Estimation of Oil-Rim Thickness with Analogue Data in the Absence of Definite Fluid Contacts: A Case study from Onshore Niger Delta

Abstract

One of the major challenges in field development is to estimate the possible oil-rim thickness, when only known contact is Gas-Down-To (GDT) or Oil-Down-To (ODT), and where seismic analysis cannot resolve the uncertainty. A reliable, repeatable, and practical technique to estimate the ranges of possible oil-rim thickness with the regional analogue would allow for making development decisions, as well as meeting the local regulatory requirements. An analog database with numerous proven hydrocarbon (gas and oil) column heights provides a trend for "gas-to-oil column ratio" by depth, which can be used for predicting possible oil-rim thickness at specific reservoirs with the contact uncertainty. For all the analog reservoirs, fluid thicknesses have been tabulated and a "gas-to-oil column ratio" has been calculated. A simple plot between the "gas- to-oil column ratio" and reservoir depth reveals a certain trend of increasing "gas- to-oil column ratio" by depth. In addition, database also provides a cumulative distribution of regional column heights, which could be used to constrain the maximum estimated hydrocarbon column thicknesses. The trend of increasing "gas-to-oil column ratio" by depth indicates that gas-columns gets much bigger compare to oil-columns as the reservoir depth increasing. With given depth information and estimated "gas-to-oil column ratio" from the database, it is possible to estimate possible range of oil-rim thickness. By investigating the scatter in the trend, it is also possible to estimate a range of "gas- to-oil column ratios". With the help of this methodology, in absence of any definite fluid contact, it is possible to estimate a range of oil-rim thicknesses, which has an impact on field development scenarios; i.e. "gas-only", "oil-then-gas", or "concurrent oil-and-gas" development options. In addition, it could also help field development plan to meet the regulatory requirements; i.e. providing a strong case not to develop or appraise the oil rim.