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Geomechanical Modeling of Pennsylvanian Carbonate Mound Complexes: Early Fracturing Related to Differential Compaction and Evolving Rock Properties

Abstract

Early fractures and faults can be long-lived conduits for diagenetic fluids and may reactivate with subsequent deformation. However, quantifying and testing the mechanisms that lead to early fracturing is challenging when examining formations that have undergone significant diagenesis and burial through time. Pennsylvanian carbonate mound complexes in the Sacramento Mountains of New Mexico are a type locality for phylloid algal mounds and analogous to several Pennsylvanian hydrocarbon reservoirs. Mound and grain-rich facies are known to experience early marine and meteoric cementation inhibiting compaction, while mud-rich and siliciclastic facies compact normally. Differential compaction in this case can cause early fracturing within the early cemented facies soon after deposition, but outcrop observations are not sufficient to constrain the key factors controlling these early processes. In this study, we investigate differential compaction as a driver for early fracture development using geomechanical models accounting for: (1) evolving mechanical rock properties by facies type and (2) initial stratal geometry and its evolution through time. A Schmidt hammer was used to measure the unconfined compressive strength (final /present day strength) for all facies associated with mound complexes in the Sacramento Mountains. Samples collected from Yucca Canyon have undergone laboratory geomecahnical tests and thin section analyses. The initial (early) rock properties are extrapolated from the final rock properties with guidance from observations of modern analogues. Strata and mound geometries are constrained from 3D point cloud models of Dry and Yucca Canyons. True stratigraphic thicknesses are calculated and then are decompacted for facies that did not experience early cementation. Constructed geomechanical models will quantify: (1) the amount of differential compaction required for early fracture development, (2) effect of mound and stratal geometries on early fractures spatial distribution; and (3) influence of pre-existing fractures on spatial distribution of subsequent fracturing. Constructed models will be validated by field observations documenting the spatial distribution and orientation of (early) fractures with sedimentary fill in Yucca Canyon. We present a process-based approach to early fractures prediction and characterization with implications on fluids migration pathways and early strain localization; both relevant to reservoir quality prediction.