Over the last 15 years or so, improvements in reflection seismic acquisition, processing and interpretation methods have enabled the geophysicist to interpret the presence of hydrocarbons directly from seismic data. This is especially true in Tertiary clastic basins.

Success in predicting the presence of hydrocarbons on seismic data depends on the interpreter 's ability to recognize the effects they have on seismic reflections. These characteristic changes in reflection pattern are called direct hydrocarbon indicators or "DHFs" Under idealized conditions, DHI analysis can yield valuable exploration information concerning the presence, thickness, lateral limits and type of hydrocarbons present in a structure. Information concerning the presence, extent and seriousness of drilling hazards related to shallow gas can also sometimes be obtained.

EPMI geophysicists have been usig DHI analysis of seismic data to assist in locating and planning exploration and delineation wells in the Malay Basin. The degree of success of DHI predictions has varied markedly. The key factors that effect success have been data quality including near surface interference problems, bed thickness versus resolution restrictions, facies conditions including the presence or absence of coals, and low gas saturation. To date, a fairly high degree of success has been obtained in predicting the presence of hydrocarbons in non-coaly environments up to depths of about 7000'. Success in mapping the lateral limits of these recognized hydrocarbon zones usually depends on bed thickness and may be complicated by the potential presence of low gas saturation below the hydrocarbon/water contacts, In thick reservoir units it is sometimes possible to locate and map gas/ oil and oil/ water contacts thus providing the explorationist with a tool for recognizing and mapping the extent of different fluid systems in multi-reservoir fields.

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