Characteristics Of Fracture Patterns In The Zagros Fold-And-Thrust Belt

The Zagros fold-and-thrust belt, in southwest Iran, exposes extensive areas of deformed Cambrian to Holocene sedimentary rocks with minimum vegetation cover. These Phanerozoic sequences are folded and faulted above the crystalline Precambrian basement, forming large doubly-plunging folds that host vast volumes of hydrocarbon in anticlinal traps. The understanding of the factors that control the characteristics of fracture patterns, such as orientation distribution, density spatial variation and chronology is fundamental to improve the methods used to characterize fractured reservoirs. Furthermore, from a regional-scale point of view, this understanding is of major interest for development plans of these reservoirs, which can constitute petroleum provinces like the Southwest Zagros.

The high productivity of these folds is related to the presence of two systems of fractures that have led to secondary effective porosity in the carbonate reservoirs. Several sets of fold-related fractures, which constitute the first system, have well-defined relationships to the fold structural elements and occur within the boundaries of the fold structures. The spatial variation of these sets of fractures is a function of the spatial variation of the fold elements and position within the fold-and-thrust belt. Fold geometry and kinematics are well known to be important factors that control fracturing.

The second fracture system includes several sets of basement faults, and their related, subsidiary fractures, that cut through the folded, Phanerozoic sedimentary sequence. While the spatial density and variation of the fold-related fractures correlate with those of the folds, the fault-related fracture system is localized along isolated linear zones, possibly marking the boundaries of blocks of basement. The fault-related fracture sets, that form around faults, which may has been active since the Precambrian, cut across the fold-related fracture system that have been developing since the Tertiary.

Knowledge of the present-day tectonic stress is an essential issue in petroleum exploration and production, and, in particular, is a key parameter in: borehole stability; reservoir drainage and flooding patterns; fluid flow in naturally-fractured reservoirs; hydraulic fracture stimulation, and; seal breach by fault reactivation. The present-day state of stress is described by determination of the stress tensor. It is commonly assumed that one principal stress acts vertically in sedimentary basins and thus the stress tensor can be simplified to consist of four components, the magnitudes of the vertical, maximum horizontal and minimum horizontal stresses in addition to the orientation of the maximum horizontal stress. Of these four components, determination of the maximum horizontal stress (SHmax) orientation has received extensive attention in recent 20 years, particularly with regards to the control of in-situ stresses on subsurface fluid flow and fault reactivation.

Fractures that are most susceptible to tensile or shear failure in the present-day stress, typically those striking approximately parallel or within 30° of the maximum principal stress orientation, are often observed to transmit the largest volumes of fluids. Furthermore, extensive analysis of flooding operations has observed that fluid flow is enhanced and pumping rates more strongly correlated between well pairs that are located parallel to the present-day SHmax orientation.

The scientific importance of understanding the present-day maximum horizontal stress orientation is further highlighted by the findings of the World Stress Map (WSM) Project, which has spent over 20 years building an extensive freely-available repository of presentday stress information. The 2008 release of the World Stress Map Project contains 21,750 present-day stress indicators from all over the world and reveals the complexity of the global stress pattern. Early studies of the present-day stress field revealed that the primary, plate-scale stress field is controlled by plate boundary forces such as ridge push, slab pull and resistance at continental collision zones coupled with large intra-plate forces such as gravitational body forces near mountain ranges. However, more recent studies have highlighted the significance of smaller-scale perturbations in the stress field, superimposed upon the plate-scale stress pattern, that are often observed at the basin to field scale. Furthermore, it is knowledge of stress orientations at smaller basin and field scales that has critical importance for petroleum applications such as wellbore stability, and hydraulic fracture stimulation.

Knowledge of the present-day stress orientation is particularly important in Iran, which has an extensive and mature petroleum exploration and production industry, and is also prone to stress related geo hazards such as earthquakes. Yet, the 2008 World Stress Map database contains very little present-day stress information for Iran and no stress data from petroleum wells. Indeed, all of the stress data currently available for Iran is derived from earthquake focal mechanism solutions from events that are typically at depths of ten kilometres or more, and which might not be relevant for petroleum applications, particularly in areas possibly detached by salt or low-angle faults. Furthermore, the majority of these earthquake focal mechanism solutions are located along the boundary between the Arabian and Eurasian plates, and there are concerns surrounding the reliability of stress information derived from earthquakes near plate boundaries. For example, stress orientations derived from earthquake focal mechanism solutions along the San Andreas Fault Zone and Great Sumatran Fault are often highly inconsistent with those obtained from more reliable petroleum industry data.