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Onshore 3D Exploration of the Upper Frio Yields Subtle Trap Techniques By: Steven Anderson

Abstract Summary

Lateral velocity changes and peak-trough-peak amplitude anomalies are two subtle indicators in onshore 3D seismic that may be used to lower risk in pursuit of the subtle trap. Interpretation of Seitel 3D seismic at the Marg tex sand interval over Maurice and Leleux Fields reveals a strong lateral velocity change oriented east-west with approximately a 300 ft/sec gradient. Similarly West Chalkley Field has a 300 ft/sec velocity gradient at the Miogyp interval as detailed in Gil Klefstad’s article Exploration Basics Led to Chalkley Gas Find published in the Sept. 30, 1991 Oil and Gas Journal. Lateral velocity changes make integration of subsurface well control with 3D seismic data more difficult. Overcoming the problems associated with lateral velocity gradients should pay off in the hunt for the subtle onshore trap. In addition, it is apparent from the three fields examined that the lateral velocity gradients are associated with fields that have above average gas reserves. Could the lateral velocity gradient be a hydrocarbon indicator? Recent exploration in the Camerina sand of Southwest Louisiana onshore trend shows a diagnostic peak-trough-peak amplitude anomaly associated with gas pay. This peak-trough-peak event coincides with an approx. 15 BCF discovery in Vermilion Parish by Amerada. The Camerina sand velocity is faster than the surrounding shale in the 12-14,000 ft depth range. A synthetic indicates the wet Camerina sand top to be a peak on the Seitel data. Fluid substitution of gas for water in the Camerina sand yields a slight dimming of the peak associated with the top of sand. The amplitude anomaly associated with gas pay is a strong trough below the peak. An amplitude extraction in the trough yields a 3 to 1 ratio of anomaly to background. Several examples illustrate this wet and gas-filled response to Upper Frio sands in the 12-14,000 ft range. It is incumbent upon the seismic interpreter to review recent onshore discoveries and apply the rich 3D data stream now available in order to find similar prospects. Both lateral velocity gradients associated with Miogyp and Marg tex sands and peak-trough-peak amplitude anomalies associated with the Camerina sand may be precursors of yet undiscovered onshore traps. .

Biographical Sketch
Steven Anderson graduated from Colorado School of Mines in 1975 with a degree in Geophysical Engineering. He began work with Union Oil of California in Lafayette as a seismic interpreter reviewing offshore data for Gulf of Mexico lease sales. He left Union Oil for Ada Exco in 1981 and joined Quintana Petroleum in 1982 as district geophysicist. He left Quintana in 1992 to consult for various clients including Flores & Rucks, American Explorer and DOR Engineering. He joined Flores & Rucks in 1994 as Onshore Exploration Manager. He is currently a prospect generator working the prolific onshore south Louisiana . Steve is a member of SEG, SWLGS, LGS and a registered geophysicist in Texas.