Juxtaposition seal

We subdivide fault seals into juxtaposition seals and fault gouge seals. Juxtaposition seals retain hydrocarbons due to the geometrical juxtaposition of a... 

Juxtaposition seal of reservoir against non-reservoir can be assessed by fault-plane diagrams. Additional seal may be developed (at reservoir juxtapositions) if fault-plane processes increase the capillary entry pressure. In Oseberg Syd, clay smearing is considered to be dominant because of the relatively shaly nature of the Brent Group and the shallow burial depths during faulting (<500 m). 

Juxtaposition seal can be complicated by a number of factors. These include unsystematic lateral lithologic variations, for instance those of channel belts. While it is common to model such lithologic variations stochastically for input to reservoir simulations, this is not to our knowledge used in fault seal analysis. For small fault throws any continuous (i.e., not wedged out) sand horizons may provide across-fault connectivity which could be misinterpreted as a seal if interpreted by juxtaposition analysis alone. Other complications relate to so-called damage and relay zones. 

Four mechanisms have been suggested to explain how faults provide seals. The most frequent case is that of clay smear and juxtaposition (Fig. 5.8)... 

Figure 5.8 Fault seal as a result of clay smear and juxtaposition...

Fig. 5. Cartoon of the main structural elements of a fault damage zone. The zone is composed of a clusters of deformation features around a large offset fault. Note that the juxtapositions present differ from those which would be present if only a single fault was present and that the presence of an array of deformation features can induce the development of micro-compartments or sealed cells in the fault zone.

The critical elements of fault damage zones which are needed for fault seal evaluation and for input into reservoir behaviour simulation include (i) the dimensions of the damage zone (ii) the fault clustering characteristics (iii) the fault offset populations, which can control the distribution of fault rocks and juxtapositions (iv) the orientation distributions of deformation features present within damage zones and (v) the total thickness of fault-rocks. Each of these aspects are reviewed below, where the data presented are part of a large database collected from the structural analysis of -90 wells, (-25 km of core) from the North Sea area (see example in Fig. 7). The final part of this section presents a simple model which demonstrates the impact of damage zone structures on flow. 

Most fault offset population analysis (see Cowie et al., 1996) have concentrated on prediction of the number of sub-seismic over large areas (>1 km ) faults rather than the distribution of the faults within the fault zones. In many cases a uniform distribution of faults across an area is assumed. The data from the structural logging of North Sea wells illustrate that the characteristics of faults found on a field scale are also present within individual fault zones identified on seismic or from well data. Because the population of small faults around larger structures will control the distribution of juxtapositions and fault rocks, detailed characterisation of the offsets is important to seal analysis. Fig. 11 illustrates the population characteristics of three fault zones with different offsets. 

The basic requirement for mapping fault seals is the generation of a realistic, maximum probability map of sealing capacities along individual fault zones. This involves evaluation of the possible juxtaposition patterns within the zone as well as an assessment of the variance of fault rock properties. 

Fig. 17. Juxtaposition diagrams for use in fault seal analysis. See text for details.

Analysis of the location and heights of potential leaky fault Juxtaposition windows which arise from variations in (i) the possible depths, geometries and locations of stratigraphic horizons and faults (ii) the difference between the cumulative throw on individual fault zones, indicated from seismic and the most likely size of the throw on the largest real fault in that zone and (iii) the sediment architecture and continuity. The end result should be a probability map of the distribution of sealed and leaky windows along the critical fault zones. 

This paper has highlighted that a number of components, important to fault seal analysis, are often either not included or not quantified in sufficient detail to allow a low risk seal evaluation. The main components which are not always considered in detail are (i) the errors in throw patterns which arise from seismic resolution and fault damage zone structures (ii) the assumption that juxtaposition of reservoir against low permeability units and shale smear are the only sealing mechanisms and (iii) that fault seal data from anywhere is directly applicable to any other sealing problem, i.e., that the geohistory is not critical... 

Seal analysis based on the assumption that juxtaposition analysis (i.e., construction of Allan diagrams, clay smear assessment and leaking sand/sand contacts) would only have been successful in -40% of the cases studied. 

Knipe, R.J. 1997. Juxtaposition and seal diagrams to help analyze fault seals in hydrocarbon reservoirs. Am. Assoc. Pet. Geol. Bull., 81 187-195. 

A pre-requisite for fault seal analysis is a consistent structural model, with sufficient detail and proper fault linkage relationships. The first step of static fault seal analysis (Fig. 2) involves the construction of a juxtaposition diagram (Allan, 1989), in which areas where reservoir is juxtaposed against a sealing lithology are identified. The retention capacity is calculated from the minimum capillary entry pressure of the juxtaposed lithology, which can be measured or... 

Fig. 1. Classification of fault seal processes. Fault seals are divided into two types juxtaposition fault seals and fault gouge seals (e.g., cataclasis and clay smear). One fault may have a combination of different sealing processes affecting its sealing capacity.

Fig. 2. Strategy for the analysis of fault seals over geological time scales. Based on a fault juxtaposition diagram, the effects of the different fault seal processes are assessed in a systematic manner for their seal capacity due to clay smear, brittle fault sealing and juxtaposition fault seal.

Due to seismically irresolvable complexities of fault zone structure, the juxtapositions of footwall and hangingwall rocks predicted from seismic data will in most cases be different from those actually present. The importance of such differences to the prediction of across-fault connectivity, of both hydraulically passive and hydraulically active fault zones, is strongly dependent on the reservoir sequence. Connectivities are calculated for hydraulically passive and active faults offsetting an Upper Brent Reservoir sequence. Shaley fault rocks within brittle fault zones often represent a spatially persistent, although variable thickness, component of the zones and provide a basis for the application of empirical methods of fault seal prediction to brittle faults. 

The fault sealing mechanisms considered are those which occur as a direct result of the faulting process, i.e., those due to either across-fault juxtapositions of reservoir and non-reservoir units or to the presence of sealing fault rocks, i.e., membrane seals. The diage-netic contribution to seals (Knipe, 1992) is not considered. 

Our approach in this paper has been to examine juxtaposition relationships and compute fault seal attributes on strike projections of fault surfaces. The analysis was carried out using FAPS software (Freeman et al., 1989 Needham et al., 1996). 

Fig. 10. Strike projections of Fault 1, viewed from the downthrown (west) side vertical exaggeration x5. (a) Juxtaposition plot. Upthrown Brent zones are shown with coloured fill (see legend) downthrown zones are shown in black outline, labelled at each end of the fault. Footwall hydrocarbon contacts are shown in black, hangingwall contacts in blue, (b) SGR for the area of Brent-Brent overlap. Upthrown zones outlined in blue, downthrown zones in black contacts as in (a). SGR is colour-coded in the ranges 0-15%, 15-20%, 20-30% and >30%. Note the area of slightly lower SGR on the upper part of the overlap zone this is the critical area for fault seal.

The calculated SGR is generally above 20%, except for the R2A unit, where it is less than 15% in areas of small throw. If the SGR thresholds that we have obtained on the G structures are representative for the C structure, the upper two-thirds of the Brent Group juxtaposition are expected to seal well, and the lower part would be open to flow (Fig. 16). Note that this is in contrast with the faults in the western area, where seal is poorest on the upper parts of the faults (Tarbert Fm). 

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