Fracture abundance and patterns in the Subandean fold and thrust belt,Devonian Huamampampa Formation petroleum reservoirs and outcrops,Argentina and Bolivia |
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| Institution: | 1. State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology, Shandong University of Science and Technology, Qingdao 266510, PR China;2. School of Engineering, Nagasaki University, 1-14 Bunkyo-machi, 8528521 Nagasaki, Japan;3. State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221116, PR China;4. Rock Mechanics and Geo-Hazards Center, Shaoxing University, Shaoxing 312000, PR China |
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| Abstract: | Opening-mode fractures in Devonian sandstones in outcrop and in several fields in the Subandean Ranges of southern Bolivia and northern Argentina compose two fracture sets, I and II that strike west–northwest and north–northeast, respectively. Abundant Set I fractures are at a high angle to local Andean structural trends, and Set II is aligned with fold axes. Crosscutting relations and quartz textures in fractures suggest that, although Set I is locally older, these sets may have opened partly contemporaneously. Sets comprise both macroscopic fractures and more abundant, millimeter-scale microfractures. Fractures with opening displacement of less than 0.1 mm are typically sealed with quartz, but wider fractures are lined with quartz and contain connected fracture porosity. Microfractures are more abundant than macrofractures, and size distributions can be interpreted to approximate power laws. Microfracture strain is an efficient method of quantifying fracture abundance. Both sets record small fracture strains of 0.00016–0.0083. In backlimbs Set I strain is higher in brittle quartz-rich sandstones. Set II strain varies markedly and is generally high in hinges and steep forelimbs of asymmetric anticlines. For individual samples, Set I–II strains in fold hinges and forelimbs are comparable, consistent with concurrent Set I–II growth. Open fractures that could augment permeability are present both on and off structure, but microfractures document fracture abundance that varies with structural position and sandstone quartz content that could account for variations in production outcomes. |
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