A downhole gas detection tool includes a housing; a first test module that includes a first fluid test chamber operable to fluidly couple to an annulus of a wellbore to receive a first portion of a wellbore fluid, the first test module further including an acoustic fluid sensor to measure a fluid acoustic velocity and attenuation of the first portion of the wellbore fluid received in the first fluid test chamber, and a fluid resistivity sensor to measure a fluid resistivity of the first portion of the wellbore fluid received in the first fluid test chamber; and a second test module including a second fluid test chamber operable to fluidly couple to the annulus of the wellbore to receive a second portion of the wellbore fluid, and a pressure-temperature (PT) sensor to measure at least one of a pressure or a temperature of the second portion of the wellbore fluid.