Petrophysical Characterization of Secondary Organic Matter and Hydrocarbons in the Early Jurassic Formation Using Laboratory NMR Techniques

The presence of solid bitumen in a deep carbonate reservoir formation, onshore Kuwait, occludes primary inter-crystalline porosity and thus impacts permeability. Here, we describe a petrophysical model to both quantify in-situ solid bitumen and estimate the effect of this bitumen on reservoir productivity. This is accomplished by adapting established nuclear magnetic resonance (NMR) workflows calibrated to laboratory NMR techniques on dolomite core samples with secondary organic matter or bitumen, primarily insoluble pyrobitumen. The NMR-based “deficit porosity” method has been tested in the industry to identify the presence of solid organic (bitumen) matter. However, this method is not universally robust for quantifying bitumen saturation, as the magnitude of the “deficit porosity” is dependent on inter-echo time (TE), in-situ temperature, and bitumen viscosity. In this study, laboratory NMR techniques are utilized to calibrate a multimineral petrophysical evaluation model incorporating available well-log data, including NMR. This evaluation scheme determines mineral volume fractions, including bitumen, porosity, and saturations. The NMR data are calibrated through a combination of both one-dimensional (1D) T2 relaxation time and two-dimensional (2D) T1-T2 map measurements. The measurements were performed at low frequency (LF, 2 MHz) for the purpose of comparing with NMR well logging, and at high frequency (HF, 23 MHz) for higher sensitivity and better map resolutions, respectively. The HF measurements with an inter-echo spacing time (TE) of 0.07 ms show that a solid-like component with its T2 < 0.1 ms can be detected at ambient temperature. In contrast, the LF maps or T2 distributions failed to detect such solid-like components due to the longer inter-echo spacing time (TE = 0.2 ms) used in the measurements, at room temperature. However, the LF signals are enhanced at the elevated temperatures, indicating contributions from the mobilized solid-like components. In order to interpret NMR logging, further data analysis was applied to the LF NMR T2 distributions at various temperatures by fitting the T2 distribution into a summation of three Gaussian functions. The fitting permitted the separation of partially overlapping T2 signal components. The results suggest that the two longer T2 components, due to bitumen and fluids in the core, are temperature dependent following the Arrhenius relationship, while the shortest T2 component (also due to bitumen) is temperature independent in the temperature range studied. The recognition that LF NMR (comparable to the logging tool measurement) can detect the bitumen component at reservoir conditions implies that the “deficit porosity” method may underestimate the volume of bitumen in situ, as the shortest peak(s) are associated with the solid-like organic matter. Further analysis on fully brine-saturated core plugs demonstrates that the magnitude of the short detectable T2 signal component from bitumen in LF depends on the relative abundance of soluble bitumen to pyrobitumen. Note that these carbonate samples do not contain any clay; hence, there is no clay-bound water present. Lastly, a permeability model for bitumen-prone intervals is presented based on the relative proportion of solid bitumen to original (or paleo-) inter-crystalline porosity. This model is calibrated to pyrolysis, to determine total organic carbon (TOC), and organic petrology available on all of the samples. Pore-filling solid bitumen generally reduces permeability by about one order of magnitude, relative to equivalent bitumen-free zones. The permeability reduction can be up to two orders of magnitude in the most bitumen-rich intervals.