Robert H. Lander

Anomalously high porosity and permeability in deeply buried sandstone reservoirs: Origin and predictability

Porosity and permeability generally decrease with increasing depth (thermal exposure and effective pressure); however, a significant number of deep (>4 km [approximately 13,000 ft]) sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability. Anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology (composition and texture), age, and burial/temperature history. In sandstones containing anomalously high porosities, such porosities exceed the maximum porosity of the typical sandstone subpopulation. Major causes of anomalous porosity and permeability were identified decades ago; however, quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature. The focus of this article is on quantification and predictability of three major causes of anomalously high porosity: (1) grain coats and grain rims, (2) early emplacement of hydrocarbons, and (3) shallow development of fluid overpressure. Grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital-quartz grains. Currently, prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets. In the absence of adequate empirical data, sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality. Such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability (Begin page 302) in a given play or prospect. These constraints include thermal history and sandstone grain size and composition. The overall effect of hydrocarbon emplacement on reservoir quality is controversial. It appears that at least some cements (quartz and illite) may continue to precipitate following emplacement of hydrocarbons into the reservoir. Our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify, prior to drilling, the potential impact of hydrocarbon emplacement on porosity and permeability. The best-case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited Tertiary or Quaternary sandstones. Our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths. The models also indicate that the potential for porosity preservation is greatest in ductile-grain-rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks. The case for significant porosity preservation associated with fluid overpressures in pre-Tertiary basins, however, is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss.

Natural fracture characterization in tight gas sandstones: Integrating mechanics and diagenesis

Accurate predictions of natural fracture flow attributes in sandstones require an understanding of the underlying mechanisms responsible for fracture growth and aperture preservation. Poroelastic stress calculations combined with fracture mechanics criteria show that it is possible to sustain opening-mode fracture growth with sublithostatic pore pressure without associated or preemptive shear failure. Crack-seal textures and fracture aperture to length ratios suggest that preserved fracture apertures reflect the loading state that caused propagation. This implies that, for quartz-rich sandstones, the synkinematic cement in the fractures and in the rock mass props fracture apertures open and reduces the possibility of aperture loss on unloading and relaxation. Fracture pattern development caused by subcritical fracture growth for a limited range of strain histories is demonstrated to result in widely disparate fracture pattern geometries. Substantial opening-mode growth can be generated by very small extensional strains (on the order of 104); consequently, fracture arrays are likely to form in the absence of larger scale structures. The effective permeabilities calculated for these low-strain fracture patterns are considerable. To replicate the lower permeabilities that typify tight gas sandstones requires the superimposition of systematic cement filling that preferentially plugs fracture tips and other narrower parts of the fracture pattern.

Toward more accurate quartz cement models: The importance of euhedral versus noneuhedral growth rates

Existing quartz cement models assume that the rate of growth per unit surface area is independent of grain size. Application of one such model to four geologically diverse data sets reveals a systematic error with grain size such that values in finer grained sandstones are overpredicted. Our laboratory synthesis of quartz overgrowths indicates that this grain-size effect results from the more rapid development of euhedral crystal forms on smaller grains. Experiments show that the rate of growth along the quartz c axis drops by a factor of about 20 after euhedral faces develop. Our numerical simulations of quartz growth in two dimensions indicate that this euhedral effect should be significant in sandstones despite the complexity that arises from the interaction of multiple growing crystals and small pore sizes. Simulations also suggest that this phenomenon is responsible for the common observation that quartz overgrowths are less extensively developed on chert and polycrystalline grains compared to monocrystalline grains. This euhedral effect may also explain the common observation that quartz growth rates are significantly faster on fracture surfaces compared to detrital grain surfaces. Most sand grains have well-developed dust rims that reflect minor adhesions of nonquartz materials or damage from surface abrasions or impacts. Our numerical and laboratory experiments indicate that such small-scale discontinuities dramatically reduce initial rates of quartz growth because they break overgrowths into separate smaller crystal domains that are bounded by euhedral faces. The paucity of nucleation discontinuities on fracture surfaces should lead to substantially faster rates of growth compared to grain surfaces.

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.

Insights into rates of fracture growth and sealing from a model for quartz cementation in fractured sandstones

A new model accounts for crystal growth patterns and internal textures in quartz cement in sandstone fractures, including massive sealing deposits, thin rinds or veneers that line open fracture surfaces, and bridge structures that span otherwise open fractures. High-resolution cathodoluminescence imaging of bridge structures and massive sealing deposits indicates that they form in association with repeated micron-scale fracturing of growing quartz crystals, whereas thin rinds do not. Model results indicate that the three morphology types develop in response to (1) the ratio of the rates of quartz growth to fracture opening and (2) the substantially faster growth rate that occurs on noneuhedral surfaces in certain crystallographic orientations compared to euhedral crystal faces. Rind morphologies develop when the fracture opening rate exceeds two times the fastest rate of quartz growth (along the c axis on noneuhedral surfaces) because growing crystals develop slow-growing euhedral faces. Massive sealing, on the other hand, develops where the net rate of fracture opening is less than twice the rate of quartz growth on euhedral faces because all quartz growth surfaces along the fracture wall seal the fracture between fracturing events. Bridge structures form at fracture opening rates that are intermediate between the massive sealing and rind cases and are associated with crystallographic orientations that allow growth to span the fracture between fracturing events. Subsequent fractures break the spanned crystal, introducing new, fast-growing noneuhedral growth surfaces where quartz grows more rapidly compared to the euhedral faces of nonspanning crystals. As the ratio of fracture opening to quartz growth rate increases, the proportion of overgrowths that span the fracture decreases, and the range in c -axis orientations for these crystals comes progressively closer to perpendicular to the fracture wall until the maximum spanning limit is reached. Simulation results also reproduce “stretched crystal,” “radiator structure,” and “elongate blocky” textures in metamorphic quartz veins. The model replicates a well-characterized quartz bridge from the Cretaceous Travis Peak Formation as well as quartz cement abundances, internal textures, and morphologies in the sandstone host rock and fracture zone using the same kinetic parameters while honoring fluid-inclusion and thermal-history constraints. The same fundamental driving forces, in both in the host rock and fracture system, are responsible for quartz cementation, with the only significant difference within the fracture zone being the creation of new pore space as well as new noneuhedral surfaces for cases where overgrowths span fractures between fracturing events. Rates of fracture growth and sealing may be inferred from fracture cement textures using model results.

A model for fibrous illite nucleation and growth in sandstones

We have developed a model for the formation of fibrous illite in sandstones where kaolinite is a primary reactant and potassium is derived from in-situ K-feldspar grain dissolution or imported into the model reference frame. Illite fiber nucleation and growth are modeled using Arrhenius expressions that consider saturation state in addition to temperature and time. Nucleation occurs on pore walls, and muscovite and detrital illite may be defined as energetically favorable substrates. The model is integrated with other Touchstone™ models to account for the influence of other diagenetic processes on surface area and reactant volumes and to provide input for permeability simulations. We evaluated the illite model performance on two data sets: (1) Jurassic quartzose samples from offshore mid-Norway with maximum temperatures ranging from 108 to 173C (226 to 343F) and (2) Miocene lithic samples from offshore Southeast Asia that have maximum temperatures ranging from 157 to 182C (315 to 360F). The model matches measured abundances of illite, kaolinite, and K-feldspar in both data sets using identical kinetic parameters. Predicted K-Ar ages are consistent with available data given uncertainties associated with detrital contaminants. Although no illite particle-size data are available from the analyzed samples, modeled crystallite thicknesses from the Norway data set are comparable to published measurements of 0.004 to 0.012 m from North Sea samples with similar temperature histories.

Combining diagenesis and mechanics to quantify fracture aperture distributions and fracture pattern permeability

Abstract Diagenesis and fracture are often linked processes in deformed rock. Empirical observations show that quartz-lined natural fractures are very common in sandstones that have been exposed to temperatures in excess of 90°C. These fractures exhibit crack-seal textures as well as cement bridges propping the fractures open and preserving fracture porosity. These diagenetic effects are examined in the context of detailed fracture characterizations generated by geomechanical modelling. Aperture, length and fracture network geometry are examined in the context of subcritical crack growth and various biaxial loading boundary conditions of varying initial anisotropy. An isotropic initial state results in more polygonal fracture patterns. A small initial anisotropy creates preferential through-going fractures that are later connected by cross-fractures. A larger initial anisotropy results in only one parallel set. The flow connectivity of isotropic and small strain anisotropic patterns appears high based on trace pattern geometry, but when the effects of diagenesis are added, preferentially filling smaller aperture fracture segments, connectivity can be significantly reduced. Finite difference, steady-state flow simulations demonstrate the permeability effects of heterogeneous fracture aperture distributions predicted by the mechanical model and permeability reduction caused by systematic diagenetic fracture sealing.

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.

The role of diagenesis in the formation of fluid overpressures in clastic rocks

We have developed a model of fluid flow and pressure development in sedimentary basins that incorporates pore volume loss due to mechanical compaction and to chemical diagenesis (quartz cementation, grain contact quartz dissolution and illitization). Mechanical compaction is modeled to be a function of effective stress. In this model, pore volume loss due to mechanical compaction will be retarded when overpressure develops. The diagenetic processes are modeled as being kinetically controlled and the reaction progress depends only on the temperature history. Hence pore volume loss due to chemical compaction is not retarded by overpressure. By including diagenetic effects on overpressure development, the pressure model should be more generally applicable than models that consider mechanical compaction to be the sole process that reduces porosity. To demonstrate the potential importance of chemical compaction in the formation of fluid overpressures in different settings, we calibrated our model with data obtained from the Halten Terrace offshore mid-Norway and from the Gulf of Mexico. In both cases, the diagenetic processes have the potential to control on the timing and magnitude of overpressuring. From 25% and up to 80% of the present-day overpressure may be caused by pore volume loss resulting from diagenetic reactions. Pressure build-up from diagenetic processes also potentially controls the timing of hydraulic fracturing. If diagenetic processes are actively contributing to overpressure generation, then unrealistically low shale permeabilities are not needed to retain overpressures for geologic time periods (>10 My).

Reservoir quality and burial model evaluation by kinetic quartz and illite cementation modeling : Case study of Rotliegendes, north Germany

Silicate reaction kinetics provide a complimentary means to other established paleothermal indicators such as organic maturation for evaluating thermal reconstructions. In this study we combine the use of an organic maturation model with kinetic models for quartz and illite cementation to evaluate burial history scenarios for five sub-salt wells in lithologically and structurally complex Rotliegendes reservoirs. Models for organic maturation are most sensitive to maximum temperature and provide no direct evidence for the time of peak temperature or the overall duration of high temperatures. By contrast, the kinetics of quartz cementation are much more strongly influenced by the duration of exposure to high temperatures compared to organic indicators. Kinetic models for fibrous illite formation similarly are sensitive to time and temperature and provide predictions for the time of illite formation that can be compared with radiometric dates. Used collectively these organic and inorganic paleothermal indicators provide improved constraints on thermal evolution compared to conventional approaches. In this study we use these indicators to evaluate two alternative burial history scenarios. Scenario one incorporates a hypothesized Jurassic heatflow peak together with significant Late Jurassic deposition and subsequent erosion. Scenario two omits the Jurassic heat flow peak and omits the deposition and erosion of the Upper Jurassic. Although both of these scenarios are consistent with organic maturation data, scenario two leads to a far better match with quartz cement volumes and fibrous illite K/Ar dates.