Weimer, Paul
University of Colorado, Boulder, Colorado

Abstract: Sequence Stratigraphy of Intraslope Turbidite Systems: Models for Exploration and Development

Increased exploration and development activities on the continental slopes are focusing primarily on turbidite systems deposited in intraslope basins. These basins form associated with salt deformation and/or faulting. Understanding the stratigraphy of these systems has required the integration of many different disciplines: 2-D and 3-D multifold seismic data, outcrop analogs, shallow late Pleistocene analogs, cores, wireline logs, biostratigraphy, production history, and reservoir simulation.

In the northern Gulf of Mexico, the Neogene sediments have complex, yet distinct, occurrences. The external shape of the sedimentary fill reflects the changing shape and syndepositional deformation of the basin. Internal stratal stacking patterns within the depositional sequences are primarily controlled by sediment delivery systems, grain size of the gravity flows, eustasy, and basin gradients. Key bounding surfaces that define the sequences include condensed sections and sequence boundaries. Condensed sections can be clay-rich or foraminiferal-rich shales, and can have high biostratigraphic abundance and diversity values. Condensed sections are laterally continuous reflections in seismic, have a low gamma ray response, and can be important pressure barriers within reservoirs. Sequence boundaries are generally nonerosional and directly overlie the condensed sections; but sequence boundaries are erosional in the upper slope, or where slides may erode into condensed sections. Within the depositional sequences, stratal stacking patterns vary.

At the base of some sequences are sheet and amalgamated sheet sands (basin-floor fans). These are characterized by 50–150 ft thick sand deposits, generally structureless sands in cores, and high-amplitude laterally continuous reflections that onlap or downlap onto the sequence boundary. Areally, these deposits can cover a portion of or up to an entire minibasin. These sand deposits have the highest sustainable production rates, which generally coincide with good aquifer pressure support.

Overlying the sheet sands or condensed sections are mud-dominated channel-levee system and corresponding overband sediments. These constitute the bulk of the sediments in these minbasins. Channels are best identified on 3-D seismic data, where they reveal a sinuous to straight pattern. Channel-fill sands are 10–80 ft thick, with blocky to upward fining/thinning patterns on gamma ray logs. Channel-fill deposits can have fairly high rates of production, especially where they are amalgamated. However, the internal fill to channel deposits can be highly variable, and complex fluid distribution can occur in these deposits. Levee facies are characterized by laterally continuous reflections of similar amplitude. Cores indicate interbedded clay and thin (1–3 cm) planar to ripple laminated sands. These sands can have good lateral continuity, as indicated by the good production rates, which generally drop after a few years. Additional nonproductive facies that are significant to basin fill are slides, hemipelagic shales, and nonchannelized mud-rich flows.

Other examples of petroleum productive intraslope turbidite systems include the Cenozoic margins of west Africa, and Upper Cretaceous Atlantic margins of Brazil. These turbidite systems vary from the deep Gulf of Mexico, primarily in the grain size of the system and internal stratal geometries.

AAPG Search and Discovery Article #90929©1998-1999 AAPG Distinguished Lecturers