This paper describes the application of horizontal well geochemistry for the 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 reflect barriers to diffusion and, by inference, possibly also to fluid flow.

Two different approaches have been chosen: (a) closely spaced head space gas samples; (b) wet cuttings-based techniques. Case studies are presented documenting the detection of sealing faults between different reservoir compartments at the time of sampling. This novel approach has several advantages: (i) the need for costly pressure measurements is reduced; (ii) high sample density allows better spacial definition of barriers compared to both seismic and pressure measurements; (iii) detection of sub-seismic barriers; (iv) it allows detection of compartments even in cases where no pressure differences exist; (v) the technique works in wells drilled with oil-based mud systems. The strong variations in the geochemical signal over short distances has implications for the filling of reservoirs. The data suggest that reservoir filling is far more complex than previously envisaged.

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