An Integrated Study of Basement Rocks in the Bayoot Field, Say’un-Masila Basin, Yemen

An integrated study of high resolution borehole images, petrophysical logs, production data, 3-D seismic, sidewall cores and cuttings was undertaken on basement rocks from six deviated wells located in the Say’un-Masila Basin, Yemen. The wells were drilled into the Rudood Ridge, a basement high positioned in the footwall of a locally significant SW-dipping fault. Hydrocarbon emplacement is through fault juxtaposition of the fractured basement against Late Jurassic organic-rich shale source rocks of the Madbi Formation.

Structural analysis of micro-resistivity and acoustic image logs focussed on fracture characterisation and determination of principal horizontal stress directions inferred from borehole breakout, induced fractures and borehole shape. Structural image facies were used to highlight fracture intensity and internal fabrics. Fracturing within the basement is intense, with in excess of 20 fractures per metre detected. Fractures have extremely scattered orientations and the fracture datasets are significantly affected by borehole bias.

Petrophysical log data show variation that implies compositional layering and image logs indicate inclined fabrics. Petrographical analyses of cuttings and sidewall core samples support a variety of rock types including diorite, granodiorite, granite and monzonite. Fracture fills include quartz, sericite, calcite, dolomite, pyrite and epidote. Many fractures contain more than one mineral phase along their trace. Rare matrix and fracture porosity was identified in some of the samples.

Rocks were classified into four broad types based on petrophysical log responses and petrographical analysis. These include felsic, intermediate, mafic and possible metamorphic rocks. Relationships between rock type and fracture properties were explored.

Geomechanical analysis shows that present-day principal horizontal stresses are directed NE-SW and that the local stress regime is likely to be strike-slip. Fractures were divided into sets based on their relationship to principal stresses. Hydrocarbon shows correlate closely with those fractures predicted to be critically stressed.

Seismic attribute analysis indicates that the seismic character of the basement is highly variable. Azimuthal anisotropy provides a tool to determine the heterogeneous spatial distribution of the fractured zones within the basement. Integration of these attributes with geological information will aid the future development of the Bayoot Field.


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