Fracture study of potential reservoirs in Northern Iraq

A fracture study was performed in Kurdistan Region of Northern Iraq. Based on exploration efforts, the Sargelu and Kurrachine carbonates are fractured reservoir rocks in the region. The study was based on traditional fieldwork. Data were collected in 6 anticlines and 9 formations. Orientation data of all fractures, faults within several blocks of 1 square metre were recorded, together with their length, aperture, frequency, filling, surface characteristics. Data were plotted on stereonets in present state, and in back-tilted (unfolded) state. Analysis was performed in latter position. Data were separated into layer-perpendicular and layer-oblique sets. Part of the fracture sets shows clear relation to local folding. Perpendicular (tensional) and shear joints form symmetrical sets. In these cases there are two oblique/shear sets: one where the acute angle bissectix points to the shortening direction and another, which has the obtuse angle bissectix in the same direction. In some other fracture sets there is strong departure from the local fold orientation. In these cases the different fracture sets can be interpreted as born of the superposition of two symmetric sets related to different fold orientations. This is supported by the measurement of NW-SE to E-W fold axes in the mentioned folds and areas, irrespective of the map attitude of the major folds. Layer-oblique sets were separated into three classes. Steeply dipping joints that were striking parallel to layer-perpendicular family of the same exposure were affiliated to fold-related joints. Flat-dipping fractures were possibly formed by compression. These fractures suggest compression directions. Finally, fractures having dips around 50° were interpreted as normal fault-like shear surfaces. These fractures suggest extension directions. Assessing the estimated structural directions we propose three probable extension events in N-S (NNW-SSE); NE-SW and NW-SE direction. The same exercise suggests (much weaker defined) NW-SE; NE-SW and N-S compression directions. Fault slip measurements in the closer and wider vicinity gave the same or very similar faulting events. In some exposures non-gravitational open fractures and others filled by bitumen were observed. These were mostly longitudinal joints, oriented perpendicular to recent local N-S or NE-SW compression, or shear joints symmetrical to these directions. Longitudinal joints may have remained open due to continuous fold growth. Shear joints may have remained open due to fold-parallel extension, or continuous shear movement. Since recent compressive stress directions play an important role in opening fractures (that are then filled by bitumen), oil migration should be a young process.