Using Diagensis Information to Augment Fracture Analysis

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This report describes relations between natural fractures and diagenesis that can be used to identify fractured beds, controls on fracture porosity, and fracture strike. By categorizing diagenesis based on the temporal relations of authigenic mineral precipitation and fracture movement, sandstones can be subdivided into those dominated by pre-, syn-, and postkinematic cements. Even sandstones that have virtually all of their rock-framework porosity occluded by synkinematic cement may retain substantial fracture porosity. An estimate of the probable degree of postkinematic mineral occlusion of fracture porosity can be obtained from study of framework authigenic cement volume. A breakthrough technique that uses photomultiplier-based imaging of electron beam-induced luminescence (scanned CL) to detect quartz-filled microfractures in sandstones having synkinematic quartz cement permits fracture strikes to be mapped in areas of the wellbore where no macrofractures are present. Because small specimens can be used to get fracture strike with the scanned CL method, this approach can be applied to samples obtained from wireline-conveyed rotary sidewall coring devices, permitting more fracture-strike data to be collected at reduced cost. Results of this study can be applied by operators to evaluation of prospects for horizontal drilling in highly cemented sandstone formations.


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