Abstract - The development of methods for 3-D modelling of syn-rift hangingwall sequences and for modelling of reservoir structural heterogeneities requires a knowledge of the systematics of faults and fault arrays, the 3-D displacement geometries associated with faults and the nature of syn-rift sedimentary sequences in a variety of tectono-stratigraphic settings. A variety of good quality 2-D and 3-D seismic datasets from NW European basins has been used to establish detailed descriptions of fault systematics, the structure of fault-related basins and the sedimentological and seismo-stratigraphic characteristics of fault-related depositional sequences. Collectively, the available datasets cover a wide range of scale, from basins down to individual hydrocarbon reservoirs.
The kinematic development of a basin is controlled by the systematics of single faults and fault systems. Several attributes of faulting are of significance: fault growth, displacement variations on faults and displacement rates together with the strain heterogeneity within basins incorporating the size and spatial distributions of faults. The growth of fault systems has been investigated in a number of ways. An expansion of our database for fault displacement and dimension data has led to a refinement of current growth models. Combined with earthquake seismological data, the systematic relationship between the displacements and the dimensions of faults provides useful constraints on the growth of individual faults and fault arrays. Displacement analysis of faults and fault arrays demonstrates their geometric and kinematic coherence. Discontinuous fault systems are linked by ductile deformation of the fault volume, e.g. the transfer of displacement between adjacent fault segments of a segmented fault array is accommodated by the formation of a ramp within a relay zone. Using a new displacement backstripping technique, fault displacement rates on North Sea faults have been measured directly from displacement variations on syn-sedimentary faults, and are consistent with data from intraplate basins, such as the Basin and Range Province, U.S.A.. Regional strain rates and fault displacement rates from a variety of areas (North Sea, Basin and Range and the Aegean) appear to be directly related, such that increased regional strain rates are accommodated by a decrease in average repeat times on faults. We can find no clear evidence for pulsed fault movements on geological, as opposed to seismological, time scales.
The heterogeneous nature of strain within a sedimentary basin is an important control on the basin geometry. Fault population analysis indicates that fault systems are scale-invariant, showing a degree of self-similarity of structure on a range of scales. The fractal nature of fault size populations has provided a basis for the development of a method for predicting sub-seismic fault populations from seismically resolved faults. These predictions account for the discrepancy between fault-derived extension estimates and other measures of crustal extension (e.g. crustal thinning or thermal subsidence). The spatial distribution of faults is an important element in the distribution of strain within basins. Using a variety of novel techniques, it has not proved possible to establish quantitatively the spatial characteristics of fault systems. Established fractal analytical techniques are considered inappropriate for the analysis of real fault systems and methods are required which integrate size and spatial distributions of faults.
The displacements and sequence geometries associated with faults have been investigated using a variety of techniques, including conventional map analysis and seismic stratigraphy. Analysis of data from a variety of study areas permits a number of general conclusions to be made. We find that the tectono-stratigraphic facies model of Leeder & Gawthorpe (1987) provides a convenient scheme within which the depositional environments of syn-rift sequences can be described. 3-D analysis shows that a 2-D consideration of fault-related geometries, although sometimes useful, is quite limiting. In detail, structure contours and isopachs associated with single faults form semi-elliptical patterns which are clearly related to the displacement variations on adjacent faults. Syn-sedimentary faults are characterised by footwall uplift and hangingwall rollover, which in 3-D can produce superimposed deformation fields and, therefore, quite complex bathymetric controls on sedimentation. The geometry of syn-rift sequences depends in great measure on the relationship between displacement rates and sedimentation rates: for our study areas these rates fall within the ranges 0.15-0.03mm/year and 0.1-0.02mm/year (for maximum sedimentation rates). The structure/stratigraphy of certain basins are, however, consistent with non-linear changes in fault displacement rates. In some basins, progressive 3-D onlap of syn-rift sequences onto hangingwall dip-slopes is an indication of growth of hangingwall basins. Relay zones are responsible for variations in syn-rift sequence thicknesses, but are transient structures which are eventually either breached or incorporated as minor irregularities along the length of longer amalgamated segmented fault systems. More complex fault geometries are responsible for marked, often unpredictable, facies variations within the syn-rift sequence. Although facies changes in post-rift sequences are likely to mimic those of syn-rift units if post-rift bathymetry is retained, post-rift units do not show fault-related bed rotations/subsidence. Seismic stratigraphic techniques provide a basis for detailed analysis of fault-related facies changes.
A soft-domino model has been developed which provides a 3-D modelling
capability with integrated faulting, sedimentation and compaction. In common
with all other combined fault/sedimentation models, the sedimentary modelling
is quite simple. Although, in that respect, the model is rather elementary,
it is the only 3-D system of its type and is, in many ways, of more value
than the still quite naive 2-D models developed previously. The soft-domino
model predicts an approximately exponential decay in vertical displacement
with distance from a fault: a geometry which is consistent with the coseismic
deformations associated with active faults and with the longer term flexural/isostatic
effects predicted for faults. The model is capable of modelling in 3-D
the main structural/stratigraphic features of our study areas. The model
predicts reverse drag within both the hangingwalls and the footwalls of
faults and, in simple cases, the formation of semi-elliptical hangingwall
lows and footwall highs. Deformed bed geometries are to be expected in
the vicinity of faults, and complex structure contour patterns can be reconciled
with a simple fault model. As indicated from our analysis of real data,
syn-rift sequence geometries are critically dependent on the relationship
between sedimentation rates and displacement rates. Variations in this
relationship are responsible for a wide range of structures e.g. 3-D stratigraphic
onlap onto hangingwall dip-slopes and footwall crests, fanning of horizons
from footwall crests and the formation of starved basins. Models incorporating
non-linear fault displacement rates are characterised by older syn-rift
sequences which are difficult to distinguish from pre-rift units in other
areas. Individual fault segments within a segmented fault array may act
independently in the early stages of faulting, but modelling shows that
their displacement geometries are consistent with eventual interaction
and behaviour as coherent geometric/kinematic packages. Although onlapping
relationships of syn-rift sequences are sometimes an indication of an increase
in the size of hangingwall basins, the extent to which fault growth related
broadening of a basin is compounded by sediment loading is not yet known. The time and scale dependence of these deformations needs to be established,
but very good stratigraphic data will be needed to resolve this problem.
In: Modelling the Earth for Oil Exploration. Final Report of
the CEC's Geoscience Program 1990-1993 (edited by Helbig, K.). Elsevier,
205-316, 1994.