Nine Jurassic and Triassic samples of two North Sea reservoirs are examined on their texture to recognise phenomenons that indicate towards the risks of pore collapse during depletion. A combination of backscatter electron microprobelement analysis and image analysis is used to create colour image element maps. These combination maps of the elements Si/AI/K and Fe/Ti/Ca, give spatial information on quartz, feldspars, clay minerals, carbonate and the weathering products at an accuracy of 3 gtm. The images are used to create binary maps on defined elements, such as gains. The resulting dataset consists of area parameters like area percentages of minerals and porosity. These false colour presentations of various minerals clearly show the integrity of the grains in the hard grain framework. Weathering of mainly feldspars and cementation by carbonates are clearly visible and contribute to the understanding of the grain framework integrity. Feature properties like distributions of grain sizes, perimeter, grain contact lengths and orientations are measured on the binary maps. Combining area and perimeter properties for specified grains give the number of grain contacts per grain and per measure frame. Recent laboratory experiments on compacted grain aggregates combined with image analysis and modelling work, indicate that the gathered data in this study also can be used for the development of a micro-model on compaction. When the scale of this investigation is considered, one can think of the use of drilling cuttings as samples for reservoir integrity characterization.
Reservoir performance with respect to pore characteristics and related mechanical behaviour is largely based on large-scale petrophysical bulk measurements. Increasing reservoir depth and the related rise in overburden stress play an important role in reservoir integrity. Depletion induced compaction is significant phenomenon to be considered. Pore/stress data information is based on log readings (ca. 15 cm resolution) and core/plug measurements (ca. t0 -tOO cm3). Mechanical test results on usually damaged and stress relaxed cores and plugs, need a careful interpretation. Long-term predictions, as needed for integrity, are also formulated by the condition of the reservoir rock texture and mineral content. These are essential when aspects such as reservoir collapse and subsidence are considered. Hence, the ensuing questions are relevant for the texture quality:
§ What is the mineral composition and content of the reservoir rock, at microscopic level.
§ Which effects of a compressive situation on the reservoir rock can be deduced from the mineral grain configuration.
§ What geo-mechanical effects can be recognized and calculated, using texture images.
Nine North Sea reservoir samples, originally from corings of two reservoirs, are used to find ways to visualise and measure relevant texture features (Wolf et al., 1998). The samples are from: A. Fulmar fmt., Upper Jurassic, depth -5. t kin. B. Skaggerak fmt., Triassic, depth -4.4 km deep. It is given, that samples from reservoir A show a growing compressibility with stresses above 45 MPa depletion. It goes with pervasive grain breakage and pore collapse. Samples from reservoir B show a constant compressibility up to 70 MPa depletion (Schutjens, 1998).
On micro-scale both optical microscopy and scanning electron microscopy (SEM) give a visual impression on the spatial relation between minerals phases and the pore related matrix. These methods only provide limited and subjective information on the relation between minerals and grain framework quality. For example, diagenetical features such as mineral weathering and cementation are difficul
