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.