There is considerable variation in the carbon intenisty of different fossil fuels
Many of the countries that have had considerable success in reducing their emissions have done so not by switching to renewables but by switching to lower carbon fossil fuels. For example, gas emits roughly half the carbon dioxide that coal does when it’s burned to produce power. Such "shallow decarbonisation" - based on a large-scale switch to shale gas - has been responsible for a 29% drop in US power sector emissions since 2005.
This is a difficult area beset with much disagreement.
Some argue that the most urgent priority is to put an end to coal and that we need natural gas as a "bridge" to renewables. Others argue that this does not get us far beyond business as usual. They want to see us leapfrog straight to zero emissions power sources - pointing out that once we build the infrastructure for gas we will get "stuck" there for many years / decades, depleting our carbon budget.
An under-utilised option in this area is to deploy carbon capture and sequestration (CCS) technologies at scale in the power sector and elsewhere in industry. Deployment has been very slow because of the high cost and limited incentives to add CCS.
A further way to reduce the carbon intensity of oil production is through enhanced oil recovery (EOR) whereby CO2 (or CO2 plus water) is injected into depleted oil fields to extract oil which would otherwise not have been recovered. The two potential advantages of this are that (a) theoretically it provides a market for captured CO2, although often the CO2used is manufactured not captured; and (b) some of the CO2 used in the EOR is captured for the long term.