Definition of Structural Patterns Using Dipmeter and Magnetism Data within the Poorly Seismic Imaged Field of Avocette, Presalt Gabonese Basin

G. Guerin, H. Lecanu, J. C. Icart, C. Dumay, and M. Lescanne

In the Gabonese southern Onshore Basin, several oil fields occur below an Aptian salt sequence. Because of the presence of diapirs and salt walls related to post aptian salt tectonics, the quality of the 2D or 3D seismic imaging of the presalt formations stays very insufficient to well define the structural patterns within the traps below the salt. This problem is particularly important in the Avocette oil field where the poor quality of the seismic is also related to very high dips within the presalt formations. Indeed the dipmeter analysis demonstrates that the values of dips can reach 80 degrees to be vertical within some of the reservoirs. To define the complex geometry of the field, depth cross-sections have been constructed using:

- magnetism and gravimetric data which allow to propose a depth and also a structural framework for the basement;

- dipmeter analysis which provides for the whole presalt sequence very good results for the geometry, but also for the age of deformation. Indeed, the biostratigraphy is well defined and it is possible to date characteristic features as growth faulting demonstrated by the dipmeter.

This allows to propose a presalt kinematic evolution for the field and adjacent area which can be summarized as follows:

- the structure corresponds to a faulted roll-over anticline, related to Upper Barremian to Aptian progressive gravity spreading of thick lacustrine deltaic sequences. During Upper Aptian the roll-over anticline is partially eroded and overlaid by a regional sandstone deposits. Last movements on the major faults deformed this horizon which constitutes now the main reservoir of the Avocette oil field.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California