Structural analysis of highly fractured, heterogeneous basement, Say’un Masila Field, Yemen

Integrated structural analyses of high resolution borehole images with various well data are necessary for detailed structural characterisation of basement reservoirs. Microresistivity and acoustic image logs together with petrophysical logs, production data, rock specimens and cuttings samples were acquired in five deviated wells that penetrate basement rocks in the Sayun-Masila Basin, Yemen. Structural analysis focussed on fracture orientation, classification, apparent aperture and intensity. Principal stress directions were inferred from borehole breakout, induced fractures and borehole shape. Structural image facies were used to highlight fracture intensity and internal fabrics.

Basement rocks are hornblende, plagioclase and quartz rich. Log data show variation that implies compositional layering and image logs indicate inclined fabrics. Fracturing within the basement is intense, with in excess of 20 fractures per metre detected. Fractures have extremely scattered orientations. The fracture datasets are significantly affected by borehole bias and no clearly defined fracture populations are evident, even when corrected for bias.

Petrographical analyses of cuttings and rock samples shows pyrite, sericite and possible calcite fracture fills, however part open fracturing is also inferred from the image logs, petrophysical log response, mud loss and production data. Part open fractures were divided into sets based on their relationship to principal stresses. Hydrocarbon shows correlated closely with those fractures predicted to be critically stressed. There is no clear relationship between fracturing style, hydrocarbon occurrence and rock type. Nonetheless, the borehole images revealed a wealth of geological data that could not be derived from conventional surface seismic.