Basement rocks tend to have negligible primary (rock matrix) porosity and, where present, almost all porosity is fracture-related. An insight into fracture type and distribution is therefore of importance when attempting to predict potential flow zones in hydrocarbon reservoirs. In the absence of core, borehole images provide a proven means of characterizing fractures within basement rocks. This paper will concentrate upon the classification and characterization of basement structural elements, including fractures, faults and breccias, versus lithological elements, such as foliations and intrusion boundaries from borehole images. In addition, suggestions of how to differentiate present-day in-situ stress indicators from natural fractures will be made as these features may provide important information regarding open fracture set orientations. Examples of microresistivity and acoustic borehole images acquired in fractured igneous basement rocks from oil wells in Yemen will be presented together with a suggested methodology for their interpretation. The discussion will be based on hydrocarbon reservoirs; however, the methodology outlined can potentially have wider applications in, for example, groundwater pollution prevention schemes or groundwater extraction, radioactive waste disposal, geothermal energy resources and deep-drilling research programmes.
To date, the Lower Jurassic Butmah Formation (also known as the Sarki Formation) has not widely been regarded as a productive target for hydrocarbon exploration in Kurdistan. However, detailed sedimentological and structural evaluation of core acquired from four wells in the Shaikan and Sheikh Adi Fields, together with test data has yielded promising results. These studies have determined that fracture style, intensity and fill vary according to lithology. Lithology types penetrated in core include a range of dolomitic facies intercalated with anhydrite and limestones.
The mid-upper section of the Butmah Formation is characterised by lithologies and textures indicative of shallow tidal / algal flat deposits whereas deposition in a shallow, locally agitated, shelfal or lagoonal setting is inferred for the uppermost section. The succession demonstrates an overall upwards deepening or transgression. This study presents the results generated from core logging and petrographic analysis and attempts to characterise the formation in terms of depositional facies and broad sedimentary trends.
A study of borehole image data acquired from eight wells drilled as open hole completions into highly fractured basement has identified open and stress-sensitive fracture sets. This information together with seismic interpretations of fault damage zones, production and gas log data has helped to constrain basement targets in the Bayoot Field, Block 53, Yemen. However the dominant productive zones within the basement appear to be within seismically resolvable fault damage zones that should in theory be closed under the present-day stress regime. Sidewall core examination has shown evidence for cement bridging between fracture surfaces and this propping effect is envisaged along these major faults. A dual porosity system is hypothesised whereby open critically stressed fractures act as hydrocarbon conduits into the seismic-scale faults.
This case study of ten oil wells drilled into highly fractured and heterogeneous crystalline basement rocks of the Bayoot Field, Say’un Masila Basin, Yemen, brings together the findings from a wealth of oil field data and shows that a multidisciplinary approach is required for full characterisation. An improved method for targeting hydrocarbons has been established for the study area. This has been achieved by drilling highly deviated wellbores into the upper basement reaches and optimally orienting them to intersect a maximum number of stress-sensitive fractures together with major seismic-scale fault damage zones that are not in direct connection to the local Bayoot Fault.
The Kurdistan region has seen a rush of exploration activity in recent years with over 30 companies operating in what many believe to be one of the last remaining frontier hydrocarbon provinces. It forms the north-western margin of the Zagros fold belt, which has seen prolific production from fields in Turkey, Iraq and Iran. Antiforms and synforms are readily interpreted from satellite imagery, but access to the region was restricted for geopolitical reasons.
The Shaikan prospect is a doubly-plunging antiform lying 85 km northwest of Erbil near Dohuk. Pre-drill indications for oil were strong. A limestone outcrop to the north and west of the structure bleeds oil, which occurs within fractures and vugs. Field reconnaissance mapping validates satellite image analysis, defining a structural closure extending around 35 km E-W and up to 6 km N-S. Interpretation of 2D seismic suggests the presence of a compressional, possibly flower structure, with reverse faults bisecting the north and south limbs.
A single well has been drilled on the crest of the Shaikan structure. The well, completed in November 2009 was a significant discovery. It encountered Cretaceous, Jurassic and Triassic hydrocarbon reservoirs with a cumulative net pay of circa 250 metres of oil in multiple reservoirs. The well was TD’d early at 2950m in the Triassic due to an influx of high pressure gas thus the full extent of the hydrocarbon accumulation is not yet fully determined.
Microresistivity image logs were gathered through much of the well section to supplement conventional petrophysical logs, and core was cut in two limited intervals of the Jurassic Mus and Triassic Kurre Chine Formations. Five drill stem tests were conducted and confirm oil flow of up to 7480 BOPD from the Jurassic.
Borehole image log and core description results provide some insight into the subsurface structure, some at variance with initial expectations, particularly the fact that fractures are not simply orientated with respect to flexure:
- Overall structural orientation differs little with depth.
- Whilst surface structure has a strong E-W grain, fractures picked from images and goniometry tend to strike N-S to NNE-SSW, across the antiform hinge. The majority of fractures are conductive and occur in clusters with depth, some of which coincide with mud loss events, suggesting they have aperture. Ten faults were inferred. Most strike NNE-SSW; only one E-W striking fault was interpreted.
- Structural core logging reveals that fractures are commonly steep, layer-bound and discontinuous. Residual hydrocarbon coats fracture apertures. Fractures appear to be of several generations. They individually have limited porosity, but form a connected network.
- Stylolites are seen in core and borehole images, where they appear as highly conductive ‘beds’. More than one generation is present, but bed-parallel stylolites are the more common type. They are predicted to reduce vertical fluid connectivity, although stylolite associated fractures are observed that may improve layer parallel flow.
- Drilling-induced tension fractures are seen in the borehole images, striking N-S to NNE-SSW. This is subparallel to many of the conductive fractures, suggesting that they could be stress-sensitive and may be enhanced, leading to better drainage.
In addition to detailed structural information the core yielded important sedimentological information:
- The Jurassic core comprises a cyclic carbonate succession of laminated lime mudstones (locally dolomitized), bioturbated and bioclastic lime mudstones (mainly undolomitized), intraclastic packstones, in-situ breccia zones and thin detrital shale laminae. The depositional setting envisaged is a shallow, sheltered marine environment, possibly a lagoon or peritidal setting. The rapid variations of lithology and thin units may indicate limited accommodation space and frequent cyclic relative sea-level fluctuations.
An anhydrite succession in the deeper Triassic core appears to be a replacement phase of precursor carbonate or carbonate-evaporite rocks. Relict grains and ‘ghosts’ of bioclasts and burrows are evident in replacive anhydrite after carbonate lithologies. Planar, disrupted or ptygmatically folded layers occur within laminated anhydrite intervals. This rock type may be a replacement of sabkha deposits of gypsum, anhydrite, limestone and dolomite.
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.
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.