THE EXPLORATIONIST AND SHELF SAND MODELS: WHERE DO WE GO FROM HERE?
Authors: Barwis John H. 
Published: October 2019

What additional insights to shelf sedimentation would significantly improve our ability to make new oil and gas discoveries, particularly in frontier basins, or in relatively unexplored stratigraphic intervals of otherwise mature basins?

Lithofacies models are important for characterizing reservoirs and are most useful on the scale of a prospect or single sandbody. Depositional models are more appropriate for larger scale work, because knowledge of environmental setting places a sandbody in its regional context. Neither of these model types, however, adequately address the explorationist’s main concern which is to predict, in areas of limited or no well control, whether any sandstone will be present in the wellbore at the target interval. The economic significance of this prediction before drilling highlights the importance of models that provide more than just recognition criteria, modern analogs of depositional environments, or lithofacies classifications.

Seismostratigraphic exploration for hydrocarbons in shelf deposits has been hampered by the lack of a basin-scale, time- and rock-stratigraphic framework which incorporates these shelf sequences. In a chronostratigraphic sequence, where are shelf-sand reservoirs most often preserved with respect to shoreline and shelf break? Where do they most often occur within third and fourth order sea-level cycles, and how are these occurrences expressed seismically? What are the tectonic controls on shelf sand location? Facies distributions on modern shelves have thus far provided insufficient data for answering these questions. This is because facies patterns on modern shelves reflect deposition under a wide range of sea levels, a fact which has often hindered consensus among sedimentologists about the depositional settings of modern shelf sandbodies. The best exploration approach to this difficulty is to first recognize the control that sea level exerts on positions and geometries not only of sandbodies originating on the shelf, but of other units likely to occupy similar stratigraphic positions, such as reworked delta-front sands or beheaded barrier island sequences. This range in diversity forms the primary physical constraint for understanding the geometries and acoustic properties of shelf sandbodies, and thus for constructing effective seismic reflection models, because in the absence of well control many other sandbodies could be mistaken for shelf deposits.

Constraints on the temporal distribution of shelf sandbodies are much less clear, as very little published work relates shelf sandbody type to relative chronostratigraphic position. However, seismic sequence analyses from several foreland basin settings, as well as published outcrop studies, suggest that shelf sands of reservoir quality are best developed at chronostratigraphic sequence boundaries, with different types forming below and above regional transgressive surfaces in response to progradational and retrogradational episodes, respectively. Greatest storm sand amalgamation on the middle and outer shelf may occur between highstands and subsequent sequence boundaries, even in cases where evidence suggests that subsequent relative lowstands are of low magnitude, because lower rates of relative sea level rise favor a progradational rather than an aggradational mode. Although reflection seismic data provide the best time-stratigraphic control necessary to establish these relationships, sequence stratigraphic techniques allow the effective testing of these concepts with only outcrop and/or wireline log data.