A new process-based methodology for analysis of shale smear along normal faults in the Niger Delta

Abstract

Three-dimensional (3-D) seismic and well-log data from the producing Okan field in the Niger Delta and a working conceptual model derived from field observations and theoretical considerations were used to map the 3-D geometry of a representative normal fault with shale smear. Seismic data show clear fault segmentation in the dip direction with extensional relays inferred to be occupied by smeared shales. Log data help to identify lithologic horizons throughout the field, and in some cases, where a wellbore crossed the fault, to quantitatively determine the amount of smeared shale within the fault zone. Conceptual models provide means to interpret crucial details of the fault geometry and the distribution of shaly fault rock beyond the conventional resolution of a 3-D seismic data set. Combining the seismic data, well-log data, and conceptual model, we developed a procedure to determine the fault geometry and to assess the nature of the smeared shales and their evolving configurations as a function of fault throw, the thickness of corresponding shale units, and the thickness of the sand units between the shales. The product is a new and improved technique to visualize fault architecture and to interpret fault rock, both of which lead to constructing a structurally realistic juxtaposition diagram and a physically sound shale-smear analysis in a reservoir.