Analysis of a Fractured Carbonate Reservoir (Thamama Group) Using Borehole Images From an Horizontal Well, Offshore UAE

Abstract

Abstract Structural analysis has been performed on borehole resistivity images from a horizontal well through the Lower Cretaceous Thamama Group carbonates, offshore UAE, in order to investigate structural compartmentalisation within a complex faulted reservoir. The study interval has been divided into four major Borehole Image Units on the basis of borehole image and openhole log response. The contacts between these units include bed boundaries and faults. Within these units, both electrically conductive and resistive stylolitic bed boundaries were identified. Faults and fractures with varying appearance and extent along the wellbore are identified from the borehole image log. These features were classified by their electrical response and image appearance. Faults were discriminated from fractures by evidence of offset, interpreted lithological juxtaposition and greater thickness. Resistive fractures, interpreted as cemented, may act as baffles to fluid movement. Conductive fractures, interpreted as having aperture filled by (electrically conductive) drilling fluid, may act as fluid pathways. Fractures with a mixed electrical response on the images may be vuggy. Fractures occur as clusters along the wellbore, with increased fracture intensity within damage zones associated with some faults. Clustering of conductive fractures is due to both tectonic and litho-stratigraphic factors. Resistive fractures show a dominant NW-SE strike, and conductive fractures show a dominant NE-SW strike. NE-SW to ENE-WSW striking fractures are under-sampled because of the borehole orientation (SW). The strike of conductive fractures is broadly similar to the regional direction of the maximum horizontal stress. Reservoir pressure data correlate with the Borehole Image Units and the distribution of resistive faults. Resistive fault boundaries support modest static fluid pressure differences and are interpreted to isolate three reservoir pressure compartments.