This payer describes the application of horizontal well geochemistry for th
e first time. Closely spaced samples, taken along a horizontal or deviated
well path, allow the identification of reservoir compartments. Significant
variations in geochemical parameters are related to the field filling and,
if they persist over geological time and over tens of metres, should reflec
t barriers to diffusion and, by inference, possibly also to fluid flow.
Two different approaches have been chosen: (a) closely spaced head space ga
s samples; (b) wet cuttings-based techniques. Case studies are presented do
cumenting the detection of sealing faults between different reservoir compa
rtments at the time of sampling. This novel approach has several advantages
: (i) the need for costly pressure measurements is reduced; (ii) high sampl
e density allows better spacial definition of barriers compared to both sei
smic and pressure measurements; (iii) detection of sub-seismic barriers; (i
v) it allows detection of compartments even in cases where no pressure diff
erences exist; (v) the technique works in wells drilled with oil-based mud
systems. The strong variations in the geochemical signal over short distanc
es has implications for the filling of reservoirs. The data suggest that re
servoir filling is far more complex than previously envisaged.