Print this pageNMR Data Integration for Improved Reservoir Fluid Contact Prediction: Case Study (Murzuq Basin - Libya)
The Murzuq Basin is a large intracratonic basin, primarily affected by the Caledonian and Hercynian orogenies, filled with continental Mesozoic and Cenozoic sediments lying unconformably on top of shallow marine to fluvio-marine Palaeozoic rocks. The main petroleum system in this basin is Pre-Hercynian traps of Hawaz and Mamuniyat sandstone reservoirs, sealed by Lower Silurian Tanezzuft marine shales and sourced by Rhuddanian age Hot Shale. The final open hole wireline logging program includes several runs. Nuclear magnetic resonance tool (NMR) is regularly included in the wireline logging programs in both exploratory and appraisal wells where some oil potential has been detected.
In the four presented wells (two exploratory and two appraisal wells) the reservoir section (Upper Ordovician Mamuniyat sandstone) was drilled using a water-base mud. Sandstone facies is very homogeneous over the studied reservoirs. Free water level depths were inferred with confidence from wireline formation tester (WFT). Unfortunately no WFT oil-water contacts were interpreted due to insufficient transition zone thickness. However, NMR helped identify the transition zone.
The NMR data is first combined with the other wireline logging data, drilling and gas data to ensure data consistency. NMR irreducible water saturation (Swir NMR) is evaluated as the ratio of the NMR bound fluid volume to the NMR total fluid volume. The NMR transverse relaxation time T2 cutoff selection is validated comparing the Swir NMR with the water saturation evaluated from the conventional resistivity, neutron and porosity logs (Sw Elan). The NMR fluid contact is defined as the deepest depth at which Swir NMR is equal to Sw Elan. NMR fluid contact is compared to the fluid contact from wireline formation tester. Water free hydrocarbons and mixed oil/water zones are clearly identified giving new insights to the reservoir. The fluid contacts prediction is used to avoid testing in the mixed oil-water zone. Drilling of an appraisal well is committed to only after an exploratory well reaches an oil production discovery rate.
NMR has demonstrated added value in an intensive drilling program in Murzuq Basin (Libya). NMR data, integrated with other acquired data in the quality assessment procedure, is currently being used as a formation evaluation tool for improved reservoir fluid contacts prediction. Identification of oil-water transition zones has helped in timely and effective well testing decision.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil