Seismic-scale Expression of Fluid Sourcing, Circulation and Expulsion in Sedimentary Series
Authors: Imbert Patrice 
Published: May 2016

Subsurface sediment remobilization has been recognized for at least a decade as a significant phenomenon in a number of sedimentary basins. Many types of anomalies have since that time been described and interpreted as evidence of the interaction between buried sediments and compaction fluids expelled through them, generally upwards. This paper proposes a way to classify these features according to the role they play with respect to compaction fluids: some of them correspond to fluids being supplied to the sedimentary column (e.g. "polygonal faults" or diagenetic horizons), while others indicate deformation by ascending fluids (like sand injectites), and others yet the expulsion of the fluids at seafloor level (e.g. mud volcanoes or pockmarks). Another key for the understanding of these systems is the role played by fluid buffers that can temporarily store fluids before releasing them. This fourfold classification into fluid providers, fluid buffers, sediment remobilization sensu stricto and fluid expulsion features is considered a key to the understanding of the implications of these anomalies. The way forward now consists in linking the anomalies along this "production / storage / deformation / expulsion" chain, much in the same way as petroleum system analysis links a source rock, a migration pathway and a reservoir / trap combination, or as clastic sedimentology views a sand reservoir as resulting from the combination of a source of sand, a topographic transit pathway and a sediment trap.

Two examples of application of this approach are developed. The first shows how the geometric relationships observed between two types of anomalies can improve the understanding of the corresponding processes, while the second attempts to integrate various observations of post-depositional features into an understanding of the plumbing system of a sedimentary basin.

Introduction

The geological interpretation of seismic data has been the primary exploration tool in the oil industry for over 50 years, and has progressively become a key for appraisal and now monitoring of production through 4-D techniques. With the increase of seismic quality over the years, this interpretation evolved from mostly structural in the beginning to stratigraphic (seismic stratigraphy, [1] on two-dimensional datasets. The development of three-dimensional seismic in the late 1980's shifted geological applications towards sedimentology, as 3D surveys granted access to the morphology of reservoirs and other sedimentary bodies in map view. Seismic geomorphology was recognized in the early 2000's as a full discipline of petroleum geology [2], and our understanding of deep-sea deposits has benefited much from seismic data interpretation.

In addition, 3-D seismic data have revealed another set of geological features, that can neither be related to sedimentation nor to tectonic deformation. These, often referred to as "subsurface sediment mobilization" or "post-burial effects", correspond to all early, seismic-scale remobilization of sediments in subsurface under the influence of circulating fluids (typically for burials ranging from a few meters to 1 km) [3]. Interestingly, many of these geological features were actually discovered by 3-D seismic interpretation: they occur in subsurface by definition, and can therefore not be observed on land or at the seafloor, like sedimentary bodies geologists can walk on while they are deposited (e.g. fluvial sands, coral reefs) or can monitor through echo-sounding techniques (e.g. turbidite systems). They also develop on very large (seismic) scales, making them difficult to recognize at the outcrop. One of the next challenges in this domain will be to identify such remobilization features at the outcrop in order to describe them at small, sub-seismic scale and improve their understanding.

Seismic-scale post-burial effects are quite varied and range from mud volcanism to sand injection, from basin-wide diagenetic transformations to layer-bound polygonal faulting. It is proposed to classify them according to their position along a fluid migration pathway, from primary fluid expulsion from its host sediment to final release at the seafloor / surface.