Page 193 - Geochemical Remote Sensing of The Sub-Surface

170 V.T. Jones, M.D. Matthews and D.M. Richers

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O0 9 ee 9 ~1'~ ~W
! wcJ, e,~., u Delta
e
~ewcme g ( wca~ ewmo ~ . .z,, 9
9
1N
Elm
9 ~ ~, Grand I.
worn e4_ I ,rt-._r--,__.r-"
~ ~,.r-~--.rd.J" I-- - ! S TImballer
_ '--7 lYermillion I Eugenel. I "
W. uameron I --.
S. Marsh :~nnp
E. Cameron Shoal
Fig. 5-22. Well locations offshore Louisiana for which gas compositions are published (from
Rice, 1980).
Dissolved gas

In offshore prospecting, "sniffers" have been used to detect anomalous hydrocarbon
concentrations in bottom waters. An extensive review of the literature was published by
Philp and Crisp (1982). Some of the most significant results reported by Williams et al.
( 1981) are highlighted here.
Gulf Research and Development Company designed and operated several marine
seep detectors that were employed aboard various research vessels, such as the RV
Hollis Hedberg and its predecessor the RV Gulfrex. These ships circumnavigated the
globe and conducted extensive detailed surveying in areas such as the Gulf of Mexico
(Mousseau and Williams, 1979). The RV Hollis Hedberg system employed three
separate water inlets which, whilst the ship was underway at normal seismic survey
speeds, continuously supplied sample streams from the near surface, intermediate depths
to 135 m (450 feet) and a deep-towed sample inlet at a depth of nearly 180 m (600 feet).
Each sample stream is analysed for seven hydrocarbon gases once every three minutes
with a sensitivity that depends upon the hydrocarbon and, for example, is about 5 x 10 1~
litres of propane at STP per litre of seawater. By using multiple depth inlets, surface
contamination can be demonstrated to have no effect on seeps observed by the deep
inlet. At sea, sniffer geochemical data from a deep tow inlet were superimposed to scale
on a seismic section to aid the explorationist in making real-time evaluations of
hydrocarbon potential of structurally-significant areas.
As for surface soil gases, a powerful confirmation of the validity of marine
geochemical data can be shown by the very close agreement between the composition of
component hydrocarbons in production gases and the composition of seep anomaly
gases in the same areas. Figure 5-22 shows the well database used for this confirmation