Abstract

Facies models for deep-water resedimented conglomerates have not changed a great deal since the 1970s and 1980s, but modern reservoir modeling for hydrocarbon fields requires a modified approach to facies classification. The Brae trend of the South Viking Graben, North Sea, comprises a whole suite of turbidite architectural styles, built from a wide range of bed types, which are interpreted to extend over the entire range of deep-water sedimentary process and product. Many questions can be answered about facies and facies models using what we know about the Brae fields, outcrop analogs, and modern sea-floor studies. What types of bodies are constructed on the slope, intraslope, and basin floor, where conglomeratic lithofacies are common? What are the basic rock fabrics and how should these be interpreted and upscaled into meaningful reservoir flow units? What can be correlated and at what scale? Are there basin margin-wide events that mark phases of particular sedimentary events, and what roles do sea-level change and tectonics have on these pulses of sedimentation?

This chapter has one central aim, to propose a comprehensive, practical rock fabric (here called “lithofabric”) model for the entire range of deep-water clastic rock fabrics. This work draws on several kilometers (several thousand feet) of detailed core descriptions from Thelma and Tiffany (CNR International operated fields), and the Inverewe prospect in the North Sea, in conjunction with outcrop and sea-floor examples from Turkey, California, Wales, and circum-Mediterranean Sea areas. Particular focus is given to conglomerates and pebbly sandstones, to capture lithofabric variations. The scheme focuses on fabric instead of on the interpretation of every event that led to every bed or bedset. This approach is particularly critical in amalgamated bedsets where the interpretation of bed boundaries is often subjective. The approach allows workers to focus on and capture vertical changes in rock fabric in core or in the field, without the need to define every bed boundary or distinguish bed boundaries from “intraevent” fabric changes. The aim is to move away from the “event bed” approach to describing deep-water facies, which involves interpretation during the description stage, particularly for coarser lithologies and structureless sandstones.

This chapter starts with a review of current facies models and process-interpretation schemes for rock fabric, outlining their strengths and weaknesses. The new approach for describing lithofabric is then presented. Models are offered for the variety of lithofabrics produced by higher energy, coarser grained turbidity currents and debris flows. The various parent flows that can produce structureless sandstones are shown diagrammatically, to illustrate how similar fabrics can be produced by different processes. A series of models are shown, comparing existing proximal to distal facies-distribution models with the new scheme, highlighting differences and consolidating commonalities.