Andrew C. Aplin

Seal bypass systems

We present an interpretational framework for the analysis of a diverse set of geological structures that breach sealing sequences and allow fluids to flow vertically or subvertically across the seal. In so doing, they act as seal bypass systems (SBS). We define SBS as seismically resolvable geological features embedded within sealing sequences that promote cross-stratal fluid migration and allow fluids to bypass the pore network. If such bypass systems exist within a given seal sequence, then predictions of sealing capacity based exclusively on the flow properties (capillary entry pressure and hydraulic conductivity) of the bulk rock can potentially be negated by the capacity of the bypass system to breach the grain and pore network. We present a range of examples of SBS affecting contrasting types of sealing sequences using three-dimensional (3-D) seismic data. These examples show direct evidence of highly focused vertical or subvertical fluid flow from subsurface reservoirs up through the seal sequence, with leakage internally at higher levels or to the surface as seeps.We classify SBS into three main groups based on seismic interpretational criteria: (1) fault related, (2) intrusion related, and (3) pipe related. We show how each group exhibits different modes of behavior with different scaling relationships between flux and dimensions and different short- and long-term impacts on seal behavior.

Compaction‐driven evolution of porosity and permeability in natural mudstones: An experimental study

This paper describes a series of experiments designed to investigate the influence of lithology on the compactional loss of porosity and permeability in mudstones. Two intact samples of London Clay with clay fractions of 40% and 67% were compacted to 33 MPa effective stress. Clay fraction, permeability, porosity, pore size distribution, and specific surface area were measured and their evolution was monitored throughout the compaction process. Electron microscopy was combined with mercury porosimetry to trace the collapse of the pore structure with increasing effective stress. In both cases, porosity loss occurred primarily by the collapse of large pores. This process is more obvious in the coarser-grained sample because throughout the compaction process it has a much broader range of pore radii and a much greater mean pore radius. Consistent with the pore size distributions, the permeability of the coarser sample ranges from ∼ 10−10 m s−1 to 10−12 m s−1 while that of the finer-grained sample ranges from ∼4 × 10−12 m s−1 to 5 × 10−14 m s−1 during progressive compaction from 2 to 33 MPa. The compressibility of the finer-grained sample is greater than that of the coarser-grained sample (0.15 as opposed to 0.07). However, in both cases the compressibility is much lower than that inferred for lithologically similar samples compacted over geological timescales. The demonstration that both porosity and lithology (clay fraction) influence the permeability of mudstones should allow the development of more realistic porosity-permeability relationships which take into account lithological variations exhibited by mudstones.

Mudstone diversity: Origin and implications for source, seal, and reservoir properties in petroleum systems

Mudstone is the most abundant sedimentary rock and variously acts as sources, seals, and shale gas reservoirs in petroleum systems. Many important physicochemical properties of mudstones are strongly influenced by the mineralogy and size of deposited grains, and by diagenetic changes (precompaction and postcompaction); these are commonly predictable. The diverse composition of mudstones reflects input and hydrodynamic segregation of detrital materials to basins, primary production within basins, and diagenetic processes (both precipitation and dissolution) in the sediment. High-magnification observations both in modern and ancient sediments demonstrate that mudstones are texturally and mineralogically heterogeneous; this variability is not always readily apparent. Although some mud is indeed deposited by suspension settling out of low-energy buoyant plumes, textural analyses reveal that it is commonly dispersed by a combination of waves, gravity-driven processes, and unidirectional currents driven variously by storms and tides. Such dispersal mechanisms mean that muddy successions are typically organized into packages that can be interpreted using sequence stratigraphy. Early bioturbation homogenizes mud, whereas early chemical diagenesis can result in highly cemented zones developing, especially at stratal surfaces. The nature of deeper burial diagenesis, which involves compaction, mineral dissolution, recrystallization, mineral reorientation and lithification, and petroleum generation, is preconditioned by depositional and early diagenetic characteristics of the mud. Although the petrophysical properties of homogeneous mudstones are reasonably well known, the quantitative implications of heterogeneity for petroleum expulsion, retention, petroleum migration, seal capacity, acoustic anisotropy, and identification of shale gas reservoir sweet spots are essentially unexplored. Future work should seek to redress this position.

INFLUENCE OF MECHANICAL COMPACTION AND CLAY MINERAL DIAGENESIS ON THE MICROFABRIC AND PORE-SCALE PROPERTIES OF DEEP-WATER GULF OF MEXICO MUDSTONES

We report on how the effects of mechanical compaction and clay mineral diagenesis have affected the alignment of phyllosilicates in a suite of Miocene-Pliocene mudstones buried to sub-seabed depths of between 1.8 and 5.8 km in the deep-water Gulf of Mexico. Mechanical compaction has reduced the porosity of the samples to 15% at 5 km, with modal pore sizes between 10 and 20 nm. High-resolution X-ray texture goniometry data show that the intense mechanical compaction has not resulted in a strongly aligned phyllosilicate fabric. The muds were apparently deposited with a weak or isotropic phyllosilicate fabric which was not substantially realigned by mechanical compaction. Unusually, X-ray diffraction of <0.2 µm separates shows that: (1) there is no illitization trend between 90 and 120°C; and (2) discrete smectite persists to ∼120°C, coexisting with R1 I-S or R0 I-S with 30–40% expandable layers. Between 120 and 130°C, discrete smectite disappears and the expandability of I-S decreases to ∼25–30%. We propose a two-stage diagenetic process involving (1) the alteration of volcanic glass to smectite and (2) the illitization of smectite and I-S; the alteration of glass results in smectite without a preferred orientation and retards the illitization reaction. We suggest that the lack of a strongly aligned phyllosilicate fabric reflects the apparently limited extent of illitization, and thus recrystallization, to which these mudstones have been subjected.

Influence of lithology and compaction on the pore size distribution and modelled permeability of some mudstones from the Norwegian margin

We have derived a permeability model which uses pore shape, pore throat size distribution and pore alignment as key inputs. The pore shape is two frustra of cones connected at their base. Both the pore shape and alignment change with increasing compaction, developing a higher aspect ratio and becoming increasingly perpendicular to the direction of maximum stress. The uncalibrated model predicts the vertical permeability of some experimentally compacted muds to a factor of three. The model is used to estimate the vertical and horizontal permeability of eleven mudstones from the Norwegian Margin for which we have also determined porosity, pore size distribution, grain size distribution and specific surface area. Samples were chosen in order to investigate the influence of both compaction and lithology on pore size distribution and permeability. Porosity is lost mainly by the collapse of those larger pores which also contribute most of the permeability. Modelled vertical permeabilities of samples buried to between 855 and 3605 m vary from 3.3 × 10−19 to 1.2 × 10−21m2 and are not simply related either to porosity or effective stress. Permeability is strongly influenced both by porosity and detailed lithology, as described for example by the % < 2 μm particles. The modelled permeability of lithologically similar mudstones decreases logarithmically with decreasing porosity but at a single level of effective stress permeabilities of lithologically different mudstones vary by more than two orders of magnitude. At lower levels of effective stress (< 10 MPa) coarser grained (siltier) mudstones appear to have a greater range of pore radii, a much larger mean pore radius and much higher permeabilities than finer grained mudstones. Vertical permeabilities calculated using the equation derived in this paper are between three and seventy times lower than permeabilities calculated from the Kozeny-Carman equation, assuming a value of one hundred for the product of the tortuosity and shape factors in the equation.

Permeability and fluid flow in natural mudstones

Abstract Mudstone permeabilities vary by ten orders of magnitude and by three orders of magnitude at a single porosity. Much of the range at a given porosity can be explained by differences in grain size; at a given effective stress, coarser-grained mudstones are more permeable than finer-grained mudstones, although the difference diminishes with increased burial. Pore size distributions illustrate why more silt-rich mudstones are more permeable than finer mudstones and also show that the loss of porosity and permeability with increasing effective stress is driven primarily by the preferential collapse of large pores. Pore size distributions can also be used to estimate permeability rapidly. None of the existing models are ideal and need to be adjusted and validated through the acquisition of a much larger permeability database of well-characterized mudstones. We also examine the role of faults and fractures as fluid conduits in mudstones. The occurrence of microscopic hydrofractures is inferred from the observation that fluid pressures in sedimentary basins rarely exceed minimum leak-off pressures. The extent to which microfractures enhance mudstone permeability, both instantaneously and over longer periods of geological time, is poorly constrained. Although fault zones in mudstones have generally low permeability, there is abundant evidence for episodic flow along faults in tectonically active regions. The role of faults as fluid conduits during periods of tectonic quiescence is less certain, and the timing and extent of any enhanced permeability and enhanced flow are not well known. In general, conditions conducive to fluid flow along muddy faults include an increase in the activity of the fault, high fluid pressures within the fault zone and the extent of overconsolidation and lithification of the mudstones.

Muds and mudstones: physical and fluid-flow properties

Muds and mudstones are the prime control on fluid flow in sedimentary basins and near-surface environments. As the world’s commonest sediment type, they act as quitards in sedimentary basins, restricting water flow, and they influence the development of overpressure. In petroleum systems they act as source rocks for nearly all oil and much gas, determine migration directions between source and trap in most settings, and act as seals to many reservoirs. In near surface environments, they not only control natural flow, but have also been used over the centuries to restrict leakage, most pertinently in recent times from waste disposal sites. This book focuses on fluid flow through muds and mudstones. Such flow controls processes such as water escape from a mud during burial, upward or downward petroleum expulsion from a source-rock sequence, leakage from a petroleum reservoir, or containment of leachate in a clay-lined landfill site. Despite the significance of muds and mudstones, their fine-grained nature means that our knowledge of their composition and properties lags behind that of other sediments. Their physical and bulk properties are poorly defined, particularly as they relate to behaviour at depth; for instance, what mudstone permeability should be applied when carrying out a particular fluid-flow modelling exercise, or under what conditions does flow through fractures dominate flow through the capillary matrix? A search of the Science Citation Index from 1981 to 1998 revealed 13 380 articles containing the word ‘mud’ or ‘shale’ in the title, keyword or abstract; about 750 per year. 5986

Influence of clay fraction on pore‐scale properties and hydraulic conductivity of experimentally compacted mudstones

We report the results of a series of hydraulic conductivity tests carried out on seven natural, well-characterised specimens of London Clay mudstone. The clay fractions of the samples range from 27% to 66% and enabled a test of the influence of clay fraction on the hydraulic conductivity, pore size distribution, compressibility and specific surface area of natural mudstones. Hydraulic conductivities were determined at effective stresses between 1.5 and 33 MPa. Hydraulic conductivities of clay-rich samples (49–66% clay fraction) decreased from ∼10−11 m s−1 to ∼10−14 m s−1 over a porosity range of 48% to 25%. At a given porosity the hydraulic conductivities of two silt-rich samples (27 and 33% clay fraction) were 40–250 times greater than those of the five clay-rich samples. Variations in hydraulic conductivity are directly related to pore size distributions and are accurately predicted by a model which uses pore size distribution as its primary input. Clay-rich samples have unimodal pore size distributions with modal throat radii around 60–120 nm. Silt-rich samples have bimodal pore throat size distributions. One modal size is similar to that observed in clay-rich samples with a second modal value at 3–6 μm. Compaction under effective stresses up to 10 MPa results in the preferential collapse of larger pores, so that the rate of loss of hydraulic conductivity is greater in the silt-rich samples. Differences in hydraulic conductivity between silt-rich and clay-rich mudstones therefore decline with decreasing porosity. The range of porosity-hydraulic conductivity relationships means that hydraulic conductivity is not easily predicted from porosity alone; additional constraining parameters such as grain and pore size distributions are required.

Combined use of Confocal Laser Scanning Microscopyand PVT simulation for estimating the composition andphysical properties of petroleum in fluid inclusions

We present a method to determine the composition and PVT properties of petroleum inindividual petroleum fluid inclusions. Confocal Laser Scanning Microscopy is used to generatethree dimensional images of single petroleum inclusions. Because liquid petroleum fluorescesunder the laser, the images readily distinguish the liquid and vapour within the inclusion and canbe used to determine the inclusions volumetric liquid:vapour ratio. Using PVT modellingsoftware, the liquid:vapour ratio is used along with the homogenisation temperature to determinethe bulk composition, phase envelope, isochore and a range of physical properties of the includedpetroleum. This is done using an iterative series of PVT calculations which match twoparameters: (1) the molar volume of the petroleum at room and homogenisation temperatures; (2) the liquid:vapour ratio of the inclusion at room temperature. Key uncertainties in the methodare explored, including the accuracy with which the liquid:vapour ratio can be determined; thecomposition of the titrant gas used in the iterative procedure; and the composition of thepetroleum chosen to model the physical properties of the included petroleum. Data from coevalinclusions suggest that the saturation pressure, Gas–Oil Ratio, viscosity, molar volume, densityand surface tension of included petroleum are determined with a precision of a few percent.Confirmation of the accuracy of the method awaits tests using inclusions grown in the laboratoryunder carefully controlled PVTX conditions. However, it is likely that the physical properties ofincluded petroleums are more accurately modelled by fluids which are genetically related tothem. If proven to be accurate, the method will provide a routine method for determiningpalaeopressure in petroleum systems. Finally, we present data from a Central North Sea examplewhich are geologically realistic and which for the first time record the evolution of fluid pressureand petroleum composition in a petroleum reservoir.

DIAGENETIC REORIENTATION OF PHYLLOSILICATE MINERALS IN PALEOGENE MUDSTONES OF THE PODHALE BASIN, SOUTHERN POLAND

We used high-resolution X-ray texture goniometry to quantify changes in the mm-scale orientation of phyllosilicate minerals in a suite of Paleogene mudstones from the Podhale Basin in southern Poland. The sample set covers an estimated range of burial depths between 2.4 and 7.0 km, corresponding to a temperature range of 60–160°C. Although mechanical compaction has reduced porosities to ∼10% in the shallowest samples, the phyllosilicate fabric is only modestly aligned. Coarser-grained (>10 µm) detrital chlorite and mica appear to be more strongly aligned with (001) parallel to bedding, suggesting their deposition as single grains rather than as isotropic flocs or aggregates. From 2.4 to 4.6 km, R0 illite-smectite with 40–50% illite layers changes to R1 illite-smectite with 70–80%) illite layers. At the same time kaolinite is lost and diagenetic chlorite is formed. The mineralogical changes are accompanied by a strong increase in the alignment of illite-smectite, chlorite, and detrital illite, parallel to bedding and normal to the presumed principal effective stress. We propose that the development of a more aligned I-S fabric results from the dissolution of smectite and the growth of illite with (001) normal to the maximum effective stress. Water released by illitization may act as a lubricant for the rotation of all platy minerals into nanoporosity transiently formed by the illitization reaction. At greater depths and temperatures, further illitization is inhibited through the exhaustion of K-feldspar. After the cessation of illitization, a further 2.4 km of burial only results in a small increase in phyllosilicate alignment. At such small values for porosity and pore size, increasing stress does not substantially reorient phyllosilicates in the absence of mineralogical change.