Do Subsurface Models Accurately Represent the Rock? Insights from Outcrop on the Often Overlooked Importance of Mechanical Stratigraphy

Ronald N. McGinnis David A. Ferrill, Alan P. Morris, and Kevin J. Smart
Southwest Research Institute, San Antonio, TX

Abstract

A valid subsurface structural geologic interpretation should (i) honor available subsurface data (e.g., well and seismic), (ii) incorporate structural styles known or expected for the mechanical stratigraphy and deformation conditions in the region, and (iii) be logically restorable to an original unstrained condition. In many cases, multiple interpretations can be developed that honor the available data, but the geology is fixed. A crucial part of any interpretation exercise is the ability of the interpreter to know what is possible and what isn't based on the constraints of the data.

H. H. Read said that "the best geologist is the one who has seen the most rocks". Whether or not that is true, it seems to be true that the more rocks a geologist has studied in the field, the more options that person will consider and test when developing subsurface interpretations for a given set of data. Structural style is highly sensitive to the mechanical stratigraphy and deformation conditions (including tectonic stress regime, stress state, fluid pressure, and temperature). In our work in a range of tectonic regimes and across scales we find that mechanical stratigraphy has predictable influences on the structural style at seismic and subseismic scales.

Here we examine two cases in Cretaceous carbonate strata in central and west Texas deformed in extensional and contractional tectonic settings, where outcrop characterization provides insights into the influence of mechanical stratigraphy and structural position on seismic- and subseismic-scale deformation in the layers. These examples illustrate the utility of considering how mechanical stratigraphy influences the development of different deformation styles, even where deformation conditions are otherwise similar.

AAPG Search and Discovery Article #120140© 2014 AAPG Hedberg Conference 3D Structural Geologic Interpretation: Earth, Mind and Machine, June 23-27, 2013, Reno, Nevada