GEOSTATISTICAL PREDICTION OF RESERVOIR PROPERTIES AND RESERVES ESTIMATION OF ‘OLAN’ FIELD, NIGER DELTA, NIGERIA

Abstract

This project described the integration of petrophysical analysis, seismic interpretation, and geostatistical method with the view to delineate reservoir sands, predict well-log properties from seismic data and estimate hydrocarbon reserves of Olan field. The analysis data consisted of 3-D seismic volume and a series of target logs from wells which tie the 3-D seismic volume. The available well logs comprised of gamma ray(GR), spontaneous potential (SP), resistivity (ILD), neutron porosity (NPHI), sonic (DT) and bulk density (RHOB) logs. These logs were used in the petrophysical analysis to determine the following petrophysical parameters; volume of shale (VSH), porosity (Ø), formation factor (F), water resistivity (RW), water saturation (SW), hydrocarbon saturation (SH), irreducible water saturation (SWIRR), bulk volume water (BVW) and permeability (K). From the seismic interpretation, twelve growth faults; A, B, C, D, E, F, G, H, I, J, K and L were identified. Eight horizons (Hor 1A, Hor 1B, Hor 2A, Hor 2B, Hor 3A, Hor 3B, Hor 4A and Hor 4B) were mapped. All the mapped horizons show similar anticlinal structures as interpreted from the time and depth structure maps. The results of this study show that the stratigraphic intervals are structurally controlled with anticlinal structures especially at the central to the north eastern part of the study area making them efficient trapping system for hydrocarbon accumulation. More well logs were predicted from the new infill wells after geostatistical relationship has been established between the existing well logs and the seismic data. The predicted logs were gamma ray (GR) log, neutron porosity (NPHI) log and bulk density (RHOB) log. The predicted logs from the new infill wells along with the logs from initially existing wells were used to derive petrophysical parameters in order to increase the accuracy of the derived petrophysical parameters used for the volumetric calculation. The result of the petrophysical analysis shows the following ranges for these petrophysical parameters; volume of shale (4.53-42.05%), porosity (14.27-24.76%), water saturation (17.36- 69.62%), hydrocarbon saturation (30.38-82.64%), irreducible water saturation (8.18-12.18%), bulk volume water (0.03-0.13) and permeability (14.80-1409.21md). Pseudo gamma log cube, pseudo neutron porosity log cube and pseudo bulk density log cube were generated from the application of derived linear multiattribute transform to the whole 3D seismic data in order to characterize the study area. The estimated reserves for delineated reservoirs A, B, C and D are; 1,653,912,816 standard cubic feet of gas, 62,709,592 stock tank barrels of oil, 116,497,057 stock tank barrels of oil and 2,912,319,046 standard cubic feet of gas respectively. This project has successfully demonstrated the integration of petrophysical analysis, seismic interpretation and geostatistical method to delineate reservoir sands, predict well-log properties from seismic data and estimate hydrocarbon reserves of Olan field.