Evaluating movable hydrocarbon in shale reservoirs using conventional logs is challenging. In shale reservoirs, the producibility is related to maturity of organic matter and ultimately the type of hydrocarbon present in the rock. Measurements such as total organic carbon (TOC) provide useful input; however, differentiating the organic carbon into kerogen and liquid hydrocarbon requires additional information. Similarly, although the insensitivity of nuclear magnetic resonance (NMR) to kerogen is an advantage, the overlapping T2 response of various fluids brings uncertainty to T2-based NMR interpretation.
Recent laboratory-based experiments have shown significant contrast in the T1/T2 ratio between water in small pores and bitumen or hydrocarbon hosted by organic pores. The two may appear as distinct signal amplitudes on a T1-T2 map. Typical downhole NMR logs have a low signal-to-noise ratio (SNR), which causes the inverted map to have broadened distributions obscuring the individual fluid amplitudes. In spite of broadened distributions, the mean signal corresponds to the relative fluid concentrations and can be converted into saturation, provided the T1 of oil and water with respect to T2 are known. The challenge is that the T1/T2 ratio of various fluid components may be nonunique and is usually unknown.
A new workflow integrates the TOC provided by spectroscopy, bulk density, and T1-T2 NMR measurements to quantify the volumes of kerogen, bound and producible hydrocarbon, and water. The workflow relates TOC to the volume fraction of kerogen, bitumen, and light hydrocarbon. The difference between NMR and matrix-adjusted density porosity provides an estimate of kerogen. A typical T1-T2 map in organic shale is comprised of the signals due to bitumen, light oil, and water. Bitumen and light oil usually have distinct T2 distributions and can be separated if the overlapping water signal is eliminated. The workflow exploits the T1/T2 ratio contrast between oil and water to eliminate the water signal. Although input of the T1/T2 ratio of water is required, the T1/T2 ratio of oil is optimized through an iterative process such that the derived volume fractions of bitumen and light oil satisfy the TOC-to-volume relationship.
The NMR sensitivity to short T1 and T2 components is imperative for the successful application of this method. The data from a new NMR tool fulfilled the requirement and provided the essential input. The integration provided a complete pore fluid volume analysis in several shale reservoirs. The predicted volumes compare well with core-derived measurements.