The paper presents the results of an integrated study comprising different disciplines and scales (core, log and seismic). The study followed the drilling of a well through a Jurassic carbonate reservoir in a NE-SW direction, which was cored near the tip of a NW-SE striking fault. Core and FMI data show a number of open and closed fractures, all striking in a NW-SE direction, contrary to what was expected from the regional structural model.
The structure grew through the Mesozoic with NE-SW extension producing a set of NW-SE striking normal faults and related fractures. A pulse of growth occurred during the Late Cretaceous-Tertiary, followed by continued Tertiary growth. 3D seismic data clearly shows the Lower Cretaceous fault pattern. The faults decrease in throw with depth, and only a limited number cut the Jurassic.
Interference tests show a preferential NE-SW communication pathway between wells. In addition, borehole and core data from other wells show open fractures, which tend to have a NE-SW strike, and a second set of closed fractures orthogonal to these.
The following were concluded from a review of the data:
Correlation between core goniometry and FMI image analysis is excellent.
The well is located near the tip of a NW-SE striking fault, and is likely to be in the transfer zone between two faults.
Two sets of fractures occur in the Jurassic. A fault related NW-SE striking set and a NNE-SSW striking set occurring further away from faults.
Fractures occur as near vertical, broken non-mineralized, and completely healed; and tend to occur in swarms.
Fractures in the reservoir are likely to provide additional permeability.
To intersect the maximum number of open fractures in the Jurassic, wells should be horizontal and oriented WNW-ESE away from faults and NE-SW close to faults.
Introduction and Objectives
The oil well OAD-1 (Offshore Abu Dhabi-1) lies in the northern sector of a major Field located in the central part of the Arabian Gulf. The Field is a gentle, slightly asymmetric domal anticline. Oil and gas have been produced from the Lower Cretaceous, Upper and Middle Jurassic, and Permian. The Upper Jurassic Arab reservoirs are the main oil producing reservoirs.
The Lower Cretaceous Upper Thamama is a highly faulted and fractured carbonate reservoir. The underlying Arab reservoirs are separated from the Thamama by the Hith anhydrite Formation.
The faults in the Arab Formation appear to be vertical continuations of four faults in the Thamama Formation with only the largest faults in the Thamama Formation extending down into the Arab Formation. This indicates that the Hith Formation represents a significant boundary to fault propagation, and therefore probably to fluid flow. There may be mechanical stratigraphy in the Field, with different deformation styles in the different stratigraphic units.
The OAD-1 well was drilled in close proximity to a NW-SE striking normal fault (Fig. 1) as a producer from the Arab D reservoir. Some 140 feet of core were cut and recovered from the Arab D reservoir. The core indicated a large number of open and closed mineralised fractures. A formation micro image log FMI was recorded across the interval and also revealed a number of fractures.
The objective of this paper is to present the results of an integrated study of the fractures encountered in this well.
The structural setting and stratigraphy are reviewed followed by the details of fracture description and comparison with FMI log data. Implications of fractures in the Arab D are also discussed.