Resistivity Independent Hydrocarbon Saturation Computation across a Lithologically Complex Jurassic Carbonate reservoir, Offshore Abu Dhabi

Abstract

The Jurassic carbonate Reservoir-X, offshore Abu Dhabi started to produce hydrocarbons in the early sixties of the last century. The reservoir is generally formed of regressive sedimentary cycles and can be divided into four highly heterogeneous carbonate units with mixed compositions of limestone, dolomite separated by dense anhydrite layers. Down-flank water injection using relatively low-saline sea water had started in the early 1970s. As of the present day, this ongoing process has highly altered the original formation water super-salinity, in a way that the Archie water saturation computation for Reservoir-X is very challenging and not a straight forward practice anymore. The objective of the presented approach is to introduce a resistivity independent saturation across Reservoir-X, using the novel Pulse Neutron Log (PNL) Spectroscopy measurements.In order to meet the subject challenges; a comprehensive logging suite consisting of gamma ray, resistivity, density, neutron and spectroscopy is used. The particular interpretation methodology is chosen such that it becomes possible to solve for the hydrocarbon saturation using elastic and inelastic capture spectroscopy. A further observed added challenge is the presence of light hydrocarbon and gas in the porous Reservoir-X layers with the unknown mixed-saline formation water in addition to mud filtrate. Here we present a method for hydrocarbon typing across Reservoir-X.The precise and accurate acquisition of spectral elemental yields allows for a detailed lithological evaluation, hence allowing for an accurate porosity determination, also taking into account wellbore fluid properties that affect the logging sensors due to the presence of formation water and mud filtrate in the formation. This helps to obtain a resistivity independent hydrocarbon saturation that is independent of Archie parameters m, n and water formation salinity. The solution has also been verified by a subsequent, downhole wireline formation tester fluid identification analysis.