Dynamics of fault-fluid-hydrate system around a shale-cored anticline in deepwater Nigeria

Abstract

[1] Gas hydrates were recovered by coring at the eastern border of a shale-cored anticline in the eastern Niger Delta. To characterize the link between faults and fluid release and to identify the role of fluid flow in the gas hydrate dynamics, three piezometers were deployed for periods ranging from 387 to 435 days. Two of them were deployed along a major fault linked to a shallow hydrocarbon reservoir while the third monitored the fluid pressure in a pockmark aligned above the same major fault. In addition, 10 ocean-bottom seismometers (OBS) were deployed for around 60 days. The piezometers simultaneously registered a prolonged fluid flow event lasting 90 days. During this time, OBS measurements record several episodic fluid release events. By combining and analyzing existing and newly acquired data, we show that the fluid-fault system operates according to the following three stages: (1) upward pore fluid migration through existing conduits and free gas circulation within several shallow sandy layers intersecting the major fault, (2) gas accumulation and pore pressure increases within sandy-silty layers, and (3) hydrofracturing and fluid pressure dissipation through sporadic degassing events, causing pore fluid circulation through shallow sandy layers and drawing overlying seawater into the sediment. This paper clearly demonstrates how an integrated approach based on seafloor observations, in situ measurements, and monitoring is essential for understanding fault-fluid-hydrate systems. Citation: Sultan, N., et al. (2011), Dynamics of fault-fluid-hydrate system around a shale-cored anticline in deepwater Nigeria,

Pore Pressure Monitoring of Active Fault-Fluid-Hydrate System in Deep Water, Nigeria

“…The hydraulic diffusivity D H value, calculated from the dissipation curve corresponding to (iii) in Figure 6-a, was found equal to 1.3 10 -4 m 2 /s for a drainage path of 5 m corresponding to the depth of the sandy layer at the northern flank of the eastern fault. This unusual high D H value (4 orders of magnitude higher than normal D H values from the study zone -see Sultan et al, 2011) confirms that dissipation occurs through fractured sediments. After the dissipation of the pore pressure, pore fluid accumulation occurs for a second time with a maximum pore pressure of around 6 kPa (iv in Figure 6-a) followed by pore pressure dissipation with normal hydraulic diffusivity.…”

“…This reference core crosses the regional reflector D60 and shows that the high amplitude characterizing this reflector can be correlated to a silty layer between 16 to 17 mbsf (Figure 2-B). In situ piezocone measurements acquired from the study area have also shown that the high amplitude reflectors (Figure 2-A) corresponds to a silty layer which is characterized by high tip resistance, high friction and low pore pressure (see Sultan et al, 2011). In the following, the Dxx reflectors presented in Figure 2 are interpreted as sandy-silty layers by extension of observations concerning D60, D50 and D30 ones.…”

“…This reflector corresponds to the Top of the Gas Hydrate Occurrence Zone (TGHOZ). Indeed, several in situ piezocone measurements were carried out along and close to this discontinuous reflector and demonstrate the presence of gas hydrates below this discontinuous reflector, as was shown from a calypso core recovered from the same area (see Sultan et al, 2011). In order to characterize and quantify the pore pressure associated to this high fluid activities and gas hydrates occurrences 4 long-term piezometers were deployed at four different locations surrounding the major eastern fault (PZS12, PZS16, PZS17 and PZS20 - Figure 1).…”

“…The important pore pressure perturbations measured by PZS12 and PZS20 (Figure 4-b and -c) at the base of the piezometer lances seem to diffuse very slowly vertically through the sedimentary column. It was show from numerical calculations carried out by Sultan et al (2011) that during the 1-year monitoring period, only a limited area surrounding the sensors P5 of PZS12 and PZS20 is affected by the pore pressure variations. For the 5 pore pressure sensors of PZS17 (Figure 4-d), a relatively small pore pressure fluctuations between -0.5 and 1 kPa were recorded.…”

“…The very detailed bathymetric data presented in Figure 1 reveal the presence of two pockmark fields separated by a major E-W structural fault. The study site displays various recent and active processes such as slope failures , deformation and fluid escape , Sultan et al, 2011, as demonstrated by a broad variety of sub-circular depression features (pockmarks). Those pockmarks are very often associated and/or rooted on very high amplitude reflectors (called Dxx in Figure 2).…”