Robert M. Reed

Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores

Matrix-related pore networks in mudrocks are composed of nanometer- to micrometer-size pores. In shale-gas systems, these pores, along with natural fractures, form the flow-path (permeability) network that allows flow of gas from the mudrock to induced fractures during production. A pore classification consisting of three major matrix-related pore types is presented that can be used to quantify matrix-related pores and relate them to pore networks. Two pore types are associated with the mineral matrix; the third pore type is associated with organic matter (OM). Fracture pores are not controlled by individual matrix particles and are not part of this classification. Pores associated with mineral particles can be subdivided into interparticle (interP) pores that are found between particles and crystals and intraparticle (intraP) pores that are located within particles. Organic-matter pores are intraP pores located within OM. Interparticle mineral pores have a higher probability of being part of an effective pore network than intraP mineral pores because they are more likely to be interconnected. Although they are intraP, OM pores are also likely to be part of an interconnected network because of the interconnectivity of OM particles. In unlithifed near-surface muds, pores consist of interP and intraP pores, and as the muds are buried, they compact and lithify. During the compaction process, a large number of interP and intraP pores are destroyed, especially in ductile grain-rich muds. Compaction can decrease the pore volume up to 88% by several kilometers of burial. At the onset of hydrocarbon thermal maturation, OM pores are created in kerogen. At depth, dissolution of chemically unstable particles can create additional moldic intraP pores.

Natural fractures in the Barnett Shale and their importance for hydraulic fracture treatments

Gas production from the Barnett Shale relies on hydraulic fracture stimulation. Natural opening-mode fractures reactivate during stimulation and enhance efficiency by widening the treatment zone. Knowledge of both the present-day maximum horizontal stress, which controls the direction of hydraulic fracture propagation, and the geometry of the natural fracture system, which we discuss here, is therefore necessary for effective hydraulic fracture treatment design. We characterized natural fractures in four Barnett Shale cores in terms of orientation, size, and sealing properties. We measured a mechanical rock property, the subcritical crack index, which governs fracture pattern development. Natural fractures are common, narrow (0.05 mm; 0.002 in.), sealed with calcite, and present in en echelon arrays. Individual fractures have high length/width aspect ratios (1000:1). They are steep (75), and the dominant trend is west-northwest. Other sets trend north-south. The narrow fractures are sealed and cannot contribute to reservoir storage or enhance permeability, but the population may follow a power-law size distribution where the largest fractures are open. The subcritical crack index for the Barnett Shale is high, indicating fracture clustering, and we suggest that large open fractures exist in clusters spaced several hundred feet apart. These fracture clusters may enhance permeability locally, but they may be problematic for hydraulic fracture treatments. The smaller sealed fractures act as planes of weakness and reactivate during hydraulic fracture treatments. Because the maximum horizontal stress trends northeast-southwest and is nearly normal to the dominant natural fractures, reactivation widens the treatment zone along multiple strands.

A 48 m.y. history of fracture opening, temperature, and fluid pressure: Cretaceous Travis Peak Formation, East Texas basin

Quartz cement bridges across opening-mode fractures of the Cretaceous Travis Peak Formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low-porosity sandstone. Incremental crack-seal fracture opening is inferred based on the banded structure of quartz cement bridges, consisting of up to 700 cement bands averaging ∼5 μm in thickness as observed with scanning electron microscope–cathodoluminescence. Crack-seal layers contain assemblages of aqueous two-phase fluid inclusions. Based on fluid inclusion microthermometry and Raman microprobe analyses, we determined that these inclusions contain methane-saturated brine trapped over temperatures ranging from ∼130°C to ∼154°C. Using textural crosscutting relations of quartz growth increments to infer the sequence of cement growth, we reconstructed the fluid temperature and pore-fluid pressure evolution during fracture opening. In combination with published burial evolution models, this reconstruction indicates that fracture opening started at ca. 48 Ma and above-hydrostatic pore-fluid pressure conditions, and continued under steadily declining pore-fluid pressure during partial exhumation until present times. Individual fractures opened over an ∼48 m.y. time span at rates of 16–23 μm/m.y. These rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings.

Grain assemblages and strong diagenetic overprinting in siliceous mudrocks, Barnett Shale (Mississippian), Fort Worth Basin, Texas

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.

Predicting and characterizing fractures in dolostone reservoirs: using the link between diagenesis and fracturing

Abstract Fracture geometries and fracture-sealing characteristics in dolostones reflect interactions among mechanical and chemical processes integrated over geological timescales. The mechanics of subcritical fracture growth results in fracture sets having power-law size distributions where the attributes of large, open fractures that affect reservoir flow behaviour can be accurately inferred from observations of cement-sealed microfractures and other microscopic diagenetic features, which are widespread in dolostones. Fracture porosity is governed by the competing rates of fracture opening and cement precipitation during fracture growth and by cements that post-date fracture opening. Combined analysis of structural and diagenetic features provides the best approach for understanding how fracture systems influence fluid flow. We review previous work and integrate new data on fractures and diagenetic features in cores from the Lower Ordovician Ellenburger and Permian Clear Fork formations in West Texas, and the Lower Ordovician Knox Group in Mississippi, together with outcrop samples of Lower Cretaceous Cupido Formation dolostones from the Sierra Madre Oriental, Mexico, in order to illustrate our approach.

Opening histories of fractures in sandstone

Abstract High-resolution scanning electron microscope (SEM)-based cathodoluminescence images were used to reconstruct incremental fracture opening in regional opening-mode fractures in sandstone. Opening is recorded by crack-seal texture in isolated mineral bridges that span opening-mode fractures formed in sandstone at moderate-great depth (c. 1000–6000 m). We restored opening histories of nine representative fractures with apertures of millimetres in five sandstones from five sedimentary basins. Gaps created by fracture widening in 11 bridges range from less than 1 μm to more than 1 mm, but nearly all are less than 20 μm and most are less than 5 μm. These are the opening amounts that could be spanned by cement growth in these diagenetic environments. Our observations are the first evidence of opening amounts from mostly porous, opening-mode (joint-like) fractures formed in diagenetic environments. Patterns are consistent with a new structural diagenetic model of bridge growth that can use opening patterns to indicate rate of fracture opening as a function of time.

How to Overcome Imaging Problems Associated with Carbonate Minerals on SEM-Based Cathodoluminescence Systems

On SEM-based cathodoluminescence systems (scanned CL), methods are needed to overcome image-quality problems caused by persistent luminescence of carbonate minerals. An effective solution to the persistence problem is to acquire images using only the shorter wavelengths, most easily done by using a broadband, short-wavelength (UV-blue range) filter. The filter used provides 80% to 90% transmissivity in the range of 385 to 495 nm and some transmissivity as low as 350 nm. The filter allows transmission of the relatively nonpersistent UV-violet luminescence present in most carbonates in the range of 350 to 425 nm, but blocks the common orange-red wavelength luminescence found in carbonates. The lack of imaging problems in subsequent images shows that persistent luminescence in carbonates is primarily in the orange-red wavelengths. Cathodoluminescence images produced using a short-wavelength filter are comparable in detail to those obtained from conventional light-microscope-based cathodoluminescence systems. In almost all examples, features visible in the orange-red wavelengths show corresponding variations in luminescence in the shorter wavelengths.

Proterozoic granites of the Llano Uplift, Texas: A collision–related suite containing rapakivi and topaz granites

Proterozoic granites in the Llano Uplift, central Texas, intruded metamorphic rocks in a southern segment of the Grenville orogen. Three compositional groups of granite are present; two of these have not been previously recognized. The most widespread group occurs in three emplacement styles: (1) irregular plutons (synkinematic) were emplaced at 1119–1116 Ma during low-pressure Grenville metamorphism and deformation; (2) associated with these, there are volumetrically minor rhyolite dikes (1098–1093 Ma) and hypersolvus granite; and (3) large 1093–1072 Ma postkinematic circular to ovoid plutons are the most abundant of this compositional group, and many show rapakivi textures. A second, more felsic group has much higher light (L) to heavy (H) rare earth element (REE) ratios. The third group is highly fractionated, contains topaz locally, is enriched in Rb, Nb, and HREEs, and is depleted in Sr, Zr, and LREEs. No other igneous rocks were emplaced at the same time as the granites at the current level of exposure. Llano granites cannot be considered anorogenic or A-type on the basis of their tectonic setting, timing, or compositions. Sr, Nd, and Pb isotopic data indicate that these granites were derived from Proterozoic mantle and crust. The granites intruded thick continental crust at one margin of Laurentia and completed the consolidation of one segment of the Grenville orogen.

Quantifying Compaction and Cementation in Deformation Bands in Porous Sandstones

Combined electron microbeam imaging techniques can be used to quantify cementation and compaction processes in deformation bands and their surrounding host rocks. Mixed secondary and backscattered electron signals can be used to definitively identify pore space, whereas scanned cathodoluminescence can be used to discriminate between detrital and authigenic quartz. Classic deformation bands from three porous sandstone units, the Cambrian Hickory Sandstone of central Texas (two bands and two host rocks), the Pennsylvanian Tensleep Sandstone of Wyoming (one band and one host rock), and the Pennsylvanian Weber Sandstone of northwestern Colorado (one band and one host rock), were examined using these imaging techniques. Cathodoluminescence images demonstrate that bands develop through a combination of grain-scale brittle processes and cementation. Point counting of scanning electron microscopy image mosaics reveals that the intergranular volume in deformation bands is higher than is apparent from transmitted light microscopy. Cementation equals or exceeds compaction as a cause of porosity decline in both deformation bands and host rocks. No evidence exists for significant pressure solution during band development. The intergranular volumes of host rocks in the range of 31–38% suggest that all of these samples have experienced burial of 2 km (1.2 mi) or less. Contrasts in the compactional and cementational states of bands and surrounding host rocks possibly reflect the differing availability of quartz nucleation surfaces in these different parts of the rock. Preferential emplacement of cement in the bands can lead to divergent paths of compactional behavior in bands relative to host rocks during the postkinematic phase of their burial history.

Nature and Age of Ductile Deformation Associated with the “Anorogenic” Town Mountain Granite, Llano Uplift, Central Texas

1.07-1.13 Ga granites comprise almost half of the rocks in the Proterozoic (Grenvillian) Llano Uplift of central Texas and thus have contributed significantly to the evolution of basement along the southwestern margin of Laurentia. Intrusions of Town Mountain Granite in the Llano Uplift have previously been described as “anorogenic”, but recent work has shown that deformation accompanied and/or postdated intrusion of some granites.