Rock microstructure

Abstract: Porosity, permeability, and total organic carbon (TOC) in a heterogeneous suite of 21 high-maturity samples (vitrinite reflectance 1.52–2.15%) from the Barnett Shale in the eastern Fort Worth Basin display few correlations with parameters of rock texture, fabric, and composition, these factors being mostly obscured by the effects of a protracted history of diagenesis. Diagenesis in these rocks includes mechanical and chemical modifications that occurred across a wide range of burial conditions. Compaction and cementation have mostly destroyed primary intergranular porosity. The porosity (average 5 vol. % by Gas Research Institute helium porosimetry) and pore size (8 nm median pore-throat diameter) are reduced to a degree such that pores are difficult to detect even by imaging Ar ion–milled surfaces with a field-emission scanning electron microscope. The existing porosity that can be imaged is mostly secondary and is localized dominantly within organic particulate debris and solid bitumen. The grain assemblage is highly modified by replacement. A weak pattern of correlation survives between bulk rock properties and the ratio of extrabasinal to intrabasinal sources of siliciclastic debris. Higher porosity, permeability, and TOC are observed in samples representing the extreme end members of mixing between extrabasinal siliciclastic sediment and intrabasinal-derived biosiliceous debris. Reservoir quality in these rocks is neither more strongly nor more simply related to variations in primary texture and composition because the interrelationships between texture and composition are complex and, importantly, the diagenetic overprint is too strong.

Abstract: Ten types of sedimentary rock were subjected to repeated cycles of freezing and thawing. In addition to monitoring sample weight loss throughout testing, a detailed graphic record was made of deterioration mode and its relationship to pre-existing rock flaws. Results suggest that the presence or absence of rock flaws alone does not control deterioration mode, but rather that it is the coupled relationship between these flaws, and rock strength and textural properties which exerts greatest influence. While some pre-existing flaws such as syndepositional deformation structures do not appear to influence breakdown, others such as incipient fractures, cavities and minor lithological boundaries frequently coincide with concentrations of deterioration. A characteristic mode of deterioration which is independent pre-existing flaws tends to develop in sandstones, indicating the influence, in this case, of rock texture. Particularly strong rocks such as crystalline limestone and metasediment tend to fracture preferentially along distinct linear weaknesses such as mineral veins, stylolites and incipient fractures. Particularly weak rocks, such as low-density chalk, break down in a random fashion without regard to pre-existing flaws. In addition to providing some insight into the role of pre-existing flaws in rock deterioration, this work also has practical implications for (i) the study of landform development due to weathering, and (ii) the selection of representative rock samples in durability testing for building stone. Copyright (C) 2000 John Wiley and Sons, Ltd.

Abstract: With the aid of published constitutive flow laws for common crustal and upper mantle rock types, it is possible to construct curves showing variation of rock strength with depth in the continental lithosphere, assuming a particular crustal stratification of rock types and a particular geothermal gradient. Such exercises lead to the conclusion that marked changes in strength can occur at major petrological interfaces, such as the Moho discontinuity, which are therefore likely to be associated with a tendency for major, sub-horizontal detachments to form. Constitutive flow laws used for such purposes are usually assumed to represent steady-state flow, at constant rock microstructure. We argue that perhaps the most significant tectonic displacements of rock masses occur across major, localized shear zones, characterized by intense grain-size reduction, and illustrate this by reference to deformation in the metabasic and ultrabasic rocks of the lower part of the Ivrea Zone of northern Italy, which is believed to provide a section through what was once part of the deep continental crust. Calculations of the variation of the flow strength with depth of rocks should therefore use flow laws which include provision for strain dependent variations of strength, arising from strain dependent microstructural changes. We use the results of recent studies of grain-size dependent flow of olivine to estimate strain dependent variations in the flow strength of the sub-continental upper mantle, arising from dynamic recrystallization. From this we argue that after an initial period of flow at high stress, parts of the upper mantle can become very weak, if grain size is sufficiently reduced. Under favourable circumstances, flow can be concentrated immediately below the Moho, or in a band below the Moho. Such behaviour is consistent with our observations of the behaviour of dunites in the Ivrea Zone, and with some interpretations of upper mantle seismic reflectors.

Abstract: A dimensionless quantitative measure of rock texture, describing grain: —shape, orientation, degree of grain interlocking and relative proportions of grains and matrix (packing density) has been developed. Data required for the model are obtained by image analysis of thin sections and concerns percentage areas of grains and matrix, length, breadth, perimeter, orientation and area of each grain in the viewing window. The results of rock strength, diamond and percussion drillability tests in eleven sandstones, marbles and igneous rocks are reported, and correlated with the developed texture coefficient. The texture coefficient returns highly statistically significant correlations with rock strength and drillability data. Sandstones have low texture coefficients and high drillability whereas igneous rocks have high texture coefficients and low drillability. With particular reference to percussive drillability it is suggested that extensional crack propagation in the sandstones in an energy efficient process since fracture paths propagate through the weak phyllosilicate matrix. Extensional crack propagation in the igneous rocks is an energy intensive process since a significant proportion of the available drilling energy is consumed in the formation of intra-granular fracture paths. Observational and correlated data are supportive of the suggestion that the texture coefficient is a measure of the resistance of the microstructure of a rock to crack propagation, whether it be inter-or intra-granular. The texture coefficient can be used as a predictive tool for the assessment of drillability and rock strength properties. The technique offers a useful approach in understanding fracture initiation and growth as controlled by the texture of intact rock samples.