces developed with possible spit complexes, as seen in the Great Salt Lake in Utah. Coarser sediments accumulated on the scarp slopes. Meter-scale cycles developed locally as the shallow lakes expanded with increased rainfall, followed by evaporation, when the carbonates and magnesium-silicates formed. Mound-like features occasionally developed in Santos Basin. (Based on multiple sources including Barnett et al., 2018.)

Why should the Barra Velha and its equivalents be so unique when there are many carbonate-bearing rift successions in the geological record? The extreme thinning of crust during the opening of the South Atlantic may have led to exhumation of the mantle, creating hydrothermal conditions linked to serpentinization, although there is no geochemical evidence for elevated temperatures in the lakes and 87Sr/86Sr data does not indicate a significant input from the alteration of the mantle or ocean basalts. More intriguing is the possibility that the extreme alkalinity in the lakes was due to high CO2 input, derived from the mantle.

The current controversy over the Barra Velha Formation involves two distinct interpretations. The first was the ‘top-down’ microbialite-platform model. This uses seismic data to identify high relief platforms with seemingly hundreds of meters of relief, which have been compared to present-day and older marine carbonate platforms. The attribution of the Barra Velha carbonates to a microbial origin has led to some companies making direct analogies with the high relief Carboniferous marine microbial platforms of the Pre-Caspian basin in Kazakhstan, where marine carbonate platforms are differentiated into margins (commonly the preferred targets for exploration) and protected interior facies. Some companies have developed reservoir models based on this concept, but there are no analogs for the key rock types in the Barra Velha and, as stated earlier, there is currently little evidence for the carbonates being largely microbial. 

The alternative ‘bottom-up model’ for the Barra Velha was based on the recognition that no viable analogs have been identified. The strategy was to start by understanding the basic components making the reservoir rock and assessing them using basic science, such as establishing the chemical conditions in which the unusual mineral suite formed, combined with integrating crystal growth data. At all times during this process, which took nearly three years, multiple interpretations were deliberately sought, modified and rejected as more data allowed a clearer understanding of the chemical environment. Only once the fundamental controls were understood, the main conclusion being that the Barra Velha formed in shallow lakes, was the seismic data evaluated; without information on lacustrine carbonate seismic facies or marine analogs, the seismic-first approach was considered unjustified. In addition, running de-risking workflows on the seismic features also showed little likelihood that they were analogous to marine carbonate build-ups. Many high relief features were subsequently interpreted as reflecting post-Barra Velha, pre-and syn-Ariri deformation. Correlations of ‘Lula’s Fingers’, a package in the uppermost 30m of the formation characterized by a series of prominent spikes on the gamma log, proved critical. The carbonates in this interval contain a series of very distinctive cycles, including all the typical Barra Velha rock types as well as clear microbial carbonates and reliable water depth indicators proving a very shallow water origin. However, these cycles, correlatable for 180km across the Santos Basin, were affected by hundreds of meters of differential local displacement prior to the main phase of salt deposition (Figure 4).

  • Figure 4: West-east correlation across the Santos Basin showing the unit at the top of the Barra Velha Formation known as ‘Lula’s Fingers’. Its thickness varies from 20.8 to 28.5m (mean 24.3m), within which 9 gamma-defined cycles (mean thickness 2.7m) can be identified. These comprise shallowingupwards cycles defined by basal laminites. These cycles are well sampled and the facies are very similar in all wells, and include a range of unequivocally shallow water sediments. Even though these cycles were deposited at comparable water depths they are now separated by over 1 km of vertical relief, indicating significant post-depositional, but pre-salt, deformation. Source: Modified from Wright and Barnett, 2017.

Important Challenge in the Interpretation of Carbonate Reservoirs in the South Atlantic

Does this difference in interpretation matter? If the platform model applies, and because most production comes from these highs, the lows between them should lack the shallow lake reservoir facies, being relatively deepwater deposits. But if the lows are simply down-thrown shallow lake deposits, they will also contain reservoir-prone rock types, and could be prospective under the right conditions. In terms of reservoir architecture, the differentiated platform model envisages likely changes in reservoir quality across a platform top, whereas the shallow, evaporitic lake model implies potentially very widespread facies continuity. The presence or absence of the magnesium clays is another critical issue to consider when exploring for and developing reservoirs in the Barra Velha.

In summary, the highly unusual composition of the lacustrine carbonate reservoirs in the South Atlantic are especially challenging, and many uncertainties still remain. Interpreting these reservoirs has presented a challenge to the accepted wisdom based on marine carbonate analogs and has required a bottom-up approach relying on first understanding the chemistry of extreme lake environments, which were heavily influenced by highly thinned crust and the effects of the mantle, and then seeking a wider range of explanations for the seismic features. 

Resolving this controversy has huge implications for future exploration and development programs in the wider region.

References:

  • Barnett A J et al. 2018 American Association of Petroleum Geologists Search and Discovery, Article #11116
  • Cross reference to 2 articles in GEOExpro in 2018, v15, pt 4 and one in 2019, v16, issue 2
  • Wright V P & Barnett, A J 2017 American Association of Petroleum Geologists Search and Discovery, Article #51439


Further Reading on Carbonate Hydrocarbon Reservoirs

Carbonate Geology of The Florida Keys
Jane Whaley
The picturesque Florida Keys are not just a favourite holiday destination, they also offer geoscientists a masterclass on carbonate geology and the sedimentary processes associated with creating carbonate reservoirs.
This article appeared in Vol. 15, No. 6 - 2019

Carbonate Hydrocarbon Reservoirs
Thomas Smith
The 2006 discovery at Tupi, offshore Brazil, opened up a new chapter for petroleum reservoirs: lacustrine carbonates capable of producing hydrocarbons at extremely high rates.
This article appeared in Vol. 15, No. 4 - 2018

Seismic Characterisation of Carbonate Platforms and Reservoirs
Imelda G Johnson and James Bishop; Chevron Energy Technology Company
Long overdue conference on the seismic characterisation of carbonate platforms and reservoirs at the Geological Society of London shows the way forward.
This article appeared in Vol. 15, No. 2 - 2018

Growth & Future of CCS in Europe
Gavin Ward and Martin Wilkes, RISC Advisory; Conor Ward, University of Edinburgh
Carbon Capture and Storage (CCS) can be used to reduce emissions, but can oil and gas save the future of CCS in Europe?
This article appeared in Vol. 15, No. 1 - 2018

Mediterranean Carbonate Potential: Lessons from Existing Discoveries
Raffaele Di Cuia and Alberto Riva, GEPlan Consulting
The opening of new areas and the changing regional political and economic situation, together with innovative technologies, new seismic data and improved imaging, have revitalised exploration activity in the Central and Western Mediterranean. Analysing the characteristics of previous carbonate discoveries can help future exploration activity.
This article appeared in Vol. 13, No. 1 - 2016

Advertisement

Related Articles