Stratigraphic Controls on Gas Migration and Pockmark Formation at the foreset of a Prograding Clinoform System west of North Island, New Zealand

Methane stored in shallow marine sediments significantly affects seafloor stability, and influence ocean-atmosphere interactions. Since methane is a potent greenhouse gas, its release influences regional biogeochemical cycles and benthic ecosystems. Along continental margins, favourable conditions promote biogenic methanogenesis and gas hydrate formation. Understanding how methane migrates beneath the base of hydrate stability is therefore essential, particularly because hydrate dissociation near the feather edge of continental slope releases methane to the seabed. Pockmarks form when gas escapes from shallow overpressure zones. Overpressure may develop through hydrate dissociation or through the accumulation of free gas below low-permeability layers. Once pressure exceeds the sealing capacity of the overlying sediments, gas can migrate upward and eventually vent at the seabed.

In the offshore Taranaki Basin, west of New Zealand’s North Island, high-resolution 3D seismic data reveal ~300 pockmarks between 300-700 m water depth. Beneath many of these pockmarks, the seismic data show tiers of near-vertically stacked shallow-gas bright spots, indicating focused migration pathways in the shallow subsurface across the foresets of a prograding clinoform system.

The theoretical stability limit for pure methane hydrates locally aligns with the shallowest bright anomalies. However, most anomalies lie within the free-gas zone landward of the methane-hydrate outcrop and beneath large parts of the pockmark field. Over the past ~16 kyr, bottom-water temperatures along the slope have warmed by ~2.25 °C, shifting the hydrate-stability feather edge downslope by ~1.7 km. This warming-driven retreat  can account for only ~20% of the observed pockmarks. While the presence of gas hydrates can deflect gas updip, there is no clear seismic evidence for a bottom-simulating reflection. Instead, gas appears to ascend upslope through a range of stratigraphic heterogeneities, such as cyclic steps that climb obliquely, scour rims, channel cuts, and levee deposits, which collectively provide localized pathways for migration.

In gently dipping (2-3°) slope, free gas beneath the hydrate stability zone would preferentially migrate updip along permeable strata toward the shelf edge. However, 3D seismic data show bright spots concentrated within scour rims, channel levees, and the crests of cyclic steps that act as effective traps updip of the upper limit of hydrate stability at the clinoform foresets. Gas is accumulated within levee deposits of vertically aggrading and laterally shifting channel-levee systems, where repeated cut-and-fill cycles build stacked fining-upward units. The climbing geometry of cyclic steps redirects gas vertically upslope along their crests, enhancing upward migration, while fine-grained scour infill inhibit lateral migration.3D visualization shows that such traps form multiple tiers of shallow-gas pockets linked by focused gas-flow. Together, these relationships demonstrate that fluid migration is strongly controlled by sedimentary architecture shaped by turbidity current-controlled depositional processes at the foresets of the prograding clinoforms. The clustering of numerous pockmarks above these vertically stacked gas zones strongly indicates that stratigraphic focusing, rather than along-slope migration at the base of the hydrate stability zone, controls gas ascent.