Bahrain Field Oil Geochemistry

Geo2004, 6th Middle East Geosciences Conference & Exhibition
Tuesday March 9th, 2003 from 08:25-08:50 in Room 2

Investigation of Fluid Transfer Mechanism in Shallow Oil Zones
of Bahrain Field using Geochemistry

Challa R.K. Murty  (Bahrain Petroleum Company), Hisham K. Zubari  (Bahrain Petroleum Company), Mark  A. Beeunas  (OilTracers, LLC), Mark  A. McCaffrey  (Oil Tracers, LLC), Keith F. Thompson   (Petrosurveys)

Abstract:

Geochemical analyses integrated with Geological and Engineering data has substantially improved in understanding the mechanisms of hydrocarbon emplacement and fluid movement in shallow oil reservoirs of Bahrain field. The field is an asymmetrical anticline, which is intensely faulted and fractured in the shallow and Bahrain zones. The shallow zones include three marine limestone reservoirs ( Rubble, Ostracod & Magwa) which belong to Upper Cretaceous Asia group. In general , Rubble zone is composed of massive carbonates while Ostracod & Magwa have muddier limestone. These reservoirs contain heavy and light oil distributed areally and vertically. Divergent Rock Types and fluid properties and Stratigraphic and Structural features could impact fluid transfer between reservoirs. It is strongly believed that syndepositional faulting might have affected these reservoirs in addition to a major unconformity which formed during the late Cretaceous. This erosional event completely removed the Rubble and part of the upper Ostracod in the central part of the field. Further, fault compartmentalization and the extent of fluid transfer from the three zones due to faulting effect the fluid properties. It is required to know the current oil distribution of light & heavy oil and if these phases occurred during migration or later , after reactivation of faults and reservoir compartmentalization ,the knowledge of which is essential for developing these reservoirs. There is also a question whether gravity segregation or de-asphalting or Bio degradation was responsible for separation into heavy & light oils across the three zones. To investigate the lateral and vertical reservoir continuity a Geochemical study was initiated involving advanced gas chromatography techniques on oil samples from 11 wells. The methods include (a) review of abundance of “ inter- paraffin” peaks identifiable, (b) “Star Diagrams” for finger printing the oils with the ratios of peaks compared with the nearby wells. The C8 to C20 range is typically the most diagnostic range for reservoir continuity assessments using these diagrams. The other technique used was the “ Slope analysis “ where in the molar concentration profiles were used to know the accumulation and alteration. The analyses have provided compositional data that : (1) Characterize the difference between oils in the shallow zones, (2) verify reservoir compartmentalization, (3) Support specific Geological models which explain the inter- and intra reservoir fluid communication, (4) Reveal reservoir oil biodegradation events, and (5) Indicate at least four discrete episodes of hydrocarbon migration into the reservoirs based on the Slope Factor(SF) analysis and several geoochemical parameters. The presentation will discuss in detail the results obtained.