Abstract
When production data are not available, gas/oil differentiation is usually performed based upon the FDC/CAL (Formation Density/Neutron Logs) response and sidewall sample (sws) information. The response of the FDC/CNL tools, however, has been regarded as the more reliable indicator of the two methods.
The formations in the Niger Delta consist of sands and shales with the former ranging from fluvial (channel) to fluvio-marine (Barrier Bar), while the later are generally fluviomarine or lagoonal. These Formations are mostly unconsolidated and it is often not feasible to take core samples or make drill stem tests. Formation evaluation is consequently based mostly on logs, with the help of sidewall samples and wireline formation tests.
This case history involves a study of an onshore field located on the West Niger Delta whereby most of the wells drilled have sws petrophysical data and no FDC/CNL and the objettive was to evaluate the adequacy of the available data, and determine a measure of confidence that can be expected when fluid differentiation (gas/oil/water) is done strictly from SWS measurements.
From the comparison between the FDC/CNL separation and SWS petrophysical data, it is possible for fluid differentiation to be done strictly from SWS measurement with a good measure of reliability, in the absence of production data and FDC/CNL logs. It is also noticed that SWS data better differentiated this marginal gas and oil sands of the order of 5-6 ft pay resulting in a potential for reserves/production addition. The FDC/CNL log response are better defined in thick clean sands as against thin marginal sands in this case study.
