Seismic stratigraphy has been used in many areas to identify stratigraphic carbonate traps such as shelf margins, pinnacle reefs, mounds, and updip porosity pinch-outs. Many large oil fields are the result of oil entrapment in these types of stratigraphic traps. Examples include fields from the Gulf Coast, Permian basin, Mid-Continent, and Rocky Mountain provinces in the United States, as well as fields in the Middle East, Canada, North Sea, and the Far East.

Where the trap shows geomorphological relief, evidence such as draping, pull-ups, dim spots, data dispersion, and other criteria can be used. Stratigraphic carbonate traps can be localized with some precision using these criteria. However, the specific definition and measurement of porosity in carbonate rocks are much more difficult. With very careful integration of geologic (rock data from cores and petrophysical data from logs) and geophysical (high resolution seismic) data, it is possible to estimate both the thickness and, in a qualitative way, the amount of porosity in a potential carbonate reservoir.

During the Carboniferous in the Mid-Continent, a sequence of depositional and diagenetic events created irregular pods of porosity in otherwise tight limestones. The areal extent, thickness, and quality of this porosity are the primary factors that determine the location, geometry, and productivity of major oil fields in the area. A twenty-million-barrel oil field has been studied in detail, and the initial production rates and overall production richness correspond closely to measurable seismic phenomena. A seismic line shot through the producing interval at a depth of 4,000 ft, using 30-fold, broad band (20-120 hertz) data has allowed the recovery of frequencies over 100 hertz. These data confirm (1) the presence of porosity and (2) field limits that correspond to the field limits k own from subsurface information.

The Geoquest System work station was used to model (1) the key porous interval as known from core and petrophysical data in the analog field and (2) evaluate similar phenomena in the surrounding play area and measure both porosity thickness and quality in prospective stratigraphic traps. Two specific trap types occur regionally. The first type evidences porosity that developed locally and has an acoustically recognizable event over it (and between it and an overlying shale). The second type shows evidence of local porosity extending vertically to the shale seal. In both types, the lateral limits can be mapped seismically. The trap types have very different characteristics; both trap types and variations of them can be modeled successfully.

The trap types discussed are very subtle and have historically been

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discovered only by operators drilling for structure or by random drilling. Many areas within the onshore United States are underexplored for these subtle type traps. With the availability of very high frequency/high-resolution data, these type plays can now be made.

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