AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Seismic Characterization and Reservoir Modeling of Turbidite Systems in Tano Basin

Abstract

The aim of this work is to describe a geophysical workflow successfully adopted to provide a detailed characterization of the Upper Cretaceous reservoir sequences located in the Tano basin. The resulting full-integrated geological and geophysical characterization of the fields would then provide quantitative inputs to reservoir models and ensure reliable constrains to facies and reservoir properties simulation, always within the boundaries of seismic consistency. The Tano Basin forms the eastern extension of the Ivorian Basin, which resulted from Aptian-Albian trans-tension associated with the opening of the Atlantic margin. The basin is bounded by St. Paul and Romanche fracture zone, which strongly affected the whole basin deposition. The province shows two important differences compared to the passive-margin basins south of the Niger Delta: the influence of transform tectonics, and the absence of evaporites and salt deformation. Thus, the province has a complex geological history, which can be divided into three stages of development: Pre-transform, Syn-transform, and the last Post-transform stage; the sedimentary sequence from Cenomanian to Holocene, is characterize by the deposition of a thick Upper Cretaceous, clastic sequence,which represents the main reservoir target of Tano Basin. The work focuses on the description of two different turbidite discoveries, a Cenomanian oil bearing and a Campanian gas bearing reservoirs, both with distinctive seismic behavior. The Cenomanianreservoi is characterized by the heterogeneous fill of a strongly erosive canyon, with “stiff” and complex elastic response. The canyon-fill sequences have been interpreted as repeated forestepping-backstepping cycles of turbiditic stages, with an overall backstepping of the system. The Campanian reservoirs are interpreted as turbiditic successions deposited in a deep water fan channel complexes, characterized by several thin sandstone fairways, commonly under the seismic resolution, with excellent petrophysical properties. The analysis of available well data allows the generation of a complete dataset of measured and modelled elastic logs and a reference petro-elastic model for each target. In order to provide a seismic-consistent characterization to be used in reservoir modelling, a leading edge Seismic reservoir characterization approach is applied, consisting in the Simultaneous Elastic Inversion and Probabilistic Facies Classification. The litho-facies volumes, resulting from the inversion activities, are later converted in depth domain using a calibrated velocity volume. Integration of the geological knowledge, seismic and well data measurements, allows the creation of reservoir models representing as much as possible the real setting. Focus on the Cenomanian field, log analysis put in evidence the presence of few log-facies, that are characterized by different petrophysical properties. In particular, diagenesis effects and strong lateral heterogeneity is shown by well data. The log facies at well represent data to honor, as output of the distribution. The seismically derived probabilistic facies volumes, coming from inversion, represent the main input to reservoir characterization and define the trend for log facies distribution; moreover, core data can highlight petrographic features affecting dynamic behavior. The applied methodology emphasizes the importance of the multidisciplinary integration in approaching reservoir characterization of complex depositional systems and the value of inversion attribute, which are related, more explicitly and quantitatively than seismic amplitudes, to rock properties such as lithology and fluid type.