Brent sequence

Across-fault juxtapositions Upper Brent sequence... 

Fig. 5a shows two fault zones which offset an Upper Brent sequence, each with an aggregate displacement of ca. 40 m. On the scale of observation, fault zone A comprises a single slip surface, while fault zone B comprises two parallel slip surfaces each of which accommodates about half of the total displacement. In this case the paired slip surfaces are separated by ca. 15 m of rock with low shear strain, as indicated by bedding re-orientation. Two slip surfaces would not be distinguished even with good quality seismic data. Across-fault Juxtapositions calculated on the basis of a single slip surface would be valid in the case of fault A, but invalid for fault B. The consequences of incorrect Juxtapositions on fault B are illustrated in Fig. 5b-d), for the Upper Brent sequence shown in Fig. 5a. 

For the Brent sequence shown here, across-fault connectivity is higher for a single slip surface than paired slip surfaces for all throws less than 80 m. For this sequence the across-fault sandstone connectivity of fault zone A is 23% of the gross reservoir thickness, and that of fault zone B is 14%. The difference in connectivity between the two fault zones is relatively small in this example. In general, however, the... 

Connectivity curves for a single slip surface are shown in Fig. 5c for a range of SGR cut-off values, for the Upper Brent sequence shown in Fig. 5b. The forms of the connectivity curves are very different. For an SGR cut-off = 20 there is no connectivity for throws >33 m. For an SGR cut-off = 30, there is no connectivity for throws from 40 to 60 m, but 8% connectivity at a throw of 65 m. For an SGR cutoff = 40, there is a sharp increase in connectivity at 45 m when the Tarbert and Lower Ness reservoirs become juxtaposed. 

Figure 6.6. Amplification of DNA by PCR. Target DNA sequence from a complex genome can be amplified by heat denaturation, providing appropriate conditions for the enzyme (Taq DNA polymerase) that allow it to cause exponential amplification of a particular DNA segment. Among components besides the enzyme that are essential for amplification process are oligonucleotide primers in opposite orientation to each other, shown by dotted arrows, deoxynucleotide triphosphates (dNTPs), Mg2+, and buffer. A 30-cycle amplification leads to a many-million-fold amplification of the discrete DNA segment, flanked by oligonucleotide primer sequences. (Reproduced from Short Protocols in Molecular Biology, 4th ed., F. M. Ausubel, R. Brent, R. E. Kingston, D. D. Moore, J. G. Seidman, J. A. Smith, and K. Struhl, eds., Wiley, New York, 1999, p. 15-1.)...

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 bifurcation mechanisms for formation of multi-slip fault zones suggest that maximum fault zone thickness will often correspond to the strike-normal distance between the traces of two overlapping slip surfaces (Fig. 2c). Fault overlaps and their breached equivalents occur on faults of all sizes as do, by implication, paired and multi-slip surface fault zones. Complex and paired slip surface fault zone structures will occur on scales below that resolvable by even high quality seismic data (lateral resolution is no better than 50-100 m at North Sea reservoir depths). The possible impact of sub-seismic complexity and paired slip surfaces on connectivity and sealing across faults offsetting an Upper Brent type sequence are briefly considered below. 

The above studies suggest that sealing by clay smear may be predicted deterministically frorn a consideration of the thickness and offset of individual shale beds. However, such an approach is difficult to apply directly in the Brent Group because of the heterogeneity of the sequence. It is not feasible to map... 

There are two approaches in the development of sequence analysis techniques for gene identification (Fickett, 1996 Brent and Guigd, 2004) ab initio and de novo methods. The ab initio gene prediction is based only on the genome sequences using the statistical... 

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