Characterisation of a Tight Gas
Reservoir using Seismic
Attributes, Ohanet Field, Algeria
By
Michael Jamieson1, Joe Boztas1, Hocine Khemissa2, Spencer Quam1
(1) BHP Billiton Petroleum, London, United Kingdom (2) SONATRACH, London, United Kingdom
The Ohanet Field is situated in the Illizi Basin 400 km. southwest of Hassi
Messaoud, Algeria. The field is a NW - SE trending, fault-bounded anticline 35
km. long by 5 km. wide, and has 2 productive horizons: Silurian fluvio-deltaic
sands and Ordovician glacio-marine sands. In July 2000 a Risk Service Contract
was executed between SONATRACH and a joint venture led by BHP Billiton Petroleum
with Japan Ohanet Operating Company, Petrofac Ohanet, and Woodside as partners
to commercialize the gas reserves in the 2 reservoir intervals, together with
reserves in 2 other reservoirs. Included in the work commitment was the
recording of ~1000 sq. km. of 3D seismic
.
The primary Ordovician reservoir consists of tight (0.5-40 md. permeability)
turbiditic sands consisting largely of 2 facies: stacked channels, and thick
lobes displaying mega-ripple marks. The sand is 30-50 m. thick, which is just
above seismic
tuning thickness, and is acoustically hard compared to the
encasing shales. Petrophysical
modeling
predicts that the presence of porosity
reduces the positive impedance contrast between the overlying shale and
reservoir sand, such that the reflection strength weakens. A crossplot of well
average porosity vs amplitude provides confidence that
seismic
attributes can
predict areas of better quality reservoir.
Seismic
attributes were extracted from the 3D
seismic
data, and were used to
statistically guide population of reservoir properties in a geocellular model.
The attribute work revealed geological features that tied with facies determined
from core in old wells, and that were substantiated in newly drilled wells. The
resulting model is being used to guide the continuing development of the
reservoir, and to predict ultimate recovery.