There are two types of gamma detectors as NaI or Ge detectors. So you can use each of them for detecting the gamma lines using gamma spectrometer. You ahve to put your sample in thinner contaner wihich take the shape of the detector to use all solid angle.
Scintillation or HPGe detector are the best choice for the detection of radionuclides.
Oil field NORM (naturally occurring radioactive materials) usually comprises Ra-226, Ra-228 and progeny. The radium is associated with the barium, calcium, and strontium salts in produced water. The salts are found as scale in the pipes. Radium is also found in sludge. Ra-226 is dominant. In some cases very little Ra-228 is found.
Gamma spectrometry will identify and can quantify the amount of the radium present. In general, there is little uranium and thorium present and it is rare to have a problem with K-40.
There is a possible problem that may be overlooked with gamma spectrometry. Pb-210 might concentrate separately from the radium when there is a large amount of lead in the produced water. If natural gas is captured at the well, Radon may be at a high concentration. Pb-210 will buildup in gas pipe lines and pumps.
If you want to conduct measurements at the well site you may need to consider regulations relating to potentially explosive atmospheres, and I don't know of a gamma spectrometry system that would be certified for use in such environments. If there are hydrocarbon residues in pipes etc this may be a consideration even away from the well head.
Not a problem if you take samples to a lab.
210Pb has a low intensity 46.5keV gamma ray, and would require HPGe spectrometry of thin samples, and a detector with good response at low energy. You can conduct alpha spectrometry for the 210Po daughter, but this requires considerable chemical extraction and questions about yield estimation (but, that may be bias because I specialise in gamma spec).