Print this pageConflicting Formation Dip Measurements Unravel Complex Subsalt Geology, Deepwater Gulf of Mexico, USA
Due to the difficulty in obtaining clear seismic images below salt, understanding complex geological structure is a difficult task in exploration and appraisal drilling in the deepwater Gulf of Mexico (GOM). It has a significant financial impact on the success of appraisal drilling, and on a larger scale in field development. Therefore, to supplement the seismic information, operators frequently depend on borehole images to delineate geological structures. With the advent of the triaxial induction tool, it is becoming common to complement dip information obtained by the borehole imaging tool with the dips from the triaxial induction tool. Generally the dips from both these two tools are in agreement. However in one recent case study on an ultra deepwater GOM well, dip azimuth from both the tools agreed but dip magnitude from the induction tool was greater than that of borehole images. At the early stage of exploration the impact of this information is significant.
Critical financial decisions regarding offset wells and delineation of geological structures depended on resolving the discrepancy between the various dip measurements. A thorough investigation was launched for imaging and induction tools, and it proved that both tools were operating normally. Two working hypotheses were suggested:
1) The well is practically parallel and close to steep Cretaceous rocks and/or salt face, thus the induction tool with deeper & larger volume of investigation is influenced by larger structure, whereas the imaging tool reveals near wellbore gentler dips;
2) There is a large scale, steeply-dipping fracture system that intersects the wellbore infrequently, and dips from induction are influenced by both the steeply dipping fractures and gently dipping bedding.
This example clearly highlights the complexity in sub-salt characterization, and also underscores the importance of examining both induction and image data. The result was that a situation that started out as a negative, with a discrepancy between two sources of dip information, turned into an opportunity to better understand a reservoir while demonstrating the value of a new source of geological information.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil