Introduction

Geomanifestations are defined as distinct expressions of an ongoing or past geological process at surface or at depth. Gas seeps and subsurface accumulations fit this definition and in this blog we show some examples of shallow gas manifestations in the Netherlands. Usually, in the subsurface gas is trapped in porous rocks when a positive structure exists (space available) and a good sealing rock (e.g. clay or salt) prevents the gas from leaking. These prerequisites are the base for economic gas fields. If one of the boundary conditions is ineffective, either volumes are too low and/or the gas can potentially leak to shallower depths. This upward (or sometimes lateral) migration stops where shallower sealing layers are met. The potential of these layers to retain the gas is called sealing capacity. Since the density of gas is lower than that of the water that normally resides in the rock pore space, it tends to migrate upward (buoyancy effect) even under normal pressure conditions. If sealing capacity is lower than the upward migration strength, the gas will leak and might eventually reach the seafloor. Worldwide, for various reasons increasing attention is paid to shallow gas accumulations and their leakage to the seabed.  Apart from significance of shallow gas as energy resource, it is a possible geohazard for drilling and wind farm locations. Also, the effect of seabed gas emissions on marine ecosystems and climate is increasingly acknowledged (Verweij et al., 2018 and references therein).

Bright spots

In the Dutch, German and UK part of the Southern North Sea (SNS) shallow gas accumulations reside in Cenozoic, shallow marine to continental (deltaic) deposits. The shallow gas appears as acoustic bright spots in seismic data, which are explained as seismic reflections with anomalously high amplitudes. This makes interpretation and delineation of the occurrences an easy task that shows the gas is either structurally trapped in anticlines above salt domes, associated with lateral fault seals, or occurs in stratigraphic or depositional traps (Ten Veen et al., 2013; 2018). Traditionally, shallow gas occurrences were regarded as hazardous or non-economic because of low gas saturations (“fizz gas”). Even though the production of shallow gas still is a challenge, to date, four gas fields are producing.

Bright acoustic events can be caused by many geological and/or physical phenomena that cause a local and anomalous impedance contrast that differ from its surrounding. Gas-filled sand layers that appear as low impedance layers with anomalously high amplitude are often regarded as a Direct Hydrocarbon Indicator (DHI). If the gas-bearing layer is thick enough, the gas-water contact might be identifiable as a flat spot (Fig. 1C).  It should be noted that the high amplitude, considering the absence of pre-stack amplitude information, is not indicative for gas saturation as even low saturations will produce high amplitude effects detected in post-stack data. The bright spots can be mapped using an auto-tracker on available 2D and 3D seismic data. If done so, most bright spots appear to be stacked, suggesting that gas is trapped in alternating clay-silt layers.  In such case it is common that the shallowest bright spot reflects most of the seismic energy back to the surface. Because of this transmission effect, the events below have very low amplitudes (Fig. 1). Consequently, bright spots below other bright spots sometimes do not meet the criteria for being an anomalously high amplitude event. However, in most cases bright spots become larger with depth (halo-shaped) and can therefore be partially mapped and extrapolated over the transmission domain. Additionally, the gas-filled sand exhibits a pull-down effect which increases with the number of stacked reservoirs (Fig. 1A).

The spatial distribution of stacked bright spots is closely related to salt domes and ridges forming the structural control on anticlinal closures.  Many of the stacked bright spots are not only salt-related, but also fault-related since the salt structures incite fault systems in the overburden as well. (Fig. 1C). Next to stacked bright spots, single elongated, bright spots occur that are associated with sandy contourites (Fig. 1D) and bright spots that are aligned with the dipping foresets of delta clinoforms (Fig. 1B); both types represent stratigraphic traps. Some bright spots types are associated with faults and if reservoirs thicknesses are above tuning thickness, gas-water contacts may be visible as flat spots (Fig. 1C).

Origin, migration and leakage

There is an on-going debate about the origin of the gas in SNS delta deposits. Either it is biogenic, thermogenic or a mixture of both sources. Geochemical and isotopic analyses on samples of accumulated shallow gas in the SNS delta show that the gas is very dry (> 99% of methane) and depleted in 13C/C1 (-70% dC13/C1). Such compositional characteristics are in accordance with a biogenic origin of the gas. One possible source is riverine organic material fetched in Cenozoic deltaic deposits which is at maximum depth and temperature today. In the deepest part of the delta, microbial generation of gas started at the beginning of the Pleistocene (onset of glacial-interglacial cycles) and continues today. However, the presence of chimneys and acoustic turbulence zones in seismic data and the fact that many shallow gas accumulations occur in association with salt domes and/or faults suggest that the gas or fluid migration from deeper levels may occur as well. Thus, the potential for biogenic generation does not exclude the possibility of mixture with thermogenic gas. At present vertical migration of shallow gas through the delta sediments takes place under normal to close-to-normal pore pressure conditions (Verweij et al., 2012) and leakage up to the seafloor is a common geomanifestation (Fig. 2).

 

Johan ten Veen
TNO – Geological Survey of the Netherlands

 

References

Schroot, B.M., Klaver, G.T., Schüttenhelm, R.T.E. [2005] Surface and subsurface expressions of gas seepages to the seabed – examples from the Southern North Sea. Marine and Petroleum Geology 22, 499-515.

Ten Veen, J., Verweij, H., Donders, T., Geel, K., De Bruin, G., Munsterman, D., Verreusel, R., Daza Cajigal, V., Harding, R. and Cremer, H. [2013] Anatomy of the cenozoic Eridanos Delta Hydrocarbon system. TNO report 2013 R10060, 142 p. plus 10 appendices.

Ten Veen, J., Verweij, H., Donders, T., De Bruin, G. and Geel, K., [2018] Shallow gas traps in the Cenozoic Southern North Sea delta, offshore Netherlands. First Break, v36, 30-36.

Verweij, J.M., ten Veen, J.H., De Bruin, G., Nelskamp, S.N., Donders, T., Kunakbayeva, G. and Geel, K. [2012]. Shallow gas migration and trapping in the Cenozioc Eridanos delta deposits, Dutch offshore. Extended abstract. 74th EAGE Conference & Exibition Copenhagen, Denmark, June 2012.

Verweij, J.M., Nelskamp, S.N., ten Veen, J.H., De Bruin, G., Geel, K. and Donders, T.H. [2018] Generation, migration, entrapment and leakage of microbial gas in the Dutch part of the Southern North Sea Delta. Marine and Petroleum Geology 97, 493-516.

 

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