The characteristics of and major factors controlling the natural fracture development in the Middle-to-Lower Ordovician carbonate reservoirs in Tahe area are studied through observations of outcrops

cores and casting thin sections

as well as logging data analysis.The natural fractures in the study area come in three types:tectonic fractures

diagenetic fractures and composite fractures.Specifically

the tectonic fractures are mainly shearing fractures

followed by tensional fractures;the diagenetic fractures are mainly bedding-parallel fractures

followed by diagenetic stylolites;the composite fractures mainly include structural-diagenetic fractures

structural-epigenetic fractures and karst fractures.The tectonic shearing fractures are dominant in the study area

developed in four groups trending NE-SW

NW-SE

nearly EW and nearly NS

respectively.However

they differ remarkably in different horizons in terms of development degree

and occur preferentially along the NE-SW direction in these horizons.High-angle fractures are dominant with a dip angle of over 70°.The longitudinal propagation length of the fractures is controlled by the rock mechanical stratigraphy

within which the fractures are developed

and on the interface of which the fractures terminate.Fracture density shows obvious heterogeneity both vertically and horizontally

mainly controlled by geological factors including sedimentation

tectonics and karstification.Along with the increase of plastic mineral contents and rock layer thickness

the fracture density gradually decreases.Faults and folds play a vital role in fracture development

leading to evident variation in fracture density at various structural locations.The fracture density tends to gradually decrease along with the increasing of distance to fault plane and fold axial plane.Besides

karstification affects the degree of fracture development in the upper strata of karst cavens

where there are mainly three types of fracture systems

i.e.nearly vertical fractures

obliquely-crossing fractures and nearly horizontal fractures.