ALBERT

Description: Crack-seal texture within fracture cements in the Triassic El Alamar Formation, NE Mexico, shows that the fractures opened during precipitation of quartz cements; later, overlapping calcite cements further occluded pore space. Previous workers defined four systematic fracture sets, A (oldest) to D (youngest), with relative timing constrained by crosscutting relationships. Quartz fluid inclusion homogenization temperatures are higher within Set B (148 ± 20°C) than in Set C (105 ± 12°C). These data and previous burial history modelling are consistent with Set C forming during exhumation. Fluid inclusions in Set C quartz have higher salinity than those in Set B (22.9 v. 14.2 wt% NaCl equivalent, respectively), and Set C quartz cement is more enriched in 18 O (20.2 v. 18.7 VSMOW). Under most assumptions about the true temperature during fracture opening, the burial duration, the amount of cement precipitated and fluid-flow patterns, it appears that the fracture fluid became depleted in 18 O and enriched in 13 C. This isotopic evolution, combined with increasing salinity, suggests that throughout fracture opening there was a gravity-driven influx of fluid from upsection Jurassic evaporites, which form a regional décollement. Fracture opening amid downward fluid motion suggests that fracturing was driven by external stresses such as tectonic stretching or unloading, rather than increases in fluid pressure.

Description: The Late Proterozoic Torridon Group Applecross Formation in the foreland of the Moine Thrust Belt, NW Scotland, contains deformation bands, three fracture sets (from oldest to youngest A, B, and L) defined by orientation, crosscutting relations, and progressively less quartz cement in younger sets, and joints. Set A crosscuts deformation bands and strikes north–south. Set B has trimodal orientation defining three linked subsets that formed concurrently. Set L strike ranges from NE–SW to ENE–WSW, in parent crack–wing crack arrays that formed progressively; these are more abundant near small-displacement, oblique-slip faults that offset the overlying Cambrian Eriboll Formation and the Moine Thrust Belt. Applecross sandstones have low fracture abundance, possibly a consequence of low elastic moduli (Young’s modulus 2.3–17.0 GPa, most values

Description: The Late Proterozoic Torridon Group Applecross Formation in the foreland of the Moine Thrust Belt, NW Scotland, contains deformation bands, three fracture sets (from oldest to youngest A, B, and L) defined by orientation, crosscutting relations, and progressively less quartz cement in younger sets, and joints. Set A crosscuts deformation bands and strikes north–south. Set B has trimodal orientation defining three linked subsets that formed concurrently. Set L strike ranges from NE–SW to ENE–WSW, in parent crack–wing crack arrays that formed progressively; these are more abundant near small-displacement, oblique-slip faults that offset the overlying Cambrian Eriboll Formation and the Moine Thrust Belt. Applecross sandstones have low fracture abundance, possibly a consequence of low elastic moduli (Young’s modulus 2.3–17.0 GPa, most values 〈6.9 GPa) and moderate to high subcritical crack index (45–78), resulting from compacted soft lithic grains and clay-mineral cements. Low abundance contradicts models that postulate persistent incipient failure by subsurface fracture. The fracture sequence resembles that found in the overlying Cambrian Eriboll Formation quartzarenites, implying that no widespread late Proterozoic fracture sets exist in this part of the Applecross Formation, an uneventful record for a rock profoundly resistant to brittle deformation.

Description: Natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low-porosity and low-permeability shale host rock. Many shale outcrops, cores, and image logs contain fractures or fracture traces, and microseismic event patterns associated with hydraulic-fracture stimulation have been ascribed to natural fracture reactivation. Here we review previous work, and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization, orientation, and size patterns. A wide range of shales have a common suite of types and configurations of fractures: those at high angle to bedding, faults, bed-parallel fractures, early compacted fractures, and fractures associated with concretions. These fractures differ markedly in their prevalence and arrangement within each shale play, however, constituting different fracture stratigraphies—differences that depend on interface and mechanical properties governed by depositional, diagenetic, and structural setting. Several mechanisms may act independently or in combination to cause fracture growth, including differential compaction, local and regional stress changes associated with tectonic events, strain accommodation around large structures, catagenesis, and uplift. Fracture systems in shales are heterogeneous; they can enhance or detract from producibility, augment or reduce rock strength and the propensity to interact with hydraulic-fracture stimulation. Burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated. The role of microfractures in production from shale is currently poorly understood yet potentially critical; we identify a need for further work in this field and on the role of natural fractures generally.

Description: Natural fractures form preferred pathways for basinal fluid flow and associated heat and mass transport. In gas sandstone reservoirs with low matrix permeability, fractures provide flow pathways between organic-rich source and reservoir layers during gas charge, and between matrix pores, hydraulic fractures, and the well bore during production. While the formation of natural fractures has previously been associated with gas generation and pore-fluid pressure increase through a process referred to as natural hydraulic fracturing, other driving mechanisms such as stress changes by tectonic or exhumation processes remained viable alternatives. To test whether these mechanisms contributed to fracture development, we investigated the spatial and temporal distribution of fracture formation and its relationship to gas generation, migration, and charge in sandstone of the Cretaceous Mesaverde Group across the entire production interval on a basinwide scale. Using fluid inclusion microthermometry of crack-seal fracture cement formed concurrently with fracture opening, we observed temperature trends that, when compared with temperature evolution models of the formation, date fracture formation between 41 and 6 Ma in the northern and between 39 and 6 Ma in the southern Piceance Basin. The onset of fracture formation 20–30 m.y. prior to maximum burial eliminates changes in stress state associated with exhumation as a mechanism for triggering the onset of fracture formation. Instead, calculated paleo–pore-fluid pressures of 40–90 MPa (5800–13,000 psi) during fracture opening and the presence of methane-rich inclusions in fracture cement suggest that fracture formation was aided by high pore-fluid pressures during gas generation in organic-rich shales and coals and associated charging of adjacent and interlayered sandstone reservoirs. A 10–20 m.y. age progression in the onset of fracture formation from deeper to shallower horizons of the Mesaverde Group is consistent with gas generation and onset of fracture formation activated by burial temperature with limited upward migration of gas at this stage of reservoir evolution. This age progression with depth is inconsistent with fracture formation triggered by changes in stress conditions associated with tectonic or structural processes expected to affect the entire formation synchronously. Our observations are thus most consistent with fracture formation by natural hydraulic fracturing in response to gas generation in interbedded source layers and reservoir charge. Based on widespread observations of fractures with similar structural and diagenetic attributes, we consider natural hydraulic fracture formation in response to thermocatalytic gas generation to be a fundamental mode of brittle failure in otherwise structurally quiescent basins.

Description: The Upper Cretaceous Mesaverde Group in the Piceance Basin, Colorado, is considered a continuous basin-centered gas accumulation in which gas charge of the low-permeability sandstone occurs under high pore-fluid pressure in response to gas generation. High gas pressure favors formation of pervasive systems of opening-mode fractures. This view contrasts with that of other models of low-permeability gas reservoirs in which gas migrates by buoyant drive and accumulates in conventional traps, with fractures an incidental attribute of these reservoirs. We tested the aspects of the basin-centered gas accumulation model as it applies to the Piceance Basin by determining the timing of fracture growth and associated temperature, pressure, and fluid-composition conditions using microthermometry and Raman microspectrometry of fluid inclusions trapped in fracture cement that formed during fracture growth. Trapping temperatures of methane-saturated aqueous fluid inclusions record systematic temperature trends that increase from approximately 140 to 185°C and then decrease to approximately 158°C over time, which indicates fracture growth during maximum burial conditions. Calculated pore-fluid pressures for methane-rich aqueous inclusions of 55 to 110 MPa (7977–15,954 psi) indicate fracture growth under near-lithostatic pressure conditions consistent with fracture growth during active gas maturation and charge. Lack of systematic pore-fluid–pressure trends over time suggests dynamic pressure conditions requiring an active process of pressure generation during maximum burial conditions. Such a process is consistent with gas generation within the Mesaverde Group or by gas charge from deeper source rocks along fracture and fault systems but is inconsistent with significant high-pressure generation by compaction disequilibrium during earlier stages of burial. On the basis of a comparison of trapping temperatures with burial and thermal maturity models, we infer that active gas charge and natural fracture growth lasted for 35 m.y. and ended at approximately 6 Ma. Our results demonstrate that protracted growth of a pervasive fracture system is the consequence of gas maturation and reservoir charge and is intrinsic to basin-centered gas reservoirs.

Description: 〈span〉〈div〉Abstract〈/div〉Cretaceous Mesaverde Group sandstones contain opening-mode fractures lined or filled by quartz and, locally, calcite cement. Fracture occlusion by quartz is controlled primarily by fracture size, age, and thermal history. Fractures occlusion by calcite is highly heterogeneous, with open and calcite-sealed fractures found at adjacent depths. In the Piceance and in other basins, processes that control the distribution of these calcite cements have been uncertain. Using pore and fracture cement petrography, fluid inclusions, and isotopic and elemental analysis, we show that host-rock calcite distribution and remobilization govern porosity degradation and occlusion of fractures 〉1 mm wide by calcite. Fluid inclusion analyses indicate calcite cement precipitation at 135 to 165°C. 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratios of calcite and the presence of porous albite suggest detrital feldspar albitization released Ca〈sup〉2+〈/sup〉 driving carbonate cement precipitation. In host rock, both albite and calcite content decreases with depth along with greater fracture porosity preservation. Although the cement sequence Fe-dolomite → ankerite → calcite is widespread, Fe-dolomite and ankerite occur as host-rock cements only, with detrital dolomite as preferred precipitation substrate. We find that rock mass calcite cement content correlates with fracture degradation and occlusion, and can be used to accurately predict where wide fractures are sealed or open.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Cretaceous Mesaverde Group sandstones contain opening-mode fractures lined or filled by quartz and, locally, calcite cement. Fracture occlusion by quartz is controlled primarily by fracture size, age and thermal history. Fracture occlusion by calcite is highly heterogeneous, with open and calcite-sealed fractures found at adjacent depths. In the Piceance and in other basins, processes that control the distribution of these calcite cements have been uncertain. Using pore and fracture cement petrography, fluid inclusions, and isotopic and elemental analysis, we show that host-rock calcite distribution and remobilization govern porosity degradation and occlusion of fractures 〉1 mm wide by calcite. Fluid-inclusion analyses indicate calcite cement precipitation at 135–165°C. 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratios of calcite and the presence of porous albite suggest that detrital feldspar albitization released Ca〈sup〉2+〈/sup〉, driving carbonate cement precipitation. In host rock, both albite and calcite content decreases with depth along with greater fracture porosity preservation. Although the cement sequence Fe-dolomite → ankerite → calcite is widespread, Fe-dolomite and ankerite occur as host-rock cements only, with detrital dolomite as preferred precipitation substrate. We find that the rock-mass calcite cement content correlates with fracture degradation and occlusion, and can be used to accurately predict where wide fractures are sealed or open.〈strong〉Thematic collection:〈/strong〉 This article is part of the Naturally Fractured Reservoirs collection available at: 〈a href="https://www.lyellcollection.org/cc/naturally-fractured-reservoirs"〉https://www.lyellcollection.org/cc/naturally-fractured-reservoirs〈/a〉〈/span〉