Source rock in petroleum exploration is rocks that produce hydrocarbon naturally at a temperature between 65 and 150 degrees Celcius for crude oil and between 150 and 200 degrees Celcius for gas. Hydrocarbon produced will expel out of source rocks and migrate to trapped porous rocks or reservoir rock. Source rocks can be identified by means of geochemical and geophysical approaches to determine the source rocks properties such as carbon content and kerogen types. Geophysically, thorium uranium ratio from spectral gamma ray log can be used to determine source rock.
Knowing the source rock of hydrocarbon is important in exploration stage to decrease risks. It is a hint to where should be the next drilling. It also can be used to identify the hydrocarbon-productive zones, lithology and reservoir content.
Typically, gamma ray logs combined with neutron and resistivity log can be used to identify source rock. It is believed that the ratio of gamma radiation emitted by Thorium (Th) to gamma radiation emitted by Uranium (U) is an indicator of hydrocarbon source rock. The ratio of Thorium to Uranium for source rock is less than 2.5. Thorium Uranium and Potassium are emitting gamma ray naturally with gamma radiation level of 2.61, 1.76, and 1.46 MeV respectively. By running those three logs, Th/U and carbon content can be concluded.
Some elements and minerals will emit different gamma radiation levels by bursting the elements or minerals with neutrons. Hydrogen will emit 2.2 MeV of gamma radiation whereas carbon and oxygen will emit 4.43 MeV and 6.13 MeV respectively. By measuring the gamma radiation of carbon, carbon content of zones can be concluded. Combination of carbon content from resistivity log, carbon content from gamma radiation or spectral gamma ray emitted by C (gamma ray logs), and the ratio of uranium and thorium (TH/U) can be used to identify hydrocarbon source rock.
Figure 1 shows a diagram of neutron-induced gamma ray instrument. The neutron source can pulse at a rate of 10000 burst/second (10 kHz) with bursting energy of 14 MeV. Gamma logging speed between 15 and 30 ft/minute is adequate.