Methods and apparatus for analyzing gas-oil-water compounds in oilfield and other applications are disclosed using terahertz radiation. A sample analyzer includes a sample chamber having a fluid communication port configured to receive the sample. The analyzer also includes a filter to filter samples and selectively remove oil, water or gas from reservoir mixture received by the sample chamber. A terahertz (THz) radiation detector is provided in electromagnetic communication with the sample. The terahertz detector provides a detected output signal indicative of the terahertz electromagnetic radiation detected from the sample. In some embodiments, the device also includes a terahertz source illuminating the sample, the terahertz detector detecting a portion of the terahertz source illumination as modified by the sample. The detected portion of the spectrum of terahertz radiation can be processed to analyze the composition of the sample.