Abstract

A simple salt dome and a few sets of graben faults characterize the structure of the El Bunduq Oil Field, where the Jurassic carbonates comprise the main reservoirs. The understanding of faults and fractures deepened after the 3D seismic survey and the drilling of a horizontal well in 1994-1995. The present study principally deals with the fault geometry mainly based on 3D seismic and the properties of fractures based on borehole image logs. Discussions shall be further extended to sealingness of fault planes, relationship between faults and fractures, and a role of fractures in the drainage systems of hydrocarbons.

Introduction

The El Bunduq Oil Field is a broad NE-SW trending domal structure of 6km×11km, straddling offshore Abu Dhabi-Qatar border in the Arabian Gulf (Fig. 1). The Upper Jurassic Arab Formation and the Middle Jurassic Araej Formation comprise the carbonate reservoirs in the Field.1,2

A shallow marine 3D seismic survey of the Bunduq Field covering a rectangular area of 128 km2 was carried on in June-August 1994 using radio telemetry system. 3D seismic data provided more detailed and accurate horizon and fault interpretations. A greatly increased number of faults were identified on both of new 3D seismic sections and time slices at the Thamama to the Uwainat horizons. The faults match geological/engineering information of wells (missing intervals, pressure test results, borehole image logs).

The graben fault system at the crestal area was clearly recognized especially on time slices. It comprises 3-4 steps faults accompanied with many diversions/conversions. The maximum fault throw there is around 100 feet. A several faults running in NW-SE direction with a few tens feet of throw were newly encountered in the NW and SE flank areas of the field.

A borehole image log (FMI) was run on one horizontal well and one highly deviated well which were drilled in 1995. The FMI images of these wells gave us a lot of precious information on the faults and fractures, which encouraged us to proceed detailed interpretation of previous 6 wells with FMS dataset. All of the faults on the 3D seismic map were cross-checked by FMI/FMS data.

Fault
Fault Interpretation - 3D Seismic.

A two cable swath shooting procedure was used, one swath of data consisting of 54 source lines of 32 shots each. The two cables recorded 112 traces each. The seismic grid is roughly square in shape with inlines oriented NW-SE and crosslines oriented NE-SW. Inlines are approximately 10.7 km long and crosslines are approximately 11.4 km long. Inline and crossline increments are both 0.25 and traces are spaced 12.5 meters apart. There are 911 live inlines and 855 live crosslines.

The 3D seismic data was expected to supersede previous 2D seismic data and provide a clear and complete picture of all resolvable faults both over the crest and on the flanks of the structure. It was expected that the true fault pattern as revealed by the 3D seismic data would be more complex and the faults more numerous than the previous 2D fault model. The basic crestal fault model of broadly parallel faults trending NEE-SWW was expected to be reconfirmed, though with significant differences in details.

Fault Interpretation - 3D Seismic.

A two cable swath shooting procedure was used, one swath of data consisting of 54 source lines of 32 shots each. The two cables recorded 112 traces each. The seismic grid is roughly square in shape with inlines oriented NW-SE and crosslines oriented NE-SW. Inlines are approximately 10.7 km long and crosslines are approximately 11.4 km long. Inline and crossline increments are both 0.25 and traces are spaced 12.5 meters apart. There are 911 live inlines and 855 live crosslines.

The 3D seismic data was expected to supersede previous 2D seismic data and provide a clear and complete picture of all resolvable faults both over the crest and on the flanks of the structure. It was expected that the true fault pattern as revealed by the 3D seismic data would be more complex and the faults more numerous than the previous 2D fault model. The basic crestal fault model of broadly parallel faults trending NEE-SWW was expected to be reconfirmed, though with significant differences in details.

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