Well....
BP bought Commonwealth Oil Refineries (COR) in the 50's, so the COR brand disappeared. Then we 'merged' with AMOCO a few years back, the AMOCO brand still exists in the US. We also bought Castrol a couple of years ago too, but the branding is so widely known it makes lots of sense to keep it.
TOTAL still exists as part of TFE (TOTAL / FINA / ELF), but has a very small marketing presence in Aus, race fuels only.
CALTEX (TEXACO) bought AMPOL and are now the same company. CALTEX also bought GOLDEN FLEECE in 1981, and gradually phased the name out.
Likewise ESSO MOBIL are all part of the massive EXXON company
I had to google NEPTUNE, but that name was acquired by SHELL in 1952 and dropped in 1959 !!!!
Try this URL if you are keen on the oil company history in australia;
http://home.austarnet.com.au/jamieson/petrol.html