Gas seepage and pockmark formation from subsurface reservoirs: Insights
from table-top experiments
Abstract
Pockmarks are morphological depressions commonly observed in ocean and
lake floors. Pockmarks form by fluid (typically gas) seepage thorough a
sealing sedimentary layer, deforming and breaching the layer. The
seepage-induced sediment deformation mechanisms, and their links to the
resulting pockmarks morphology, are not well understood. To bridge this
gap, we conduct laboratory experiments in which gas seeps through a
granular (sand) reservoir, overlaid by a (clay) seal, both submerged
under water. We find that gas rises through the reservoir and
accumulates at the seal base. Once sufficient gas over-pressure is
achieved, gas deforms the seal, and finally escapes via either: (i)
doming of the seal followed by dome breaching via fracturing; (ii)
brittle faulting, delineating a plug. The gas lifts the plug and seeps
through the bounding faults; or (iii) plastic deformation by bubbles
ascending through the seal. The preferred mechanism is found to depend
on the seal thickness and stiffness: in stiff seals, a transition from
doming and fracturing to brittle faulting occurs as the thickness
increases, whereas bubbles rise is preferred in the most compliant,
thickest seals. Seepage can also occur by mixed modes, such as bubbles
rising in faults. Repeated seepage events suspend the sediment at the
surface and create pockmarks. We present a quantitative analysis that
explains the tendency for the various modes of deformation observed
experimentally. Finally, we connect simple theoretical arguments with
field observations, highlighting similarities and differences that bound
the applicability of laboratory experiments to natural pockmarks.