Fault seal analysis

Fault seal analysis successful methodologies, application and future directions... 

The critical questions which now require evaluation in order to advance fault seal analysis are listed below ... 

Fundamental to a successful fault seal analysis is quantification of the petrophysical properties of the different fault rocks present in the hydrocarbon field under investigation. The critical properties which require quantification are permeabilities, capillary entry pressures, transmissibility, fault-rock thickness and the strength of the fault rocks. One of the reasons why fault seal analysis and reservoir modelling has proved difficult has been the absence of data on these properties. Analysis of the petrophysical properties of... 

A large proportion of the structural features which can impact on the flow properties of a reservoir are below the resolution of seismic data. Therefore, understanding the spatial distribution and internal structure of fault zones can be fundamental to the flow behaviour of hydrocarbon reservoirs. This section reviews the important geometrical characteristics of fault zones and sub-seismic deformation features which can influence fault seal analysis. 

Is the structure of fault zones usually assumed in fault seal analysis valid and what is the impact of fault damage zones ... 

What fault zone properties need to be incorporated into a robust fault seal analysis ... 

Fig. 6b illustrates the type of simple interpretation of the fault structure often used as a basis of fault seal analysis. This interpretation is based on representation of the fault as a single fault plane, where the offset is assumed to be equal to the seismic (cumulative) offset. This is clearly not valid as a representation of throw distributions for use in fault seal analysis. Fig. 6 also illustrates that sub-seismic fault populations can be clustered around larger faults with extensive areas of low fault densities away from the large faults. This has important implications for the spatial distribution of sub-seismic faults and emphasises that uniform distributions of small faults are not always applicable (except perhaps in areas where more uniform straining is associated with doming). The information contained in Fig. 6 reinforces the results of other recent studies which have noted different fault... 

Fig. 6. (a) An example of the complex architecture of a fault damage zone as imaged on a seismic attribute map (dip magnitude in this case). Note that the fault zone is composed of linked segments and domains where different sub-structures are present, (b) The simple interpretation of the fault zone which would be misleading if used as a basis for fault seal analysis. 

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

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... 

It is interesting to evaluate the success of the simple fault seal analysis, based on the list of common assumptions outlined above, after a more detailed and integrated seal analysis has been performed. The following results from our analysis of >25 North Sea seals are applicable here ... 

The aim of this review has been to demonstrate that although a complex problem there are techniques which can be, and should be, applied to fault seal analysis as they allow a clearer understanding, quantification and therefore predictability associated with a fault seal analysis. 

Jones, G. and Knipe R.J. 1996, Seismic attribute maps application to structural interpretation and fault seal analysis in the North Sea Basin. First Break, in press. 

Knott, S.D. 1993. Fault seal analysis in the North Sea. Am. Assoc. Pet. Geol. Bull., 77 778-792,... 

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... 

The second stage of static fault seal analysis is the evaluation of the properties of the fault gouge (Fig. 2). In the presence of clay layers, the introduction of clay into a fault is one way of strongly increasing the capillary entry pressure. A common process is the... 

Fig. 6. Brittle fault seal analysis strategy. The strategy aims to quantify the sealing capacity of brittle faults by first predicting the deformation mechanism. Particulate flow faults are treated as non-sealing. Cataclastic faults have variable sealing properties according to fault throw and matrix properties. The chart in the lower left of this figure is reproduced at a larger scale in Fig. 8.

We strongly recommend a rigorous, integrated strategy to fault seal analysis, including all possible aspects of fault as well as top seals. The rigour of the... 

All fault and fault-seal analysis was performed in the depth domain, since this allows ... 

The poor seismic data quality in the C and J structures limits the reliability of the fault seal analysis in these areas. Consequently, the description below is focused upon the good quality seismic of the western area of Block 30/9 (Omega, B and G). The eastern area is covered in more general terms. 

Fault seal analysis in hydrocarbon exploration and appraisal examples from offshore mid-Norway... 

The hydraulic properties of faults determine whether they act as migration barriers or pathways across the region. Fault seal analysis was applied, therefore, to predict the degree of fault seal across the area and hence define migration pathways and also to risk trap integrity. The fault seal analysis is also used to explain earlier well results and to calculate likely hydrocarbon column heights in undrilled prospects. 

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