Subcrop Mapping Using Seismic Attribute
Ouazar, Abdelkader, Amin Bellahcene, Sonatrach - Exploration Division -
Triassic province is characterized by one of most important play of the algerian mining domain. This play is part of the petroleum system which is generally structurally controlled. The traps are fault related. The reservoir is represented by sandstones of continental deposits type, they are charged by the Silurian source rock. The juxtaposition of the Silurian source rock immediately under reservoirs units represents one of the major risks of this play. The assessment of the presence of the source rock (Silurian Shale) under The Hercynian Unconformity, either by conventional technics of interpretation, or by drillings remain very difficult. Nevertheless, a fine analysis of seismic amplitudes may help to solve this problem. This technic allows the identification of the different underlying formations and in particular units of the Silurian. The approach can be subdivided in four (4) steps: -1st step: Modelling of reflection coefficients according to the different lithologies. -2nd step: Characterization of the seismic signature bounded to the presence of the Silurian under The Hercynian Unconformity. -3rd step: Assessment of the “ Tuning effect “ on the seismic signature. -4th step: Removal of the shallow layers effect on the seismic signature (absorption, heterogeneity…) by normalizing of amplitudes at the ‘Hercynian Unconformity’ relatively to the amplitude effect bound to the Lias “S3”. Construction of ratio map (Hercynian Unconformity / S3).
This method allows the mapping in Pseudo - subcrop map at the Hercynian Unconformity characterizing then the distribution of the source rock in the absence of wells.