Experimental Investigation of Solid Organic Matter with a 2D NMR T₁–T₂ Map

Abstract

Organic matter (OM), as the source of the hydrocarbon in shale plays, should be studied extensively from various perspectives. In this regard, using a 2D NMR T 1–T 2 map is becoming a popular method that can be used to distinguish proton populations of different sources, including the OM in shale samples. In this study, 17 shale samples with different maturities from the Bakken Formation in the United States were collected and analyzed by using the 2D NMR T 1–T 2 map method to assess the physicochemical properties of the OM. The results showed that the OM in these samples belongs to kerogen type II and is in the immature to early oil generation window stages. While the 2D NMR T 1–T 2 maps were separated into four different regions representing different sources of protons, the focus of the study was only on the signal for region 2 that is indicative of the solid OM. Based on detailed analysis of the spectrum from this region, two new parameters named T 1p and T 2p, respectively, representing the peak of the T 1 and T 2 relaxation time distributions, were proposed. The detailed analysis demonstrated that T 2P did not vary much with maturity, while the T 1P value increased as thermal maturity advanced. This resulted in an overall increase of the T 1P/T 2P ratio versus thermal maturity. T 1 relaxation time distribution more clearly exhibits the effect of thermal maturity on OM compared to T 2 relaxation time distribution in region 2. Finally, the fractal analysis of the T 1 relaxation time spectrum truncated of region 2 showed that as the maturity increased, the fractal dimension declined, which suggests that the T 1 relaxation time attributes a more chemically homogeneous structure to the OM.

Quantification of Imbibed Heptane in Shale Rocks Determined by Edited T1–T2 Nuclear Magnetic Resonance Relaxation Experiments at High Magnetic Field

“…Figure A and B shows the T 1 – T 2 maps for the sample without heptane and a partially saturated one, respectively, where four regions can be distinguished, following the separation of relaxation pools introduced by Liu et al., Khatibi et al, Liu et al, and Yang and Kausik . Region 1 is not affected by heptane’s presence and can be attributed to the hydroxyl group present in the inorganic components of the shale, such as clay, clay-bound water, or other minerals.…”

“…This led to performing separate experiments where dipolar interactions were refocused to detect solid-like components that were correlated to organic matter content. − When performing experiments at higher magnetic fields, the inductance and capacitance of the components from the probes resonant circuit are lower, resulting in a more rapid decay of the transients between the application of radio frequency pulses and detection, namely, the receiver dead time. At ∼20 MHz, for instance, echo times are reduced to values of about 70 μs, where signals from kerogen and bitumen are detected, and fluids are clearly resolved. − …”

Application of Gaussian and Exponential Inversion for 2D Relaxation Maps in Source Rock Oil and Gas Reservoirs

“…A synthetic data set was generated based on the results reported for source rock oil and gas reservoirs and used to analyze a priori known distribution of both Gaussian and exponential decay functions. The T 1 −T 2 maps for shales can be divided into four regions, following the separation of relaxation pools introduced by Liu et al, 19 Khatibi et al, 20 Liu et al, 21 and Yang and Kausik, 44 which correspond to OM or confined fluids. Figure 1A shows two scanning electron microscopy (SEM) images of a sample from the Vaca Muerta formation where OM, organic pores (OPs), IPs, and pores in clays (CP) are shown.…”

“…Two-dimensional (2D) T 1 – T 2 correlation maps are sensitive to the molecular environment and molecular dynamics. Moreover, they can separate information originating in different molecular mechanisms and enable for example the differentiation between OM and the fluids in the rock sample. − However, in shale relaxometry, solid and fluid signals coexist, which requires the design of specific sequences for the acquisition and tailored methods for data processing . Liquid signal spin–spin decays are well described using a distribution of exponential decay functions ( T 2E ), while signals from solid spin–spin decays, which correspond mainly to kerogen, are described by Gaussian decay functions ( T 2G ) which are mainly driven by strong intermolecular dipolar couplings .…”