Abstract: Characterization and Significance of a Stylolitic Fracture System Determined From Horizontal Core and Borehole Imaging Data, Hanifa Reservoir, Abqaiq Field, Saudi Arabia

Stephen T. Luthy, George Grover, Elliot Wiltse

The Hanifa reservoir at Abqaiq Field, eastern Saudi Arabia, consists of microporous (up to 30% porosity) lime mudstones with low matrix permeability (< 10 md). SEM imagery reveals a crystal framework texture of micro-rhombic calcite crystals with 2-5 am-sized intercrystalline pore spaces. Fluid transmissibility was preliminarily identified as via fractures as indicated by no stratigraphic predictability to fluid flow, high flow over thin stratigraphic intervals, little relationship between high flow and high porosity intervals, large disparity between core Kh and well-test Kh, and observation of fractures in cores and borehole imaging logs from horizontal Hanifa wells. Integration of descriptions from over 4000 fractures observed in borehole images together with descri tions of over 500 fractures identified from vertical and horizontal cores has resulted in further characterization of the fracture system. The fractures are open to partially-open, with an east-to northeast orientation, and they cluster in low porosity zones which are characterized by intense stylolitization. These sub-parallel, nearly vertical, discontinuous fractures terminate at stylolites, or pinch out locally into tight carbonate matrix, and contain appreciable amounts of dead oil and calcite cement. The fractures commonly originate off high amplitude stylolite column margins which also may contain dead oil stain and calcite cement. Fracture surfaces show minimal grain shearing or cataclasis, indicating little mechanical or chemical alteration. In zones of particularly intense stylo itization, fracturing may be locally pervasive, giving the rock a brecciated appearance. Petrographic relations indicate a complex history in which fractures, associated with stylolitization, were cemented and then filled with hydrocarbons. Together, the stylolites and stylolite-related fractures form the primary permeability system of the Hanifa reservoir. This fracture system architecture is critical to understanding the production characteristics of the reservoir, which include anomalously high fluid flow in low porosity zones or transition zones between high and low porosity, radial flow behavior from well tests, smaller than expected differences in well productivity between vertical and horizontal wells, and limited injection water breakthrough.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France