API gravity = 141.5/(specific gravity - 131.5).
By rearranging this, the specific gravity (SG) may be deduced as:
SG = 141.5/(API gravity + 131.5).
Hence, the Tupi oil has an SG of: 141.5/(28 +131.5) = 0.887 kg/m^3
However, the case of Tupi is complex, and recovering the oil is going to be a considerable task. The oil lies under a layer of salt, which lies under a layer of rock, which lies under great depths of sea. Probably the rock (pre-salt layer) lies under 2 - 3 kilometres of water, and is itself maybe 2 kilometres thick. The salt layer is gauged-in at around another 2 km in thickness, and having got through all that, there lies the reservoir of oil, and probably gas too, since oil cooked at such depths (and according temperatures) in all likelihood has produced gas.The company working on the TUPI project, Petrobras, is currently exploring for oil and gas in waterline-to-reservoir depths of 5 km, and is the world leader in offshore hydrocarbon exploration.
The salt-layer itself poses some particular challenges. Drilling through salt has been done before, but at Tupi the salt-layer is of an unprecedented thickness for drilling and the depths involved are greater than have been tackled before. To get some idea of the pressure, we can note that the pressure of water increases by about one atmosphere for every 10 metres depth. For solid crustal material, it is nearer three atmospheres for each 10 metres of descent (and an average of 4.5 atm./10 metres at much greater depths). Hence, if the sea layer is 3 km, that imposes 300 atm., below which is 2 km of rock, i.e. 3 x 200 = 600 atm, and then the salt itself, which yields another 600 atm. say, a grand total of 1500 atm. pressure (around 1.5 x 10^8 Pascals or 0.15 Gigapascals, GPa).
The salt is also heated by geothermal energy (from the interior of the Earth), and under these combined conditions of pressure and heat it behaves less like a solid and more like a jelly, with properties of flow, and so a hole may be drilled through it but it then closes. At the Coordination of Post-Doctoral Engineering Programmes (COOPE) hosted at the Fedreal University of Rio de Janiro, there are three high-pressure chambers that permit the simulation of the conditions at depths of 6000 metres, and where drilling equipment is tested.
Another problem is that when oil is pumped-up from the reservoir, it is hot (100 degrees C) and fluid, but at the sea floor temperatures are only around 2 degrees C. where the oil becomes "thick" and this can block the flow of oil to the surface. It is possible to get around this by heating the pipes, but this all adds to the costs of recovering the oil. The weight of the very long, 7 km, pipes can also impose mechanical stress on the steel they are made of and one suggestion is to use titanium instead, but this is a far more expensive material, adding further to the final bill.
On a final note, even at the predicted production from Tupi of 400 kb/d by 2015, this amounts to just 0.4/80 x 100 = 0.5% of the 80 million barrels a day of oil that the world currently uses, and who knows what it will cost per barrel to produce. Either way, as it becomes necessary to drill in increasingly difficult places to get it, oil will become a very expensive commodity, and perhaps other forms of "oil", e.g. as produced from Coal to Liquids (CTL) plants, now considered expensive, or Biomass to Liquids (BTL), thought to be operational technology by 2020, may become economically viable. Either way, the age of cheap oil is well and truly over.
(1) "Tupi, the new kid in town", By Luis de Sousa: http://europe.theoildrum.com/node/3269
[In general, oils with an API gravity of 40 to 45 have the highest market price and those with values outside this range sell for less. Above an API gravity of 45, the molecular chains become shorter and are less valuable to a refinery. Crude oil classified as light, medium or heavy, on the following basis:
Light crude oil has an API gravity of above 31.1 °.
Medium oil has an API gravity in the range 22.3 ° and 31.1 °.
Heavy oil has an API gravity less than 22.3.
In contrast, the US Geological Society uses slightly different definitions, but put simply, bitumen sinks in fresh water, while oil floats.
Oil which will not flow at normal temperatures is defined as bitumen for which the API gravity is normally less than 10 °. Bitumen derived from the oil sands deposits in the area of Alberta in Canada, has an API gravity of around 8 °. It is 'upgraded' to an API gravity of 31 ° to 33 ° by dilution and the upgraded oil is known as synthetic crude].