Page 173 - Geochemical Remote Sensing of The Sub-Surface

150 v.T. Jones, M.D. Matthews and D.M. Richers

Bound gas

Bound gas, which is adsorbed on both the organic and inorganic matter contained in
the sediment by means of physicochemical binding, introduces new complexities into
defining the appropriate sample for analysis. The difficulty with defining this bound gas
is forced by the reality that rocks and/or sediments contain gases of multiple origins. By
their very nature, sediments contain both migratory (epigenetic) and indigenous
(syngenetic) gases. Migratory gases (biogenic and thermogenic) have migrated to the
surface from a deeper, more concentrated source.
Indigenous gas is related to biogenic, diagenetic and thermogenic generation within
the rock sampled at the surface and to recycled materials which may contain some
physically-transported hydrocarbons tightly bound in inclusions or other interstitial sites
within the sediment matrix. The nature of the bonding of the hydrocarbons to the grain
surfaces leads to two categories, adsorbed and chemi-adsorbed. These form an important
part of this discussion because of misnomers involved with the use of the word
"adsorbed".
True adsorbed gases are by definition bound to the surfaces of sediment or rock
particles. As defined by Greenland (1981) adsorption is the process by which a chemical
species passes from one bulk phase to the surface of another, where it accumulates
without penetrating the structure of the second phase. Because the light hydrocarbons are
so labile, they do not strongly adhere to surfaces and are easily desorbed if the source of
these gases is removed. The gas must be replenished by continuous migration in order to
maintain the presence of adsorbed gases on the available surfaces.
Bound within the rock matrix, or within certain minerals (calcite, oxide coatings,
etc.) gases are chemi-absorbed. They can be removed only by a chemical attack that
completely dissolves the rock or sediment matrix. Sometimes these more tightly-bound
gases not only include indigenous gases, but also might integrate the signal over time,
mixing the products of "dead" or "non-active" seepage with those gases actively
migrating today. The non-active seeps are often coupled to the lithologies of transported,
non-residual sediments (Richers et al., 1986). These last considerations provide two of
the main reasons why "free" and "chemi-adsorbed" gases are often found to have no
obvious spatial correlation.

Choice of ree gas or bound gas
f
Any prospector would generally agree that it is desirable to measure only the gas
which has migrated from depth, since this is clearly the gas signal which is related to
buried reservoirs. The difficulty in doing this begins with choosing the method of sample
collection, because there are few sample-collection techniques that do not mix the
syngenetic and epigenetic gases. Both "free" and "adsorbed" hydrocarbons can often be
related to a migratory source, and thus can yield useful exploration information. The free