Organic matter detection in shale reservoirs using a novel pulse sequence for T1-T2 relaxation maps at 2 MHz

Quantification of organic matter and fluids contained in oil source rocks from shale plays is a fundamental goal for the petrophysical and geochemical assessment of the production potential of a well. Laboratory H-1 nuclear magnetic resonance (NMR) is a fast, reliable, and non-destructive method widely used in the oil industry. Measurement of T-1-T-2 correlation maps have been found to be critical for identifying the presence of solid organic matter, liquid hydrocarbons, and brine in these formations. However, the inherent long echo times associated with standard laboratory equipment for large sample volume challenge the detection of contribution from kemgen and viscous bitumen. In this work, we use a commercial low field NMR rock-core analyzer where a novel T-1-T-2*&T-2 pulse sequence is implemented to enable the detection of solid organic matter by acquiring the free induction decay after the first excitation pulse. This acquisition is followed by a train of refocusing pulses that generate multiple echoes from which liquid components are detected. A set of outcrop and well samples from the Vaca Muerta Formation in Argentina, with a varying amount of total organic content, were measured. The signal intensity assigned to solid matter was correlated with RockEval 6 pyrolisis. The novel pulse sequence presented here can be implemented in any commercial apparatus without the need for hardware modifications.

Automated summary

Quantification of organic matter and fluids in oil source rocks is crucial for evaluating the production potential of a well. In this study, a commercial low field NMR rock-core analyzer was used with a novel pulse sequence to detect solid organic matter and liquid components. The effectiveness of this method was verified by correlating the results with pyrolysis analysis.