Modes of fluid migration and accumulation

Fluids may laterally flow at the top of continuous permeable beds, in contact with the impermeable seals. These carrier beds and the lateral migrations they allow are important for the concentrations of fluids, especially hydrocarbons, at the top or on the flanks of high regional zones.

In stable margins, lateral migrations are able to continue over long distances, ranging from 10 to

>100 km. Hydrocarbon migration pathways within carrier beds are determined by the three-dimensional geometry of the top boundary of bed, along which fluids migrate by taking the structurally most advantageous routes (Hindle, 1997). For buoyancy driven hydrocarbons, that means migrating updip along directions perpendicular to the strike of the top boundary (England et al., 1987). Discontinuities such as sealing faults or pinch-outs may create a permeability barrier, in which case hydrocarbons might accumulate and be trapped downdip of this seal. Otherwise hydrocarbons will accumulate at structural crest traps, and if the sourcing of hydrocarbons continues at a rate that surpasses any eventual leakeage, fluids will eventually fill the trap.

Excessive fluids could then spill and continue migrating updip by taking other paths within the carrier bed until arriving at other traps.

Vertical migration called seepage occurs across stratified sediments, including very low permeability sealing sediments (figure 8). Given the high capillary resistance of sealing sediments, this migration mode is restricted to areas where fluids have built high capillary entry pressure, or to where heterogeneities on the sealing sediments make for more permeable pathways for fluid migration. Typical areas of focused overpressure are at structural crests and at updip limits of aquifers, at these parts fluids are trapped and may be susceptible to pressure build up. Lithological and structural heterogeneities in the sealing sediments are also important in the context of vertical migration, since it is natural for fluids to be drained through the most permeable part of the seal.

A different scenario for vertical migration is when large scale geological features promote bypass of the pore network. Some of these features have only recently been described with the advent of 3D seismic data. Cartwright et al. (2007) based on 3D seismic interpretation, recognizes three main groups of seal bypass systems: (1) fault related, (2) intrusion related, (3) pipe related.

Faults are known to have an ambiguous role when it comes to fluid flow, since they can be both sealing and trap defining or focused vertical fluid migration pathway (Aydin, 2000). A difficulty then appears when trying to identify which faults are acting as a seal and which faults act as fluid flow pathways. There is also the possibility of faults being transient fluid flow pathways, that is, the role of fluid flow pathway is restricted to periods of active fault slip. As one major example of fault related vertical fluid migration, polygonal fault systems have been indirectly linked

as the main conduits to gas leakage in many parts of the Mid Norwegian margin (e.g. Berndt et al., 2003; Hustoft et al., 2007), while at the same time the ultra low permeability sediments which host these faults overlie many prolific reservoirs in the same area (Stuevold et al., 2003). Another way fluids migrate vertically is by creating their own permeability through hydrofracturing the sealing sequence (Hustoft et al., 2007). Hydraulic fractures develop when pore pressures are sufficiently high to cause mechanical fracture of the sealing sediments. This fracturing opens a permeable pathway through the sealing sequence, allowing fluid migration.

Intrusive structures allow fluids to flow through by breaching the integrity of the sealing sequence. Within intrusive bypass structures we find sandstone intrusions, igneous intrusions, mud diapirs and diatremes, and salt diapirs.

Sandstone instrusions are newly recognized geological phenomena, by which high pressured sandstones are liquefied and emplaced as sills and dikes in low permeability sequences (Huuse and Mickelson, 2004). Sandstone intrusions affect fluid flow through inserting several meters wide permeable conduits through a sequence with low permeability (Cartwright et al., 2007). Igneous intrusions, in contrast, have generally lower permeability than the seals they intrude, but in turn cause intense fracturing associated with their forceful intrusion, hydrothermal linked metamorphism and subsequent thermal contraction (Cartwright et al., 2007). These fractures provide permeability for fluid flow around the intrusion, potentially allowing the bypass of the sealing sequence (e.g.

Svensen et al., 2003).

Mud diapirs and diatremes and associated mud volcanoes, are characterized by episodic mud intrusions and extrusions and are known to cross impermeable sediments. Mud volcanoes in the Vøring basin, are found to cross 1 km of otherwise intact sealing sequences (Hansen et al., 2005). Mud diapirs or mud volcano conduits which pierce hydrocarbon accumulations are normally associated with leakage of hydrocarbons at the surface, above or surrounding the mud intrusion (e.g. Planke et al., 2003).

Salt diapirs influence fluid flow through the development of fractures and faults associated with the piercement of sealing sequences by the episodic salt movement. These faults and fractures develop mainly at the crest of the diaper and are often associated with amplitude anomalies around them and presence of fluid flow expressions on the seabed above them (e.g. Egeberg, 2000; Chand

crestal regions, tilted fault block crests, fold crests, or crests of sand bodies with positive topography, or any other focusing element at depth” (Cartwright et al., 2007). The detailed geologic structure of the pipes is poorly understood, and could be highly variable (Cartwright et al., 2007). Some pipes appear to consist of stacked pockmarks or stacked amplitude anomalies related to gas accumulation, while other pipes appear to consist of near circular zones of sediments deformed by minor folding and fracturing. By analogy to published descriptions of breccia pipes discovered in outcrops or in mines, pipes seen on seismic are likely to consist of brecciated seal facies with zones of intense fracturing and intruded by material transported along the conduit (e.g.

Gernon et al., 2007). According to Cartwright et al., (2007), there are four families of pipes:

Dissolution pipes, which are associated to dissolution of salt at depth and concomitant collapse of sediments above; Hydrothermal pipes, associated with igneous intrusions; Blowout pipes, associated with overpressured reservoirs at depth and which terminate in seafloor fluid expulsion;

and Seepage pipes, which are similar to blowout pipes but are not associated with seafloor fluid flow expressions. Of particular relevance to this study are the blowout pipes and seepage pipes, both of which have been observed and described from seismic data in the study area by many authors (Mienert et al., 1998b; Bouriak et al., 2000; Buenz et al., 2003; Berndt, 2005; Hustoft et al., 2007).

Figure 8 - Sketch showing different fluid flow systems discussed in the text (from Berndt, 2005).