Oil majors Shell and Eni will be carefully monitoring a first ruling this week by a Milan judge in one of the energy industry’s biggest corruption scandals for clues to what might be round the corner for them.

The two companies are embroiled in a long-running graft case revolving around the purchase in 2011 of one of Africa’s biggest oilfields—Nigeria’s OPL 245—for about $1.3 billion.

The case, which involves Eni CEO Claudio Descalzi and four former Shell managers including one-time Shell Foundation Chairman Malcolm Brinded, has spawned legal cases spanning several countries and is expected to drag on for months.

But Sept. 20, in a case running parallel to the main trial, a judge will decide, for the first time, whether $1.1 billion of the sum paid was siphoned in bribes to win the license to the field.

Eni and Shell, as well as their managers, deny any wrongdoing.

While the ruling will not tie the court’s hand in the main trial, it will nonetheless constitute a sort of prejudgment, a legal source involved in the case said.

“It’s clear the ruling will become a first building block in favor of the prosecution or the defense ... it will be a first verdict by a third-party judge on the matter,” the source said.

In a fast-track procedure that began last November, the judge will be called on to decide whether Nigeria’s Emeke Obi and Italian Gianluca Di Nardo, who the prosecution says were middle men, should be convicted in the case or acquitted.

Prosecutors allege Obi received a mandate from former Nigerian oil minister Dan Etete to find a buyer for OPL 245, collecting $114 million. Di Nardo, they said, took $24 million of that amount for putting Obi in touch with Eni.

Those alleged payments were illegal kickbacks, prosecutors say.

Obi and Di Nardo, who have previously denied any wrongdoing, asked for a fast-track trial which under Italian law allows any eventual sentence to be cut by a third.

Etete has previously denied charges of bribery for channeling money from the OPL 245 deal to Nigerian politicians and officials.

Nigeria’s OPL 245 is one of the biggest sources of untapped oil reserves on the African continent with reserves estimated at 9 billion barrels. Because of the ongoing dispute, it has never entered into production.

If found guilty, the individuals on trial face possible jail terms for bribery while the companies face hefty fines.