Cross-plotting of rock properties for fluid discrimination using well data in offshore Niger Delta

Abstract

Differentiation of hydrocarbon from non-hydrocarbon reservoirs using rock-properties and their attributes (combination of rock properties) cross-plots have been attempted using well data from an offshore field of the Niger Delta as a case study. Numerous cross-plotting techniques of rock- properties/attributes derived from literature were extracted and used to validate their potential and applicability as pore fluid discriminants. Gamma-ray, dipole shear wave, sonic and density well log data were used. The zones of interest (6642-9220ft) for sands, and shale/sand/shale sequences were obtained by combined analysis of attribute Vp/Vs ratio to depth cross-plot and gamma ray log. The result of the analysis of the composite plots for the sand bodies reveals distinct separation between the non-hydrocarbon sands and gas charged sands, with the oil portion intermediate between the water- wet sand and the gas charged portions. Between the oil charged portions and the water- wet sand lies the mixed sand cluster, possibly slight oil mixed with water. The shale/sand/shale sequences reveal the encasing shales have higher attribute values. The analysis reveals possibility of a Class 3-AVO response between the overlying shales and the encased hydrocarbon charged sand reservoirs. The Lambda-Mu-Rho attributes and their combinations cross-plots are more sensitive and robust for fluid discrimination.

Keywords: amplitude-versus-offset (AVO), Vp-Vs ratio, rock properties, fluid discriminant, Lambda-Mu-Rho

Nigerian Journal of Physics Vol. 17, 2005: 16-20