Methane in Martian Soil
Studies shows there is Methane on Earth's Crust. The chemical form how it is stored is called Methane Clathrate,

&quot;a solid clathrate compound (more specifically, a clathrate hydrate)
in which a large amount of methane is trapped within a crystal
structure of water, forming a solid similar to
ice.&quot;
(source:wikipedia).

There are two main origins for this structures on Earth's Crust, mainly developed at the Oceanic Crust and stored some centimeters or meters below the water interface, after the sulfate reduction zone:

Microbial reduction of CO2.

Thermal decomposition of organic matter.


Both sources imply organic activity so possibly your question encloses that much-contested question about whether there was or not life on Mars in the past.
Regardless there were or there were no life, Lefèvre et al.(2009), clarify the occurrence and possible sources for methane on Mars from what we know with our current observation possibilities:

&quot;The detection of methane on Mars has revived the possibility of past
or extinct life on this planet, despite the fact that an abiogenic
origin is thought to be equally plausible&quot;

They continue...

&quot;An intriguing aspect of the recent observations of methane on Mars is
that methane concentrations appear to be locally enhanced and change
with the seasons.&quot;

And finally:

&quot;We find that photochemistry as currently understood does not produce
measurable variations in methane concentrations, even in the case of a
current, local and episodic methane release. In contrast, we find that
the condensation–sublimation cycle of Mars’ carbon dioxide atmosphere
can generate large-scale methane variations differing from those
observed.&quot;

This is still on debate and maybe someone can contribute quoting more recent articles, but the occurrence of Methane on Mars soil is measured, but this do not provide a proof that there was life on the planet.
That condensation-sublimation cycle may lead to CH4 storage on soil around the poles, and form a kind of &quot;Methane permafrost&quot;, but formed for physical-chemical processes with no life interaction.
Also asking for &quot;large amounts of Methane on soil&quot;, the answer possibly is no too, because either Mars had life, Earth's organic activity has developed big Methane Clathrate deposits on Oceanic Crust because of the listed organic associated process.
To recognize an organic origin of CH4, isotopes of C of the samples should show low concentrations of heavy C14, as life prefers to use lightweight ones: C13/C12
.
That hypothetical biologic Methane would be found on the big depression that was supposed to have a water ocean. The latest formed on soil would have been liberated to the atmosphere at the death of the ocean (because of pressure decrease), but rests of organic CH4 could maybe be found drilling the zone.
The new of past life occurrence has not yet been published, and could come from other chemical compounds.
I usually cross my fingers when I think about it.
Lefèvre, F. Forget, F. &quot;Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics.&quot;Nature (2009)