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 interpreters ability to recognize the efforts they have on seismic reflections. These characteristic changes in reflection pattern are called direct hydrocarbon indicators or "DHI's". Under idealized conditions, DHI analysis can yield valuable exploration information concerning the presence, thickness, lateral limits and type of hydrocarbons present in a 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 using, DHI analysis of seismic data to assist in locating and planning exploration and delineation wells in the Malay Basin. The degree of success 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.
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 represents a major advance in our ability to explore for oil and gas.
Interpreting the presence of hydrocarbons on seismic data first started when geophysicists noticed that some anomalously strong reflections or "bright spots" on the seismic sections were associated with gas accumulations in the substance. It was soon recognized that several other seismic characteristics could also indicate the presence of hydrocarbons and now the more general term direct presence of hydrocarbons and now the more general term direct hydrocarbon indicator or "DHI" has gained acceptance.
Recognizing these direct hydrocarbon indicators on the seismic sections is only the first step in present day DHI analysis. The ultimate objective is to accurately predict the geologic conditions that have caused these characteristic geophysical responses. Under idealized conditions it is sometimes possible to predict with seismic the presence, thickness, lateral limits and even type of hydrocarbon present in a reservoir. present in a reservoir. Unfortunately what appears to be a typical seismic response from a hydrocarbon accumulation is sometimes caused by something completely different and explorationists who have experienced unfortunately results have had reason to label DHIs as dry hole indicators. Separating these false DHIs from true DHIs and correctly predicting the actual geological conditions that cause the true DHIs predicting the actual geological conditions that cause the true DHIs are some of the more difficult tasks facing geophysicists today.
Over the last few years geophysicists from Esso Productor, Malaysia (EPMI) have been using DHI analysis to aid them in their exploration efforts in the Malay Basin. In some cases it has proven to be an extremely valuable exploration tool while in other cases the results have been rather discouraging. By looking back at how DHI analysis has been applied in EPMI's exploration effort one can obtain a better understanding of the uses and limitations of this exploration tool. The purpose of this paper then is to look at the practical applications and limitations of DHI analysis in the Malay practical applications and limitations of DHI analysis in the Malay Basin.
The Malay Basin is located in the South China Sea off the east coast of Peninsular Malaysia (Figure 1-1). It is approximately 250 miles long by 80 miles wide and contains in excess of 20,000 feet of tertiary sediments. These sediments are oligocene to mio-pliocene shallow water clastics that contain highly variable reservoir sands. These sands have been deposited in environments ranging from fluvial to lagoonal to nearshore. Reservoir sand thickness vary from less than 10 feet to a few hundreds of feet. These sands have been found to contain significant amounts of hydrocarbons in various different structural traps.
The presence of hydrocarbons in these sands can alter the physical properties of the sand, which in turn alters the seismic response properties of the sand, which in turn alters the seismic response resulting from the sand/shale interfaces. This effect is demonstrated in Figure 1-2. This model was constructed to show the effect that gas can have on the acoustic impedence properties of a reservoir sand. The structured sand represents typical conditions encountered at a depth of 4000 feet in the Malay Basin.
Consider first the case where water is present in the reservoir sand. Here shale velocities are in the range of 6500 ft/sec while a porous water sand might have an internal velocity of 7300 ft/sec. The velocity contrast across the shale/water and interface is 800 ft/sec. The percentage of the seismic energy that will be reflected from this interface is given by the "Reflection Coefficient" which is +4% at the shale/water sand interface. A relatively low amplitude positive pulse would be reflected from this surface. pulse would be reflected from this surface.