Fracture Development Related to Differential Compaction and Mechanical Stratigraphy, Sacramento Mountains, NM

Abstract

Phylloid algal mound complexes of the Pennsylvanian Holder Formation in the Sacramento Mountains, NM, were examined for fracture distribution, mechanical stratigraphy, and current rock strength. Early cementation of mound facies can create brittleness contrast with younger rocks that may fracture when strain is exacerbated by differential compaction. Finite-discrete element 2D Elfen models under the realistic constraints of gravity, evolving mechanical properties, pore pressure, and mechanical stratigraphy can be utilized to quantify the effect of differential compaction, mound geometry, and mechanical unit thickness on fracture attributes and distribution. The validity of such models can then be tested by comparison to field observations. Fracture density maps of Yucca Canyon northern wall show high fracture density values located on beds overlying the steep side of a mound (up to 0.36 frac/ft²) as well as in the middle of the mounds (up to 0.24 frac/ft²). However, beds that are indirectly above the mound and do not exhibit depositional draping have low fracture density. Fractures are bounded or encompassed by 4 types of mechanical units. Each mechanical unit is associated with specific depositional facies: (1) massive phylloid algal boundstones; (2) massive grainstones and grain-dominated packstones; (3) thinly bedded recessive fusilinid-phylloid algae wackestones and packestones; and (4) recessive shale/mudstone. Unconfined compressive strength (UCS) of all facies was estimated from a total of 285 Schmidt hammer hardness measurements. There is no discernable difference in the UCS values between facies (Median UCS = ∼50 MPa) except for the significantly stronger boundstones located in mud mound (Median UCS = ∼ 70 MPa). UCS values are consistent with previous cement stratigraphy work indicating that mounds were lithified by early meteoric water cementation and the remaining facies underwent compaction and late cementation in episodes that predate and postdate fracturing. The highest mechanical contrast is expected to be present soon after deposition of rocks younger than phylloid mounds. This highlights the utility of geomechanical modeling to unravel the evolution of fractures as mechanical properties evolve temporally. Thus, we present 2D Elfen models that accounts for the deposition and compaction of strata above the cemented rigid phylloid mounds.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016