Mechanical stratigraphy in Mesozoic rocks of the San Juan Basin: Integration of stratigraphic and structural terms and concepts

Abstract

We characterize relationships between stratigraphy and natural fractures in outcrops of Mesozoic strata that rim the San Juan Basin in New Mexico and Colorado. These outcrops expose fluvial and shallow-marine siliciclastic deposits and calcareous mudstones deposited in a distal marine setting. We focus primarily on a regionally extensive fracture set formed during the Eocene to minimize localized tectonic effects on fracture development. Where possible, we supplement our observations with wireline log- or laboratory-derived measurements of rock properties. Our goals are twofold: 1) to illustrate how direct integration of data and concepts from stratigraphy and structural geology can lead to better fracture characterization, and 2) to develop thought processes that will stimulate new exploration and development strategies. Genetic beds form one scale of stratification in the outcrops we describe. For example, sandstone beds can be arranged into coarsening and thickening upward successions that are the depositional record of shoreline progradation. In fluvial settings, cm- to dm-scale sandstone beds can also be part of m-scale single-storey channel complexes that, themselves, can be arranged into amalgamated channel complexes 10s of m thick. In these and other settings, it is important to distinguish between beds and features that can be defined via wireline logs because it is the former (cm- to dm-scale) that are usually the primary control the distribution of natural fractures. The extension fractures we describe are typically bed-bound, with bedding being defined by lithology contrasts and the associated changes in elastic properties. Fracture spacing distributions are typically lognormal with average spacing being less than bed thickness. Although mechanical bedding and depositional bedding are commonly the same, diagenesis can cut across bed boundaries and complicate this relationship, especially where lithologic contrasts are small. Deposits from similar depositional environments which undergo different diagenetic histories can have substantially different mechanical properties and therefore deform differently in response to similar imposed stresses.

The mechanics of initiation and development of thrust faults and thrust ramps

“…Mechanical stratigraphy is the stratal expression of the contrasts in the cohesive, compressive, tensile, and frictional strengths of rock, along with the thicknesses of rock units and interface properties between mechanical units (Corbett et al, 1987;Gross et al, 1995;Underwood et al, 2003;Laubach et al, 2009;Ferrill et al, 2017;Hart and Cooper, 2021). Mechanical stratigraphy affects fault nucleation and propagation (Teixell and Koyi, 2003;Underwood et al, 2003;Laubach et al, 2009;Roche et al, 2013;Ferrill et al, 2017;Bracken, 2020), fracture nucleation location (Eisenstadt and De Paor, 1987;, fault length, width, and the aperture across a slip surface (Laubach et al, 2009;McGinnis et al, 2017), fault-growth directions (King et al, 1988;Mitra and Mount, 1998), the proportions of folds and faults that form (Morley, 1994;Erickson, 1996), fold geometry (Fischer and Jackson, 1999;Gutiérrez-Alonso and Gross, 1999), fault-fold interactions (Chester, 2003), and fault shape and related fault zone deformation (Woodward and Rutherford, 1989;Pfiffner, 1993;Ferrill and Morris, 2008).…”