Cyclic Attributes on Seismic Data and Sequence Stratigraphy - New Criteria for Exploration, New Interpretation Styles

Barbara J. Radovich: Consultant, Houston, Texas

Abstract

The application of sequence stratigraphy on seismic data has long centered about the careful, and often time-consuming process of interpretation of seismic reflection geometries of onlap and downlap, and the tying of well data to seismic. But in many basins and especially in deepwater areas, well data may be rare or nonexistent. Tying shelf sequences to basin sequences is often impossible because of long distances, gaps in seismic data, and complex structures. The idea of cyclic sequence architectures offers new criteria for exploration, and new ways to interpret the seismic data on 2D and 3D datasets with modern visualization tools. The focus of interpretation shifts from finding seismic reflection geometries to finding cyclic vertical stacking patterns even if geometries are absent or subtle. This framework can give insight into the sediment delivery system of margins and to the aggradation of sediments in deep water in areas of sparse or no control. These criteria have been applied for almost a decade to the Offshore Nigeria exploration areas and key discoveries have been made using these techniques. Other areas of application include the Gulf of Mexico, the Northwest Shelf of Australia, the Gulf of Thailand and Bangladesh.

The key criteria that guide the interpreter in these settings is the repetitive cyclicity of seismic reflection attributes and seismic facies patterns. The most useful cyclicities consist of changes in seismic instantaneous amplitude and frequency. Cyclic seismic facies patterns often change from laterally continuous reflections to mounded or chaotic patterns. These seismic criteria correlate to the 3rd order sequence and the different depositional energy and styles that predominate as sea level falls then rises. The seismic data is resolving the aggradation of sediments with changing lithology assemblages, thick to thin-beddedness, and depositional styles such as sheet-forms or sinuous channel-forms. In deep water settings, these cyclicities are often a very prominent part of the data. Full analysis of the seismic data typically reveals the framework on three scales; the mega-architecture basin scale of 2nd order sea level rises and falls and tectonic events, the 3rd order 'building block' sequence scale of ~1-10 my, and the parasequence scale suitable for well prediction and reserve calculation. The repetitive nature of the cyclicities implies a time of balance for important parameters like sedimentation rate, subsidence, sea level, and the development of a matured, efficient sediment delivery system. These patterns also imply a high potential of recycled sediments stored in an intermediate position ready to be efficiently delivered to the basin at each lowstand of sea level. Thus, the more repetitive the cycles the better is the potential for good quality reservoir sands.