Abstract: Stratal Slicing and Seismic Facies Imaging

ZENG, HONGLIU and NOEL TYLER, Bureau of Economic Geology, The University of Texas at Austin

Summary

Seismic attributes must be picked on a depositional surface (geologic time surface)to represent a genetic depositional unit. We call such a seismic surface display a stratal slice (Zeng, 1994; Zeng et al., 1996). Time slices and horizon slices (Brown, 1992) are the most commonly used seismic surface displays aimed at extracting stratigraphic information. The value of these approaches has been discussed in numerous publications on 3-D interpretation in recent years. For the purpose of depositional facies analysis, however, both methods have limitations. A time slice is not a stratal slice unless the sliced formation is fiat lying and sheetlike. A horizon slice can be picked at or against a dipping surface, but (1) the seismic surface is merely an impedance surface that may fluctuate around true time surface; (2) the dipping formation must be sheetlike; and (3) if traceable horizons are rare, it is very difficult to pick enough slices to replicate the basin's depositional history.

Some realistic 3-D seismic models built in lower Miocene reservoirs of Powderhorn field, Calhoun County, Texas, demonstrate that a seismic event does not necessarily follow an impedance boundary defined by a geological time surface. Instead, the position of a filtered impedance boundary relative to the geological time surface may vary with seismic frequency due to inadequate resolution of seismic data and to the en echelon or ramp arrangement of impedance anomalies of sandstone. Except for some relatively time-parallel seismic events (sequence boundaries and maximum flooding surfaces), the correlation error of event picking is large enough to distort or even miss the majority of the target interval on stratal slices. In some cases, reflections from sandstone bodies in different depositional units interfere to form a single event.

Real 3-D seismic data from the Gulf of Mexico Tertiary also show a close dependence of seismic events on data frequency. While some events remain frequency independent, many events exhibit different occurrences with changing frequency, and therefore are not parallel to geologic time surfaces. In the data set we have studied, observed maximum time transgression of seismic events is at least 120 ms traveltime in lower frequency data. Severe interference in lower frequency data may produce false seismic facies characteristics and obscure the true stratigraphic relationships. This phenomenon has important implications for seismic interpretation, particularly for high-frequency sequence stratigraphic and reservoir characterization studies.

This time transgression problem is mitigated to a large degree by the stratal slicing technique. Experiments on seismic models suggest that instead of event picking, a phantom mapping [or using the term of Posamentier et al. (1996), proportional slicing] between reference events is a better technique for picking stratal slices. Stratal slicing on a workstation is done by first tracking frequency-independent, geologic time equivalent reference seismic events, then building a stratal time model and an amplitude stratal slice volume, based on linear interpolation functions between references. The new volumes have an x, y coordinate system the same as the original data, but a z-axis of relative geologic time.

Stratal slicing is a useful new tool for basin analysis and reservoir delineation by making depositional facies mapping an easier task, especially in wedged depositional sequences. Since 1994, the technique has been applied to more than 50 3-D seismic volumes acquired from the Gulf of Mexico basin, offshore West Africa, Venezuela, North Sea, Offshore Thailand, and China. The utility of the methodology is best illustrated by many high-resolution depositional facies images like fluvial channels, deltaic systems, barrier bar-lagoon systems, and submarine turbidite deposits that are very difficult, if not impossible, to make by using other approaches. For examples of the colored facies images on stratal slices, please refer to Zeng et al. (1995), Zeng (1996), and Zeng et al. (in press).

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah