While Chinese regulatory approval was long considered a potential stumbling block, Shell's $10 billion takeover attempt for Woodside was famously blocked by the Howard government in 2001.

In this recent case, Credit Suisse unsurprisingly viewed that the combination of Shell's half-owned Arrow Energy CSG resources in Queensland - once designated for an Arrow LNG project - with BG's Queensland Curtis LNG operation and CSG resources could be an issue for regulators.

"There probably is the potential for a case to be made for a ‘negotiated remedy', where part of the reserves are sold off, either via an asset sale or contracted volumes," Credit Suisse said in a client note over the issues raised from a possible Arrow tie up with QCLNG.

The broker also considered that PetroChina, the other half owner of Arrow, might agree to a deal to capture a stake in a third expansion train of QCLNG - which could also be an issue to Australian regulators who were worried about the domestic supply front.

"We expect no significant hurdles in Brazil, but both China and Australia are likely to come with greater scrutiny and uncertainty," Credit Suisse said.

Shell is not revealing any cards yet in regards to Arrow-BG synergy opportunities.

The oil giant's chief financial officer Simon Henry was reportedly non-committal during a press conference in April "when pressed on the potential" for a third train at QCLNG that could use Arrow's uncommitted CSG reserves.

Shell's takeover of BG is expected to be completed in early 2016.