FRACTURE ANALYSIS OF THE BOLSHOI KARATAU UPPER PALEOZOIC CARBONATE PLATFORM AS A WELL-EXPOSED ANALOG TO THE TENGIZ FRACTURED CARBONATE PETROLEUM-RESERVOIR

Abstract

The Tengiz oilfield in western Kazakhstan is one of the major oilfields in Kazakhstan. It is a fractured carbonate reservoir, a lower Paleozoic carbonate platform. TengizChevrOil is presently investigating the fracture properties of the outer carbonate platform slope of the reservoir. As rocks at Tengiz are not exposed and only accessible through drilling, fracture properties were studied in the well-exposed carbonate platform in the Bolshoi Karatau mountains which is regarded as an analogue platform. The carbonate platform in the Bolshoi Karatau mountains are of the same age and similar lithology as in the Tengiz platform. It is well exposed, because the platform was uplifted to form a mountain range during Hercynian and Alpine deformation phases. The outboard lower slope of the Bolshoi Karatau platform is represented by the Baktysai Formation, in which open fractures had turned into completely mineralized fractures, i.e. calcite veins. These were analyzed in grainstones and rudstones in three key outcrop locations: Aktobe, Akuyuk and Zhankorgan sections. Scanline measurements of 6.0 ± 2.0 m long were collected to characterize the veins, while collecting orientations and vein dimensions (kinematic aperture, length, height). From this data set, spacing was derived. In addition, Schmidt hammer measurements were collected to determine Youngs Modulus of the various rock types that host the veins. It should be noted that the vein width is aperture and since they are fully filled, we also might refer as "kinematic aperture". In the Aktobe section, the dominant vein set has an average width of 3.7 ± 1.0 mm, an average spacing of 0.2 ± 0.1 m, an average height of 0.3 ± 0.1 m and occur in beds of 1.0 ± 0.5 m thick. In the Akuyuk section the dominant vein set has an average vein width of 1.0 ± 0.6 mm, with an average spacing of 0.1 ± 0.02 m and an average height of 0.3 ± 0.1 m in beds of 0.2 ± 0.1 m thick. No significant correlation was found between bed thickness and either vein intensity or vein spacing, with R2 values of 0.11 and 0.21 for grainstone and rudstone, respectively. Similarly, Dunham classification and facies types did not show a strong relationship with vein characteristics. The calcite vein coverage, expressed as a percentage of the wall rock for the dominant set is 2.0 ± 0.7 % in the Aktobe section and 0.4 ± 0.1 % in the Akuyuk section. Young’s modulus of the grainstones and the rudstones derived from Schmidt hammer rebound values is 66 ± 12 Gpa, with slight difference between the two rocktypes It was concluded on the based on theoretical calculations of the maximum vein width and relative age relations that the calcite veins in the Baktysai Formation are not burial veins (i.e. did not form due to the burial nor during the burial), but formed as 'tectonic' veins during the Hercynian mountain building. It further appeared that the presence of Hercynian faults near the sections exerts a significant role on the vein statistics as does the presence of stylolites that developed before and after the veins. Vein width of dominant sets was plotted in cummulative log-log plot. Both sections exhibit multi segmented power-law behavior, with segmented trendlines. The slope of linear parts of the curves falls in the range of –0.43 to –2.03 in the Aktobe section, while Akuyuk section shows slopes of –0.11 and –0.91. Nonlinearity of the curves was shown to be possibly due to insufficient length of the scanlines. The results suggest that, ideally, they should be longer than 10 m to avoid abnormalities caused by variations in local stress conditions.