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Seismic Diagenesis, A New Perspective for Seismic Interpretation

Patrice Imbert
Total

Abstract

Diagenesis is defined as "The process of chemical and physical change in deposited sediment during its conversion to rock". The term is commonly restricted to chemical changes and conveys an idea of "processes to be observed through a microscope or by chemi-cal analysis". However, such physical changes as:

  • Mud liquefaction in subsurface and its expulsion as mud volcanoes to the seafloor;
  • Pockmark formation;
  • Subsurface sand liquefaction and subsequent intrusion (emplacement of "injectites");
  • Development of layer-bound contraction faults ("polygonal fault systems") in the shallow subseafloor;
  • Co-freezing of migrating gas and pore water into hydrates in the sub seafloor, as well as hydrate dissolution and resulting collapse/remobilization of the host sediment;
  • Basin-wide transformation of amorphous opal (e.g. in diatomite) to crystallized forms of silica;
  • Development of carbonate crusts at the sulfate-methane interface a few meters below seabed;
  • All correspond to the definition and are observable on seismic profiles. The first four processes are usually referred to as "sediment remobilization" and the last two are commonly recognized as diagenetic, while methane hydrates are currently more studied as a poten-tial resource than as a special agent of post-depositional sediment modification ("diagenesis" in the wide sense retained here).

    The integrated study of these phenomena on seismic data is what we propose to identify as "seismic diagenesis" (or perhaps better "seismic-scale diagenesis"?). It has a profound unity, in that it addresses the large-scale migration of geofluids (water, liquid and gaseous hydrocarbons, other gases) from their source at depth up to the seafloor or land surface. Migration is influenced by the distribution of permeable beds in the basin, but it can in turn affect the geometry of carrier beds through remobilization.

    A key point is that many of the phenomena mentioned in the list above can be dated from geometric relationships, obeying or not the principle of superposition. Their study in an area thus provides an insight not only into the present-day situation of fluid circulation (e.g. presence of a hydrate-related BSR), but also in many cases into its evolution though geological time.

    These are key indicators of the present and past occurrence of fluid flow in sedimentary basins. They are in particular of primary importance to a good understanding of petroleum systems, provided we learn how to safely differentiate hydrocarbon-related effects from those resulting from water circulation, and also provided quantitative aspects are dealt with. In that respect, and in order to come to maturity, "seismic diagenesis" would now deserve recognition as a specific discipline of Earth Sciences, like seismic geomorphology (Posamentier, 2004) or seismic stratigraphy (Mitchum et al., 1977). Although literature on specific features or processes has now become abundant, the discipline still lacks a comprehensive approach based on the study of the physical and chemical processes that accompany fluid expulsion and sediment remobilization in the shallow sediment section.

    R. M. Mitchum Jr., P. R. Vail , J. B. Sangree (1977). Part 6. Strati-graphic Interpretation of Seismic Reflection Patterns in Depositional Sequences: Section 2. Application of Seismic Reflection Configuration to Stratigraphic Interpretation. In AAPG Memoir 26, Seismic Stratigraphy and Global Changes of Sea Level.

    Posamentier HW (2004) Seismic geomorphology: imaging elements of depositional systems from shelf to deep basin using 3D seismic data, In: Davies RJ, Cartwright JA, Stewart SA, Lappin M, Underhill JR (eds.) 3D Seismic Technology: Application to the Exploration of Sedi-mentary Basins. Geol Soc Spec Publ 29:11–24.

    AAPG Search and Discovery Article #90200 © AAPG Geoscience Technology Workshop, Fifth Annual AAPG-SPE Deepwater Reservoir, January 28-29, 2014, Houston, Texas