Recent discoveries open an exciting new frontier
Joshua H. Rosenfeld - Yax Balam Inc.John F. Hood - Veritas DGC
Because of its geological similarity to the adjacent and prolific Campos basin, the Santos basin of Brazil is an increasingly important hydrocarbon province. Most exploration in the Santos basin has been in shallow water, where fewer than 100 wells have been drilled with some oil and gas discovered (~ 6.3 MMbbl of oil in Tubarão and related fields, and ~0.17 tcf gas in the Merluza field). This compares to over 1,130 wells drilled in the shallow and deepwater portions of the Campos basin (Figure 1).
The deepwater portion of the Santos basin, however, may prove to be the most important area for new reserves. Based on early 2D seismic and the results from shallow-water wells, the deepwater Santos basin was considered to share important geological characteristics with the neighboring Campos basin, including syn-Rift lacustrine source rocks, a thick salt section and post-salt turbidites. A recent non-proprietary 3D seismic survey by Veritas DGC on 20,000 sq km of the Santos basin provided the incentive for Petrobras to drill two deepwater wells in the area. In 2005, Petrobras drilled the RJS-617D well (ANP designation 1BRSA329DRJS) in 2,038 m of water to a TVD of 6,407 m (sub-seafloor), and discovered gas and oil in the deepwater syn-Rift section. A second Petrobras well, the RJS-628A (ANP designation 1BRSA329DRJS), is drilling in 2,140 m of water toward a PTD of more than 6,000 m and already has found light oil. These two Petrobras wells confirm the exploration potential of the deepwater area by dispelling doubts about the presence of mature source rocks; Petrobras has programmed three more deepwater wells to follow the RJS-628A. The approximate locations of the wells, the current Petrobras acreage, and the area covered by the Veritas 3D seismic are shown in Figure 2.
Petrobras was awarded its Round 2 acreage blocks in 2000 and was required to relinquish 50% of the original area in 2003. Fifty percent of the remaining acreage will be relinquished in September 2006 or shortly thereafter. The open acreage probably will be offered in future bid rounds, and companies who wish to explore this basin should contact ANP to express their interest.
Geological history and potential analogs
As part of the rifted Atlantic margin of South America, the geological history of the Santos basin can be divided into pre-Rift (pre-Cretaceous), syn-Rift (Neocomian to Barremian), and post-Rift (Aptian to Recent) stages, as shown on the seismic-based interpretation in Figure 3.
The lithology and age of pre-Rift rocks in the deepwater Santos basin are open to speculation. Reassembly of the African and South American cratons suggests that Santos’ pre-Rift units are a crystalline complex that may contain late stage pre-Rift basaltic flows and intrusions. The presence or absence of pre-Rift source and reservoir rocks has not been ascertained.
The specific composition of syn-Rift units in deepwater Santos basin generally is unknown since Petrobras has not released the details of its wells. Field modeling based on interpretation of the Veritas 3D seismic data, plus the fact that this basin is adjacent to the Campos basin, suggest that the syn-Rift units probably comprise lacustrine, continental, and neritic facies with possible basaltic intrusions and lava flows.
Excellent syn-Rift lacustrine source rocks are present in the Campos basin (Lagoa Feia Fm.), which supports deepwater Santos basin models that propose a similar facies named the Guaratiba Fm. The existence, maturity, and generating capacity of the Guaratiba has been confirmed by the hydrocarbons in Petrobras’ wells.
The RJS-617D and RJS-628A discoveries prove that hydrocarbons are generated and trapped in syn-Rift, and possibly pre-Rift, rocks beneath the Aptian salt. A possible analog for this trapping configuration is the Tertiary Suez Rift of Egypt, which has produced several billion barrels of oil. In both the Suez and the deepwater Santos areas, traps in pre-Rift and syn-Rift units are related mainly to horsts and grabens, with both areas containing syn-Rift reservoirs overlain by salt. Syn-rift source rocks are major contributors to Gulf of Suez production and this should hold true for the deepwater Santos. The multiplicity of trap types related to block faulting in the Gulf of Suez is shown in Figure 4, and the presence of analogous syn-Rift block faulting in the deepwater Santos basin is shown on the Veritas 3D seismic section in Figure 5.
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In much of the deepwater area, the basal post-Rift unit is Aptian Salt more than 2,000 m thick, which forms an excellent seal for the syn-Rift sequence. In some areas, the salt has been evacuated, leaving welds through which hydrocarbons may migrate into overlying post-Rift strata. Overlying the salt are Albian deepwater carbonates and marls, which are overlain in turn by Late Cretaceous through Recent turbiditic clastics. The thickness of these clastics varies within minibasins floored by salt and/or salt welds, but in general, the clastics thin seaward.
While the post-Rift section of the Campos basin is relatively sandy, the equivalent Santos basin section is thought to be argillaceous, although actual sand/shale ratios across the deepwater Santos are unknown. This is an important consideration since post-Rift turbiditic sandstones in the Campos basin are excellent reservoirs, and the presence or absence of similar sandstones in the Santos could have a significant effect on this basin’s productivity. It may be that most sand derived from the shelf was trapped within the first major salt evacuation basin upslope of the Cabo Frio Fault (Figure 3). On the other hand, this salt-evacuation basin may never have been a bathymetric low, in which case sandy distal turbidites could have reached the deepwater area as they did in the northern Gulf of Mexico.
If post-Rift structural-stratigraphic traps are present, they would relate to turbidite-filled minibasins formed by salt evacuation. This hydrocarbon environment is well known in several analogous areas around the world, the foremost of which is the US sector of the GoM. The critical factors for this play type are the presence of sandy turbidite reservoir facies and hydrocarbon migration pathways across salt welds.
In the shallow-water Tubarão and Merluza fields, the predominant source rock for hydrocarbons is the post-Rift (Cenomanian) Itajai Fm. Geochemical modeling by the ANP indicates that this potential source unit, if present in the deepwater Santos basin, probably is immature.
Conclusions
Petrobras’ recent discovery of hydrocarbons in the pre-Salt syn-Rift sequence has markedly improved the outlook for the deepwater Santos basin. Much of this area should become available in future bid rounds, and interested parties should contact ANP to encourage movement in this direction.
Both syn-Rift and post-Rift plays could prove rewarding: Veritas seismic data and preliminary models indicate that syn-Rift plays may be analogous to other rifted, evaporite-sealed petroliferous basins such as the Gulf of Suez, while post-Rift plays may be similar to those in the adjacent deepwater Campos basin and US sector of the GoM.
Twenty thousand sq km of high-quality 3D seismic is available for evaluation of the source kitchens, migration pathways, trapping geometries, direct hydrocarbon indicators, and other seismic attributes that can guide acreage selection and minimize exploration risk.
