The cores from formations (Image 1) have a lot of small/tiny characteristics that will scale up to the field study. Here, we expect to identify signatures for a faulted zone and also differentiate between the two geologic time zones (Jurassic and Triassic).
Image 1: Structural cross-section along the 15/9-19 SR well path with the cored zone indicated.
The fractures in the core are mostly small faults with displacements from a few mm to cm. Fracture frequencies are generally higher in the Jurassic than in the Triassic (Image 2). Recorded frequencies include syn-sedimentary features which seem to be more pronounced in the Triassic.
Image 2: Fracture frequencies along one meter intervals of cores 2 and 3. The
Jurassic-Triassic boundary (unconformity) is at 4343.60 m MD RKB.
Deformation Bands
The small faults generally precede large faults and these are better identified by color. Deformation bands here are lighter in color (Image 3). However, deformation bands can be dark in color if oil seeps into them.
The effects of grain crushing lead to porosity reduction which can be seen in the thin section of the deformation bands. The porosity reduction of one order of magnitude generally occurs. (If the formation had a porosity of 20%, then the porosity is around 2% at the deformation bands). Also, the crushing of grains means that there must be some overburden pressure already on the formation before faulting.
Image 3: Core photograph from the Hugin Formation, at 4336.59-4336.80 m MD
RKB. The dark rock is the oil-stained sandstone, the light zones are deformation bands.
Strain Hardening basically happens when you apply stress to a material and it hardens due to that stress. Example: Steel
Strain Softening happens when the material loses hardness, yield strength due to application of stress. Example: Plastic
The formation of deformation bands with grain crushing/ porosity hardening is a result of strain hardening.
The presence of large faults with localized slip surfaces represents strain softening.
Grain Orientation
There is a preferential orientation of the long axis of the quartz grains (Image 5). The preferential growth of clay minerals like Kaolinite, Illite and Smectites are generally identified in the thin sections.
Image 4: Thin-section photograph 100X magnification from the Hugin Formation at 4336.84 m MD RKB. The photograph shows quartz grain crushing and strong porosity reduction.
Clay Smearing
Clay smearing occurs when clay from an overlying formation gets smeared/ incorporated near the fault zone (Image 5).
Image 5: Clay smear in normal fault zones
Clay smears are important to understand the fluid flow, as zones with clay smearing considerably reduce permeability and determine the fluid behaviour near the fault zone.
Clay Striations are basically scratches etched on the surface as the formations move. These are important as they indicate the direction of movement in the fault plane. (Image 6)
Image 7: Fault with clay smear and striations in the Triassic at 4345 m MD RKB.
Interpretations
Fault Drags
Fault drags are basically deformation of the sand/formation due to the faults and are generally identified using a dipmeter log (Image 7).
Due to operational issues, a Dipmeter log could not be recorded. So we try to extract this information from the core. The study reveals two major zones with fault drag (also termed as rotation):
- 4345-4348 m MD RKB (Triassic): This zone coincides with the area with clay smearing at 4345 and 4347 m.
- 4355-4362 m MD RKB: We do not have a fault zone in the core, yet we do see a dip angle. The possible reason for this dip would another phenomenon called slumping.
Slumping is a more localized phenomenon like cliffs forming as the landmass beyond a points falls away. Unlike faults that are indications of tectonic activity, slumping occurs when the bedrock fails due to weathering, earthquakes, storm events.
Fault Sealing
The Jurassic formations have the majority of the fractures where displacements are mostly due to normal faulting. However, in the Triassic formation we see syn-sedimentary faults. These faults are both reserve and normal.
Syn-sedimaentary faults are faults that a present only within a single formation.
Image 9: Small faults in the Hugln Fm. at 4334 m MD RKB. The dark colour of deformed zone (band) indicates oil invasion.
In the Jurassic formations, we have dark and light colored fractures. The light colored fractures indicate a seal but the dark colored fractures indicate the presence of oil in them (Image 9). The deformation bands have low porosity due to the presence of grain crushing and can act as a seal. But the presence of dark bands indicates that the oil has broken this barrier – possibly due to capillary displacement pressures.
Induced Fractures
Induced fractures due to drilling are present in the core. These are mostly defined by the absence of clay smear, striations or deformation bands of less thickness relative to the natural fractures.
Joint means a fracture/ fault without any visible movement mainly parallel fracture, swarms means in large numbers.
The fractures perpendicular to the core axis are probably due to bending while those parallel to the core axis and ones with joint swarms to the core axis may be caused by torsion (this is basically twisting effect).
