Formation Evaluation Of Fresh Water Shaly Sands Of The Malay Basin, Offshore Malaysia

Heavysege, R. Grant (ExxonMobil Exploration and Production Malaysia Inc.)


Formation water chemistry is a critical variable in the Malay Basin formation evaluation, and a major cause of the low contrast between pay zones and wet sands. A recent understanding of the distribution of brine composition has led to a systematic use of different saturation calculation methods in different parts of the basin. In the easternmost Malay Basin, where salinities approach that of seawater (30,000 ppm TDS), conventional shaly sand equations adequately reduce uncertainty in hydrocarbon saturation estimates. However, some of the largest fields are located in a fairway of fresh water (<3,000 ppm) in the middle of the basin. At these low salinities, and in the presence of clay minerals, Ro is insensitive to Rw and the excess conductivity term will predominate in all commonly used shaly sand equations. Capillary pressure methods have been used increasingly in this interpretation environment. In some fields lacking ample representative, high quality capillary pressure data, saturation-height functions have been synthesized as a means of arriving at more robust fluid saturation values. Petrographic data (mineralogy, shale distribution, and grain-size analysis) can help determine minimum amounts of both clay and capillary bound water at irreducible conditions. Clay bound water can be determined from mineralogy, and from routine core/log measurements in shale. Capillary bound water can be estimated from the empirical bulk volume of"effective" water (BVWE) based on grain size data, or analog clean sand reservoirs. Where available, NMR data can be used to calibrate the total combined value of bound water (BVI). From an operational perspective, low resistivity contrast pay poses a serious limitation on real- time fluid-type prediction, particularly in high angle, oil based mud wells where logging suites are limited. Time lapse LWD logging has been used to identify fluid type and reduce completion uncertainty. Increases in resistivity between log passes have provided excellent qualitative evidence of the presence of a movable water phase.