Total especutlation mode about the origin of this disaster: Venezuela is suffering oil export sanctions. When that kind of sanction happens it's not uncommon to transport oil in ships with "rented" flags or no flags to ports of close countries, so the oil is incorporated to that country's production and can be sold.
It is a tiny quantity, but generate some money to some people. As everything clandestine, this transportation doesn't care much about safety and environmental precautions.
It is tough investigation to make, so maybe we will end only with those especutlations
You’re right on. Chemical analysis has already confirmed that the oil comes from Venezuelan fields, and the quantity is smaller than you’d expect, they are currently investigating a handful of suspect ships, but most likely the responsible one would have had its comm equipment turned off and hence not recorded by radar.
The hard limit on naval radar is the horizon which is determined by the height of the mast it's mounted on so much of the sea's surface at any given time isn't actually covered by radar. Even when a ship happens to be on radar there's no indication of what ship it is without a transponder like AIS.
Think of the air traffic controller screens in Movies or TV Shows. It shows Delta 1234 from the aircraft’s transponder. Similar to ships, the ones with radars would still see a blip but there wouldn’t be an info on screen. The Automated Identification System (AIS) wouldn’t be broadcasting.
That is “stealth”. It’s obvious because it is expensive and secret tech. But also because if you look at the ship you see a ship, whereas the radar shows an albatross or something small. Transitions are also notable for radar operators that are really paying attention. A ship that turns into a goose and then back into a ship, for example.
I think you may be mixing up active jamming (which works like shining a searchlight in someone's eyes--they can't see or track you, but it's very obvious that there's someone out there) and passive stealth (which tries to reduce the emissions that the vehicle emits or reflects).
Anyone can make a jammer, but it only helps if you can evade your pursuer before they get a look at you optically; it's not much use for a lumbering oil tanker.
As zentiggr says, you can get some degree of passive stealth just by turning off your own radio/radar, but anything more than that requires a custom-engineered (and strangely-shaped) hull, propulsion, etc. that takes military-level money to design and would be extremely obvious in any port. So, again, not practical for this kind of smuggler.
I'm not sure if data from the analysis will be made available to the large public anytime soon, but in the links below you can find more information about the protocol in use, measures taken so far and a statement from Petrobrás about the type of oil found in the region.
English tl;dr: Gas Chromatography and “Positive Ion Electron Impact Low Resolution Mass Spectrometry” allow them to compare the samples with known samples from other oil fields. Since petroleum has organic origins, it’s footprint is pretty much specific to the environment it comes from.
A common hypothesis in South America is that Maduro did that on purpose. Maduro and Bolsonaro are on the opposite side of the political spectrum and I don't find that hard to believe.
This is not common at all, except in a few fringe groups. Also, if you don't find that hard to believe, maybe you should check out your information sources.
It is a tiny quantity, but generate some money to some people. As everything clandestine, this transportation doesn't care much about safety and environmental precautions.
It is tough investigation to make, so maybe we will end only with those especutlations