Structure and Regional Context of Onshore Fields in Abu Dhabi, UAE

Abstract

Abstract High quality 3-D seismic and well data allow detailed structural analysis of the major onshore fields of Abu Dhabi. Most of the large onshore fields were examined for this study. The fields are generally asymmetric anticlines with reservoir units in the Mesozoic section. All are interpreted as deepseated (probably Proterozoic) grabens and half grabens that were gently inverted to various degrees in Late Cretaceous time. The timing, distribution, and geometry of structural relief link trap formation to foreland compression during orogenesis and ophiolite obduction along the Arabian margins, particularly in Oman. Traps are cut to varying degrees by remarkably regular sets of cross faults that grew as strain developed in more competent units during structural growth. Some traps display welldeveloped conjugate cross faults indicative of NW-SE or WNW-ESE compression. Other traps have only one set of cross faults, indicative of more oblique transpression at an angle to the underlying basement structure. On 2-D seismic data the cross faults are quite linear and often appear to cut the entire structure for distances of several kilometers. However, 3-D data reveals that the faults are actually fault zones composed of arrays of smaller en echelon faults. These observations indicate oblique offset and show that the fault zones can be interpreted as conjugate shears. Some of the faults are detached, limited stratigraphically, and their vertical offset diminishes from 10's of meters near structural culminations to zero off structure. If true for the majority of faults, this would indicate an origin related to internal strain within the fold, not a major, through-going basement shear zone. However, some faults may not terminate at the structural limbs, but instead could extend much further off structure. This, combined with possible evidence for basement involvement, would support a regional shear or wrench model. Ongoing interpretation is focused on deciding which of these models is appropriate. The cross faults can influence fluid flow and are thus of interest from a trap segmentation and production perspective. In general, steps and relays between fault segments allow diminished fluid communication across the fault zones, but do not completely partition the reservoirs into compartments.