Characterization of the Sligo (Lower Cretaceous - Aptian) Platform Margin in South Texas: Understanding Facies Distributions Using 3-D Seismic and Modern Analogs

Christopher J. Modica and David J. Katz
Pioneer Natural Resources, Irving, TX

3D seismic data coupled with an understanding of modern carbonate platform geometries can yield realistic characterizations of subsurface facies distributions in carbonate rocks. This very straightforward and distinctly geologic approach employs high-density data sets (3D seismic isochrons and satellite imagery) to map facies distributions and can be used to vastly improve prospect and play definition. This approach has advantages over those that rely on older low-density data sets (2D seismic and well logs) or more current semi-quantitative geophysical techniques (i.e., inversion or AVO/AVA). Often, geophysical attribute geometries do not resemble modern systems and are therefore of dubious value. In complex carbonate systems where reservoir prediction is paramount, such as the Aptian Sligo platform margin in South Texas, these simple techniques can be applied to re-visit plays that failed or were inadequately explored in the past with only 2D seismic and well data.

We describe the Sligo carbonate platform margin as an over-steepened and dominantly aggradational rimmed shelf margin. The main reservoir target is a very narrow (~ ¼ to ½ mile wide) barrier reef and flat complex. The platform margin reef and reef-flat facies tract is not a continuous ribbon, as often depicted, but is dissected by tidal inlets and slope-failure scarps. This complexity cannot be predicted without 3D seismic data combined with an understanding of tidal inlet and barrier island geometry from modern environments. In this case we used a stretch of platform margin near Darby Island on the Bahamas Platform. This type of analysis is important because not all “bumps” or positive structural features are reefs or buildups. Such generalizations are far too simplistic and they ignore scaling constraints provided by modern analogs.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas