Abstract Microstructural studies using a dual beam FIB-SEM instrument reveal dimensions of pores within shales, which are consistent with macroscopic-averaged dimensions resolved by nuclear-magnetic resonance and mercury-injection capillary pressure. These dimensions are on the order of nanometers to hundreds of nanometers. We compare observations on a limited number of samples from the Haynesville to observations on the Woodford, Barnett, and Marcellus Shales. The FIB-SEM imaging uniquely resolves where the pores lie, that is, mainly within kerogen in the Woodford and Barnett and between clay platelets for the Haynesville samples. Measurement of velocities as a function of pressure and calculated anisotropies display a pressure dependence that reflects difference in the microstructure studied. The Woodford samples show a weak velocity dependence on pressure whereas the Haynesville samples show a very strong dependence. Coupled with the validity of the effective pressure law, the pressure dependence of anisotropy may prove useful in monitoring pressure depletion and compartmentalization in the Haynesville Shale.

Abstract Integrated reservoir characterization relies increasingly on vastly improved log-based results from new technologies such as Nuclear Magnetic Resonance, NMR . Our experimental study is designed to extract more petrophysical information from NMR for reservoir characterization. We compared the empirical permeability estimation based on NMR with direct measurements; evaluated the use of NMR observations in providing capillary pressure estimates; and the use of NMR to classify rock types. Using a 2MHz NMR spectrometer, we analyzed 90 clastic cores from five different wells. Cores were measured at 100% brine saturated and at irreducible saturation achieved through centrifuging the core plugs at 5800 rpm. High pressure mercury injection was performed on parallel samples from the same plug. The porosity of the cores studied ranged from 4% to 23% while measured permeabilities ranged from 0.01 md to 900 md. The measured T 2 cutoffs (i.e. , the boundary between free and bound water) ranged from 6 ms to 100 ms, which represents significant departures from the typically assumed 33 ms cutoff for clastics. Mineralogy appears to have an influence on the T 2 cutoff value. In general the permeability estimation based on the weighted geometric mean of the T 2 time is better than the model based on the ratio of free fluid index to bound volume index. Additionally, mapping NMR and mercury measurements provided estimates of surface relaxivities, which ranged from 16 to 50 μm/sec. Measurements of surface relaxivity allow the empirical mapping of NMR data to capillary pressure data. The mismatch between the cumulative NMR and mercury data at lower and higher T 2 times reflects differences in how the pore space is accessed between the NMR and the Hg measurements. The applicability of NMR T 2 distribution for rock typing is discussed. It is observed that NMR is more sensitive to subtle pore characteristics (dimension, shape, and composition, etc. ) as compared to Levertt J-function derived from capillary pressure and may provide an alternative method for rock typing.

Abstract The fluvial-deltaic sandstones of the Cretaceous Ferron Sandstone, Utah, provide an opportunity to document and compare petrophysical properties of outcrop and subsurface rocks. We find that the processes that generate outcrop exposures — uplift, erosion, and exhumation — can overprint patterns of velocity, porosity, and permeability developed in the subsurface. Burial to depths of 3000-3400 m (9800-11,100 ft), with associated compaction and carbonate cementation, was followed by uplift, which exhumed different portions of the Ferron Sandstone by 0 to >3400 m. A complex diagenetic history culminated with the development of secondary intergranular porosity by carbonate dissolution during exhumation, because of increasing groundwater flux at depths shallower than ~2 km (7000 ft) subsurface. Velocity logs show velocity decreases larger than expected from porosity increase; we attribute the excess to presence of microcracks. Outcrop plugs exhibit even higher porosity and lower velocity than shallow logs, probably because of enhanced leaching of carbonate cement. Ferron core-plug and log velocity responses to this secondary porosity are comparable to that of primary inter-granular porosity, but these samples lack permeability anisotropy and sensitivity of velocity and permeability to clay content, both of which are typical of primary porosity. Ferron Sandstone permeability is very closely related to porosity, and therefore exhumation increases permeability by porosity enhancement. The influence of grain size on porosity and permeability persists after both initial compaction/cementation, and subsequent exhumation and secondary porosity development. Consequently, Ferron outcrop stratigraphy can provide useful clues to fluid-flow patterns in other deltaic formations, despite its complex diagenetic history.