Natural fractures in the spraberry formation,midland basin,texas:the effects of mechanical stratigraphy on fracture variability and reservoir behavior

摘要

Horizontal cores from sandstone-siltstone reservoirs in the Spraberry Formation have documented tow systems of dramatically different yet dynamically compatible natural fractures,in reservoirs separated vertically by only 145 ft.Each system is capable of producing a different degree of the northeast-trending permeability anisotropy recognized in Spraberry reservoirs.One fracture system consists of two vertical fracture sets with an apparent conjugate geometry.The other system consists of evenly spaced,northeast-striking vertical fractures,nearly bisecting the acute angle of the first system.Although lithologically similar,differences in quartz-overgrowth and clay content in the layers resulted in the two reservoirs desgrowth and clay content in the layers resulted in a yield strength of the lower bed that is only half of that of the uppper layer,producing different fracture systems in the two reservoirs despite their proximity.Such differencesin the mechanical properties,due to variations in diagenetic and depositional histories of the strata,are probably widespread within the formation.They have the potential to cause significant vertical and lateral variation in the Spraberry fracture system across the basin.Low present-day in-situ stresses in the reservoirs allow the fractures to open,to become more conductive,and even to propagate,under very low injection pressures.