Impact of wellsite biostratigraphy on exploration drilling in the deepwater offshore Nigeria

Role of biostratigraphy in reconstructing the subsurface during drilling highlighted. Calcareous nannofossil is an excellent group in delivering real time data. Wellsite biostratigraphy assists in stratigraphic monitoring and confirmation of well prognosis. Biosteering, coring and casing point selection are aided by wellsite biostratigraphy. This has been applied in the Niger Delta, Benin Basin and JDA of Nigeria/Sao Tome and Principe offshore areas. The application of wellsite biostratigraphic technique has aided the reconstruction of the subsurface geology in the Nigerian deepwater region. It has further aided decision making during drilling operations including the planning of well paths. Critical decisions regarding casing setting, coring point selection, overpressure zone determination and total depth picks, are easily achieved through confirmation of well prognosis and correlation to offset wells. This technique is very important in the tectonically active Nigerian deepwater region which is characterized by varying degrees of both syn-depositional and post-depositional deformation. Accurate interpretation of basin architecture, lateral variation and facies change is required before drilling. Confirmation of well prognosis during drilling operation is equally important.It has been proved that wellsite biostratigraphic technique helps to "get it right at first" when integrated with the traditional lithologic description, log signature correlation and seismic profile interpretations. The modern and rapid processing technique of calcareous nannofossil, for instance, provides 'real time' result for the exploration team for confirmation of, or adjustment to, the drilling program. The attendant contribution in saved time, cost and safe and successful drilling operation makes the technique beneficial for all operators.This paper presents practical experience of wellsite biostratigraphy application in three oil prolific Nigerian deepwater regions: the Niger Delta, Joint Development zone of Nigeria/Sao Tome and Principe area (JDZ) and Benin (Dahomey) Basin.