Jonny Hesthammer

Geometric analysis and scaling relations of deformation bands in porous sandstone

Displacement, length and linkage of deformation bands have been studied in Jurassic sandstones in southeastern Utah. Isolated deformation bands with lengths (L) that span more than three orders of magnitude show similar displacement (D) profiles with more or less centrally located maxima and gently increasing gradient toward the tips. Soft- and hard-linked examples exhibit steeper displacement gradients near overlap zones and immature hard links, similar to previously described fault populations. The deformation band population shows power-law length and displacement distributions, but with lower exponents than commonly observed for populations of larger faults or small faults with distinct slip surfaces. Similarly, the Dmax-L relationship of the deformation bands shows a well-defined exponent of ca 0.5, whereas the general disagreement for other fault populations is whether the exponent is 1 or 1.5. We suggest that this important difference in scaling law between deformation bands and other faults has to do with the lack of well-developed slip surfaces in deformation bands. During growth, deformation bands link to form zones of densely spaced bands, and a slip surface is eventually formed (when 100 m < L < 1 km). The growth and scaling relationship for the resulting populations of faults (slip surfaces) is expected to be similar to ‘ordinary’ fault populations. A change in the Dmax-L scaling relationship at the point when zones of deformation bands develop slip surfaces is expected to be a general feature in porous sandstones where faults with slip surfaces develop from deformation bands. Down-scaling of ordinary fault populations into the size domain of deformation bands in porous sandstones is therefore potentially dangerous.

Possible absence of small faults in the Gullfaks Field, northern North Sea: implications for downscaling of faults in some porous sandstones

Faults and fractures have been studied in more than 6 km of cores from the Gullfaks Field, northern North Sea, and compared to fault populations determined by stratigraphic correlation of well logs and seismic data. Statistical analysis indicates a power-law correlation between displacement and fault frequency for displacements down to 5‐10 m. Observations of deformation bands, with displacements ranging from 1 mm to 10 cm, perfectly fit the low-end extension of this power-law model. However, integrated use of well-log correlation data and core data indicates that few faults exist with displacement between 020 cm and 5 m. If this data gap is real and representative for other sandstones, uncritical downscaling of seismic-scale fault sizes for use in oil reservoir description or strain estimation may yield erroneous results. The possible gap may be related to the high density and coalescent nature of (larger) faults, excluding the tip portions of many faults and thus the likelihood for wells to intersect low-displacement tip regions. Deformation bands, on the other hand, seldom develop displacements much in excess of 010 cm, thus providing a distinct population of displacement data in the lower end of the displacement scale which, by coincidence, falls close to the extrapolated trend of the well data in log‐log space. 7 2000 Elsevier Science Ltd. All rights reserved.

CSEM performance in light of well results

During the past several years, we have seen an increasing focus on the use of CSEM technology for hydrocarbon exploration in marine environments and, recently, a number of success stories have been published. The technology has been demonstrated to aid both detection and delineation of hydrocarbon-filled reservoirs.

Fault interaction in porous sandstone and implications for reservoir management; examples from southern Utah

Different types of fault interaction are examined and compared to a single fault situation with respect to density, distribution, and orientation of subseismic structures. Fault branch points are found to be considerably more complex than single faults. The damage zone in these areas shows a wider range in orientation of deformation bands and fractures, and the damaged volume extends far into the fault blocks. Overlapping structures develop wide damage zones at early stages, typically with structures that are oblique to the faults and, thus, represent potential flow barriers. The damage associated with relay structures is inherited by later stages, when the fault segments are coalesced and behave as a single fault. At advanced stages, the damage zones are uncommonly wide in breached relay locations. Such locations can be recognized as places where faults make abrupt steps or bends. The extent to which complications associated with both single-tip and double-tip interactions affect reservoir performance depends on the nature of the minor structures in the damage zone. It is thus crucial that the physical nature of minor structures is investigated so that their influence on reservoir performance can be evaluated.

Evolution and geometries of gravitational collapse structures with examples from the StatfJord Field, northern North Sea

Abstract Gravitational collapse structures may range in scale from centimetres to hundreds of kilometres and affect both loose sediments and consolidated rocks. The area affected by gravitational failure will commonly be amphitheatre-like in map view, whereas a cross-sectional view will typically display a listric and concave upwards detachment surface. The degree of deformation increases in the direction of sliding. If movement of the sliding rocks is sufficiently slow, several intact slump blocks may be identified within the slide area. The movement of blocks may be translational or rotational. Two types of gravitational collapse structures are identified. In Type A, the newly-formed detachment reaches a free surface at the toe of the slide. In Type B, however, the listric detachment fault follows a weak layer and its displacement is accommodated by simultaneous slip along a major, steeper fault. This results in a ramp-flat-ramp fault geometry. Gravitational failure is observed along the east flank of the Statfjord Field, northern North Sea. The triggering mechanisms were probably earthquakes and high fluid pressures. Listric faults detached within soft shales and are associated with several rotated slump blocks that decrease in size away from the break-away zone. The slumping occurred in several phases. First, parts of the Brent Group failed. The detachment surface was within shales of the Ness Formation. Next, the slumping cut into the Dunlin Group and detached within the lower parts of the group (shales of the Amundsen Formation). Renewed slumping of the Brent Group occurred at the new break-away zone created by the Dunlin slumping. In the final stages of gravitational failure, slumping reached into the Stattjord Formation and detached within shales at the base of the unit or within shales of the uppermost Hegre Group. The relief created at the head (break-away zone) of Statfjord slumping caused renewed slumping of the Brent and Dunlin Groups. A study of gravitational failure analogues demonstrates several similarities in geometry in spite of differences in scale, lithology, degree of consolidation, and triggering mechanism.

Uncertainties associated with fault sealing analysis

Recent advances in understanding how faults restrict fluid flow in sandstone reservoirs have led to improved models for reservoir simulation. Nevertheless, there are still many uncertainty factors that can render even the most detailed simulation model useless. On a detailed scale, these uncertainties include variations in lateral continuity of faults, properties and thickness of fault zones, and the influence of deformation bands within and outside damage zones. Subseismic features such as small-scale relay zones, drag features and frequency and distribution of small faults around the fault zone further decrease the confidence level of simulation modelling results. Detailed analyses of seismic and well data from the Gullfaks Field, Northern North Sea, have helped understand the detailed structural reservoir characteristics. The results from these analyses can, in many cases, be used as input to further enhance models for reservoir simulation in order to increase the validation of the models. Furthermore, the studies carried out on the Gullfaks Field demonstrate that a sound approach to knowledge management for increased oil recovery based on fault seal analysis requires sharing of gained knowledge from many oil and gas fields rather than monopolizing information that cannot be fully utilized by studies from a single field.

Structural geology of the Gullfaks Field, northern North Sea

Abstract The large amount of structural data available from the Gullfaks Field have been used to unravel the structural characteristics of the area. Two structurally distinct subareas have been revealed (a major domino system and an eastern horst complex) that show significant differences with respect to fault geometry, rotation and internal block deformation. The main faults have very low dips in the domino system (25–30°) as compared to the horst complex (65°), whereas most minor faults are steep in all parts of the field. Forward modelling indicates that the horst complex balances with rigid block operations. However, the domino area underwent significant internal deformation, reflected by the low acute angle between bedding and faults, and by non-planar bedding geometries. The internal deformation is modelled as a shear synthetic to, but steeper than, the main domino faults. This deformation explains a large-scale (kilometre sized) drag zone that has a triangular geometry in cross-section. Much of this shear deformation occurred by strain-dependent grain reorganization in the poorly consolidated Jurassic sediments, which led to a decrease in porosity. A strain map is presented for the domino area, indicating where the porosity is likely to have been decreased due to internal shear. Hangingwalls are generally more deformed (sheared) than footwalls. This is seen on both the kilometre scale (large-scale drag) and the metre scale (local drag).

Seismic attribute analysis in structural interpretation of the Gullfaks Field, northern North Sea

Seismic attribute maps provide a useful tool in interpreting faults, particularly those close to or below seismic resolution. Dip, relief, azimuth and amplitude maps are most useful. Optimal use of such maps requires careful filtering and appropriate use of colours and light sources. One of the challenges is to distinguish between anomalies related to real geological features and to seismic noise--both of which may occur as linear or curvi-linear, continuous features on the attribute maps. This challenge must be solved by use of independent data. In the North Sea Gullfaks Field, a family of (curvi-)linear features on the attribute maps are subparallel to contour lines on time maps. Core data, dipmeter data, stratigraphic log correlation and forward modelling show that these features are related to seismic noise rather than real faults.

Spatial relationships within fault damage zones in sandstone

Abstract Orientation analyses of deformation bands in sandstones from the Gullfaks region, northern North Sea, show that different sets of bands are a common feature within damage zones associated with faults with displacement in the order of several tens of metres or more. The different sets may coexist within the same depth intervals, or may occur in cluster zones located at different depths. The strike of the deformation bands is generally subparallel to the main faults, but may deviate from this in complex areas where larger-scale faults with different orientations coalesce. Analyses of smaller-scale faults (less than a few tens of metres displacement) do not display the same clear “conjugate” sets of deformation bands. A possible explanation is that deformation bands in the early stages of fault development are subparallel to the main faults. As the displacement along the smaller faults increases, the bands may intersect and form a single slip surface with a more complex geometry. At this stage, a second set of deformation bands may develop in order to accommodate the different stresses that exist along and near the fault plane. During slip along a main fault, smaller splay faults may develop in the footwall or hanging wall. The splay faults will be associated with abundant deformation bands with orientations that may differ from that of the “conjugate” sets, depending on the orientation of the splay faults with respect to the main faults.

Are relay ramps conduits for fluid flow? Structural analysis of a relay ramp in Arches National Park, Utah

Abstract Relay ramps associated with overlapping faults are commonly regarded as efficient conduits for fluid flow across potentially sealing intra-reservoir fault zones. The current study demonstrates that structural heterogeneity in the often anomalously wide damage zone of relay ramps may represent potential baffles to intra-ramp fluid flow. A network of ramp-parallel, ramp-diagonal and curved cataclastic deformation bands causes compartmentalization of the ramp studied in Arches National Park, Utah. Harmonic average calculations demonstrate that, although single deformation bands have little or no effect on effective permeability, the presence of even a very small number of low-permeable deformation band clusters could reduce along-ramp effective permeability by more than three orders of magnitude. Thus, although relay zones may maintain large-scale geometric communication, the results of this study demonstrate that caution must be exercised when considering relay ramps as fluid conduits across sealing faults in a production situation. Although relay ramps clearly represent effective migration pathways for hydrocarbons over geological time, the extent to which they conduct fluids in a production situation is more uncertain. Quantitative approaches include adjusting the transmissibility multipliers for faults in reservoir models to allow for increased cross-fault flow. If, however, the effect of internal structural heterogeneity is not taken into consideration, this type of adjustment may lead to gross overestimation of the effect of relay ramps. Sedimentology, stratigraphy, burial history and deformation mechanisms are some of the controlling factors for the formation of such structural heterogeneities.