An Integrated Geological, Chemical and Engineering Approach to Determine the Reservoir Continuity of the B Field, Onshore Abu Dhabi
Authors: Mohamed Awad A. , Kim Jeoung S. , Al-Khiami Kamel 
Published: December 2007

The B field tested oil in the Upper Cretaceous multi-stacked reservoirs of the Thamama Group. It has been producing oil mainly from Thamama Zone B of the Kharaib Formation (Early Cretaceous) for the last thirty years. The reservoir has a thick oil column with a large gas cap and an extensive aquifer. The geological, structural and stratigraphic analyses, have shown no significant evidence of discontinuity, and therefore the reservoir was considered a massively continuous interval. Initially minor indications of possible compartmentalisation can be inferred from the variability in formation water salinity measured in different part of this reservoir.

The structural configuration of this field is based on very limited seismic data and the use of alternative techniques to determine the reservoir continuity is adopted.

A detailed geochemical investigation was conducted using long range gas chromatography. Initially, a pilot study was carried out involving three samples, one from the northern and two from the southern part of the field. The chromatograms were digitised and the peak heights were measured. the correlation of the molecular composition of the oil was achieved through peak ratios and star diagrams representation. The results indicate the possibility of lateral compartmentalisation. Follow-Up studies carried out by independent laboratories have confirmed these preliminary results. Three possible compartments were identified laterally across the field, at Zone B level. Thamama Zone B Upper and Lower proved to be vertically continuous. Thamama Zones A, B, C & E show strong signs of vertical isolation from each other.

These results were substantiated by extensive reservoir engineering data analyses. Lateral strike slip faults (escaped detection by seismic) filled with calcite, are the most likely sealing structures that cause the observed areal discontinuity. Vertically, the dense limestones between Thamama Zones A, B, C &F may have acted as lithological seals and caused the vertical isolation among these zones.

Introduction

The B field is located in north central Onshore Abu Dhabi, approximately 85 km to the south of Abu Dhabi (Fig. 1).

The field discovery in 1953 and subsequent development were based on a very limited seismic and geological data.

The B field tested hydrocarbons in a series of stacked reservoirs mainly from the Thamama Group (Early Cretaceous). It produces oil predominantly from the Thamama Zone B of the Kharaib Formation. The Zone B reservoir has a thick oil column with a large gas-cap and an extensive aquifer. The initial reservoir pressure was 4,178 psig at a datum of 8,300 feet subsea. The original gas-oil contact of the reservoir was estimated to be at 8,220 feet subsea, while the oil-water contact of the reservoir was observed to range from 8,600 feet to 8,700 feet subsea.

Production from B reservoir of the B field began early October 1963. Water injection into the downdip flank area started in March 1974 to maintain reservoir pressure and control gas-cap expansion. Most oil production came from the southern and central portion of the field. A recent development plan that calls for injecting lean hydrocarbon gas into the B-lower Zone of the crestal area near the gas-cap was approved for the northeast side of the field.

The primary objectives of this study were to examine the areal and vertical continuity of the Thamama Zone B reservoir. The secondary objective was to check the vertical communication among the stacked Thamama reservoirs of Zones A, B, C & E (Fig. 2). Geological, geochemical and engineering techniques were integrated for this purpose.