Comparative Studies of Modern, Quaternary and Ancient Seaways: Building Depositional Models for Hydrocarbon Exploration and Production - A Review

Stuart Blackwood
Department of Geological Sciences, The University of Texas at Austin
Austin, Texas, USA
s.blackwood@mail.utexas.edu

Ancient seaways, narrow/elongate passageways connecting open marine areas, are sometimes mistaken for estuaries, inlets or open shelves. There is a severe lack of data to build generic depositional and sequence stratigraphic models for seaways. One of the reasons for this is that Seaways are often short-lived within relative sea-level cycles, particularly during ‘icehouse’ times, less so during greenhouse times. During transgression, estuaries often evolve into embayments, then seaways, and finally open shelves. This is the case for the evolution of the Mesozoic/Quaternary rift basins in the North Sea. During sea level falls in narrow basins, some open-marine shelves evolve to seaways (both ends open). In some instances, further contraction of the seaway leads to embayment(s) with one end open. The Holocene/Quaternary analog for this is in SE Asia (e.g. Malacca Strait).

Distinguishing between Seaway and other shallow-marine deposits is crucial in reconstructing paleoshoreline trends, for example through the use of clinoform geometry from seismic data, and therefore in hydrocarbon exploration and production, for the following reasons:

1. The margins are often strongly controlled by tectonic elements
2. The extent and possibly thickness of the sand ridges within the seaway is much greater than in its estuarine or open shelf counter parts.
3. Offshore sand ridges are rather homogeneous because they form in relatively clean water, whereas deltaic/estuarine complexes along the seaway margins tend to be heterolithic.
4. The deltas debouching into the seaway are often severely deflected due to strong tidal currents, and are often prograding sub-parallel to the shoreline.