Petrophysical Sampling Research Articles

Определение остаточной водонасыщенности и ее распределения в горных породах по данным компьютерной томографии с использованием интегральных интенсивностей спектров поглощения рентгеновского излучения

This article describes the methodology and results of studies to assess the distribution of residual water saturation in real core samples using X-ray computed tomography. The algorithm for determining residual water saturationis based on the analysis of integral intensities of absorption spectra, which is also used in other methods of studying the chemical composition of a substance, for example, infrared spectroscopy, nuclear magnetic resonance.This technique has been tested on standard petrophysical samples of terrigenous and carbonate deposits, characterized by various filtration-capacitance properties and collector type. The developed technique made it possible to determine the distribution of residual water in core samples, which can serve as additional information for development engineers of oil and gas fields.

Joint inversion of gravity, magnetic, and petrophysical data — A case study from a gabbro intrusion in Boden, Sweden

We have studied a gabbro intrusion in northern Sweden, using 3D inversion of airborne magnetic data, ground-based gravity data, and petrophysical measurements on outcrop samples. Gabbro intrusions are of interest because they are potential hosts of Cu-Ni and platinum group element mineralization. We developed a joint inversion algorithm and applied it to both potential-field data sets to obtain spatial distributions of density and magnetic susceptibility. The distributions were coupled through a nonrigidly enforced parameter relationship determined from the petrophysical samples. We managed the problem of balancing the influence of the two data sets by a novel adaptive reweighting scheme that enforced the discrepancy principle for each data set independently. We demonstrated in tests with synthetic data that neither individual nor joint inversions gave reliable estimates for the depth extension of the intrusive body, the near-surface details, or any complex geometrical features. However, the joint inversion improved the image of the interface between the intrusion and the surrounding rocks and revealed that the density and susceptibility models satisfied the observed petrophysical relationship, which, in turn, caused the structures in the models to align. The geometry of the intrusion was an intrinsic result of the inversion, based on the two distinct petrophysical trends for the gabbro and the surrounding rocks. The inferred shape was simple and concise, and was therefore a useful and testable hypothesis about the subsurface geology that was in agreement with both potential-field data sets and the petrophysical information.

New compilation of top basement and basement thickness for the Norwegian continental shelf reveals the segmentation of the passive margin system

Abstract We present the first complete top basement map for the passive margin system of the Norwegian continental shelf, which covers the Northern North Sea, the mid-Norwegian margin system, and the western Barents Sea. This compilation is based largely on a review and synthesis of previously published portions on detailed depth to basement and 3D modelling studies, which in turn are based on a wealth of seismic profiles, commercial and scientific drilling on the shelf and mainland Norway, petrophysical sampling and a dense coverage of gravity and aeromagnetic data. The top basement defines the base of the sedimentary strata. Sedimentary thickness is larger than 14 km on the mid-Norwegian margin and the Barents Sea, but varies strongly along the different segments of the margin. In the northeastern North Sea, the Danish–Norwegian Basin, and over the Trøndelag Platform the top basement does not exceed 8 km in depth. The top basement map highlights the transition between the different segments of the Norwegian margin, between different basement domains (e.g. Caledonian and Precambrian basement), and shows a clear correlation with the normal faults mapped on the margin and prolongated from onshore Norway into the offshore realm. The distribution of basement domains provides a new regional understanding of the evolution of the entire passive margin system. We also present an updated Moho depth compilation, which allows calculation of a thickness of crystalline basement map for the entire margin. The basement thickness map enhances differences in crustal architecture for the different margin segments and shows extremely thin crust (less than 12.5 km) for large areas on the mid-Norwegian margin and westernmost Barents Sea, intermediate thin crust (15–20 km) below the Viking Graben, but moderately thin crust (20–30 km) below the Norwegian–Danish Basin and the remaining part of the Barents Sea.

Strategies for overcoming linewidth limitations in quantitative petrophysical NMR measurements

The simple pulse-acquire experiment has been used to evaluate the level of accuracy and precision achievable in NMR fluid saturation measurements for a range of rock core samples saturated with either brine or hydrocarbons. For a set of more than 70 cores measured at 0.66 T the mean error in the NMR measurement is only 0.35% porosity when the sample linewidths are less than 50 ppm. However, for a significant portion of cores, those with very broad NMR linewidths (>50 ppm), difficulties associated with nonuniform excitation are encountered. The magnetic susceptibility difference between pore fluid and rock matrix translates into relatively broad NMR line-widths, and this feature of petrophysical samples is the major difficulty in performing quantitative NMR experiments. Numerical simulations are used to complement the experimental results in order to develop strategies for obtaining accurate NMR results with these difficult samples.

The Magnetisation of the Elura Orebody, Cobar, NSW

Petrophysical sampling of the steeply plunging, roughly pipe-like Elura Zn?Pb?Ag orebody demonstrates a dominantly remanent magnetisation with steep negative inclination; the magnetisation is carried by central monoclinic pyrrhotite lenses. The remanence data suggests a mid-Cretaceous low grade thermal overprint possibly related to final cooling associated with minor uplift. Susceptibilities are high, averaging about 4200 x 10-6 (about 52800 x 10-6SI), and markedly anisotropic with k1:k2:k3 = 1.6:1.2:1.0. The anisotropy reflects a preferred crystallographic orientation of pyrrhotite grains ? the basal planes are vertically oriented suggesting plastic flow or crystal growth in a vertical direction. The high susceptibilities coupled with a Koenigsberger Ratio of 9.4 produce a ground magnetic anomaly 140 gammas (nT) in magnitude. This anomaly can be simply and correctly modelled by vertically plunging ellipsoids using equations that incorporate remanence, anisotropic susceptibility, and demagnetisation.