--> Analysis of Fracture Style and Development Associated With Differential Compaction Around Mississippian Carbonate Mounds, Sacramento Mountains, New Mexico

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Analysis of Fracture Style and Development Associated With Differential Compaction Around Mississippian Carbonate Mounds, Sacramento Mountains, New Mexico

Abstract

The effects of early-lithified carbonate mounds on subsequent sediment deposition are well understood. However, the localized deformational impacts of early-lithified mounds during burial have not been well-studied. This project hypothesizes that rigidity differences between early-cemented carbonate mounds and subsequent sediment deposits result in differential compaction and potentially significant fracture development in the overlying sediments. To substantiate the hypothesis, this study characterizes fractured strata overlying Mississippian Waulsortian-style carbonate mounds that outcrop in the Sacramento Mountains of southeast New Mexico. Understanding such fracture networks could improve production in tight carbonates, tight sands, and unconventional reservoirs. Field work involved vertical and lateral facies mapping (mechanical and stratigraphic), and measurement of fracture spacing along line-transects. Fracture length, orientation, and intensity were compared against mechanical facies, bed thickness, lithology, and stratigraphic architecture of strata overlying the mounds. Thin sections from field samples enabled close assessment of facies and petrophyscial characteristics, whereas mechanical properties were measured directly on the outcrop or on 1-inch core plugs extracted from hand samples. (The core plugs were subjected to Unconfined Compressive Strength and Triaxial Compression tests, which model in-situ confining pressures and gauge corresponding rock strength characteristics.) Two and three-dimensional fracture maps of the study area were generated by combining field mapping with remote sensing (i.e. using ground-based LIDAR and newly acquired high-resolution images from an unmanned aerial vehicle). Fractures range from 0.2-48m in length, and fractures longer than 2m (n=1045; 75% of data) have an average strike of 13°, while those longer than 10m (n=85; 6% of data), have a mean strike orientation of 40°. Importantly, the average northeast strike trend aligns with regional faulting and compression, implying that tectonically-influenced fractures may be intermingled with local fractures generated by differential compaction. Furthermore, there exists an marked increase in fracture intensity above mound cores. Finally, the project develops predictive guidelines regarding fracture development around carbonate buildups, and demonstrates how such fracture networks affect hydrocarbon flow in analogous subsurface reservoirs.