Understanding the reservoir fluids, hydrocarbon-generating potential, and formation characteristics of a reservoir rock formation is important to estimate the reserve and to improve the efficiency of oil field operations such as drilling, well completion, and production. To this end, various types of formation and reservoir analysis are performed using downhole tools. Further, various types of rock sample analysis are performed using laboratory tools at earth's surface. One example of formation or rock sample analysis involves nuclear magnetic resonance (NMR) phenomena.
An example NMR tool includes a magnet assembly that produces a static magnetic field (B0), and a coil assembly that generates radio frequency (RF) electromagnetic signals (including a perturbing magnetic field (B1)) and that detects magnetic resonance phenomena. The behavior of nuclei in presence of B0 and B1 has been correlated with formation rock and fluid properties such as the amount of hydrogen in pore space fluids and/or the porosity.
Total organic carbon (TOC) is an important reservoir quality indicator, particularly for unconventional source rock reservoirs. It is well-known that TOC can be obtained from laboratory mineralogy and fluid analysis using techniques such as the combination of X-ray diffraction (XRD) and X-ray fluorescence (XRF). However, laboratory tests are expensive and time consuming. Performing such tests for a large number of samples corresponding to a particular reservoir or even a single well is impractical. Nuclear logging based mineralogy analysis can also be used to estimate TOC, provided that all elements in the formation can be resolved, which is not always possible. Combining density and NMR logging measurements can also be used to estimate TOC, with an assumption that NMR logs do not include any signals from organic matters, liquid or solid, in the source rocks, an assumption not always valid.
It should be understood, however, that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims.