A quantification of fracture dimensions and spacings has been obtained from a survey of outcrops of Middle Cretaceous Natih Formation carbonates in simple fold structures in North Oman. From the distribution of cross-axial and longitudinal extensional fractures in thickly-bedded limestone, our geological model has average matrix block dimensions of:
50 m (length) × 5 m (width) × 10-20 m (height) in gently dipping areas,
50 m (length) × 0.5 m (width) × 10-20 m (height) in steeply dipping areas.
These blocks contain and/or are intersected by cross-axial shear fractures with variable spacings.
These results have been applied to the fractured limestone reservoirs of the Natih Formation of the Natih Field, located some 50 km to the west of the jebels studied. This field is a NW-SE trending anticline which contains a STOIIP of 500 million m3. Open, cross-axial (NE-SW trending) fractures dominate as shown by electrical borehole imagery, core, and tracer testing studies.
From the outcrop and Natih Field subsurface data, the open, cross-axial fractures are inferred to be regularly distributed over the Natih structure with a spacing of 1 to several tens of m (extensional fractures), and 0.1 to 2 m (shear fractures). The spacing of the closed or only partly open longitudinal and shear fractures varies probably more over the structure because it is more sensitive to lithological and dip changes than that of other fracture types. These data provided realistic fracture parameters for the Natih Field integrated reservoir modelling that was carried out to study the production mechanism of the field.