Depth Maps Calibration Using Fluid Contact Evidences: A Case History

By

M. Serazzi¹, F. Marchini¹

(1) ENI SpA - Agip Division, S. Donato Milanese, Italy

 Conformity to trap geometry is usually required for validating seismic amplitude anomalies and flat spot reflections as Direct Hydrocarbon Indicators.

This case history refers to an offshore field, where the interpretation of reprocessed seismic, supported by seismic modelling and sedimentology revision, allowed to recognise DHIs with high confidence.

Inconsistencies with structural map have therefore been interpreted as zones of poor map precision, and utilised for depth map calibration, providing a tool for improving geophysical maps precision.

The field is characterised by two culminations separated by a saddle. The reservoir is very uniform in terms of thickness and petrophysical properties, and extends continuously all over the area. The pay area shows clear amplitude anomaly.

Two types of calibration have been implemented:

1. saddle area. We modelled the relation between amplitudes and gross pay, due to interference between top reservoir and OWC reflections. An amplitude-based gross pay map has been produced, that, added to the OWC depth, provides the reservoir top in the OWC area. The maximum correction was -80 ft.

2. flank area. A clear flat-spot is visible, which towards the Northern structure significantly departs from depth contours. The map has been forced to OWC depth along the intersection between reservoir top and flat spot reflections, significantly modifying the mapped dip and azimuth. The maximum correction along the flat-spot is -150 ft.

A well drilled close to the flank hit the top reservoir with only 9 ft error, compared with 35 ft of the map calibrated with wells only.