ABSTRACT

Consolidated reservoir formations are defined as those having consistently higher acoustic impedance (density and velocity product) than similarly aged, normally pressured shales. For such formations, porosity and hydrocarbon effects can be effectively imaged via seismic velocity changes over a substantial range of depths (to 16,000+ feet/4877+ m). Additionally, the velocity variations that could be noted had quantitative significance in terms of reservoir quality and production potential.

The goal of this exposition is to illustrate the technology, which has been verified in a number of diverse examples. We shall note consolidated sand and carbonate reservoirs from South Texas, northern Oklahoma and the North Sea. Fractured reservoir plays of current interest encompassing such formations would include the Austin Chalk (Cret.), Mississippian Osage Lime, and Niobrara as well as others.

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The effectiveness of spatially dense seismic velocity measurements and colour inversion sections scaled in velocity is clearly demonstrated by the variety of successful examples reviewed. Velocity drops observed for gas presence (greater than 1,000 ft./sec) approach those typically seen for unconsolidated reservoirs. Smaller, but significant velocity drops are noted for oil-filled reservoirs (400 to 800 ft./sec) as in the case of the Austin Chalk. Such effects can not be viewed on conventional black and white displays of normally processed seismic data. "Coarse" processing loses the detailed information, and visual dynamic range limitations are severe.

Colour inversion displays from properly processed seismic data can image these target zones quite clearly. Additional analysis such as "DIVA" can also indicate the orientation of fracture zones using conventionally processed seismic data.

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