Outcrop study

To derive a reservoir geological model various methods and techniques are employed mainly the analysis of core material, wireline logs, high resolution seismic and outcrop studies. These data gathering techniques are further discussed in Sections 5.3 and 2.2. 

Lindsay, N.G., Murphy, F.C., Walsh, J.J. and Watterson, J. 1993. Outcrop studies of shale smears on fault surfaces. Int. Assoc. Sedimen-tol. Special Publication, 15 113-123. 

Fault seal prediction in hydrocarbon reservoirs requires an understanding of fault seal mechanisms, fault rock petrophysical properties, the spatial distribution of seals, and seal stability. The properties and evolution of seals within fault zones can be evaluated using the combined results of structural core logging, microstmctural and physical property characterisation, together with information on fault populations from seismic and outcrop studies and well test data. 

Background and methods. This paper reports on an outcrop study of clay smears in synsedimentary normal faults that were exposed in the open-cast lignite mines at Frechen near Cologne, Germany. The observations made are interpreted in terms of a mechanism of clay smear emplacement. 

Data for characterisation of faults in the subsurface are limited to two sources, seismics and wells. Seismic reflection data allow the displacement distribution over a fault surface to be mapped while well and core data may allow determination of fault rock types and deformation mechanisms at specific points, in addition to characterising the lithologies of the host sequence. It is evident from outcrop studies that the internal geometries of fault zones are usually complex, in terms of the numbers of individual slip surfaces, the partitioning of slip between them and in the distribution of different fault rocks, all of which vary over a fault surface. This 3-D complexity of fault zone structure may not be apparent from either seismic or core data but is nevertheless crucial to the bulk hydraulic properties of a fault. 

Lindsay et al. (1993) describe outcrop studies of shale smears in a carboniferous fluvio-deltaic sequence in northern England. As in the study described above, Lindsay et al. concentrated on the effects of individual shale beds in the sequence rather than the bulk properties of the sequence. Smear is observed to be thickest when derived from thicker source layers and with small fault throw values smear thicknesses commonly decrease with distance from the shale source bed. From a study of 80 faults they conclude that shale smears may become incomplete when the ratio of fault throw to shale layer thickness exceeds 7. Smaller ratios are more likely to correspond to continuous smears and therefore to a sealing layer on the fault surface. 

Fault displacement is usually taken as fault throw from depth structure maps. A relation has been determined, from empirical oil and gas field data (Knott, 1993) and outcrop studies (Knott, 1994), which shows that there is a positive correlation between the probability of a fault sealing and fault throw divided by reservoir thickness (Dn). 

Miall, A.D. (1988) Reservoir heterogeneities in fluvial sandstones lessons from outcrop studies. Bull. Am. /lii. Petrol. Geol., 72, 682-697. 

There is an important qualification to answering the question When did a solid crust first cover Earth In every outcrop studied to date, there are hints of still older crustal cycles. The age estimates for Earth s oldest rocks provide a constraint on the timing of crustal formation embodied in the statement Earth s first crust formed at the same time or earlier than the age of its oldest known rocks . 

We have emphasized that the modelling and the associated rock mass characterization should be determined by the rock engineering objective. This means that different types of information will be required for THMC modelling for the different types of application, as illustrated by the themes of this conference and the associated papers. However, site investigation has limited methods of approach, e.g. surface outcrop studies, borehole and borehole core measurements, and geophysics, which means that we will not have as much information as we would like and concessions have to be made on both the site investigation and modelling sides. 

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