Quantifying the impact of the structural uncertainty on the gross rock volume in the Lubina and Montanazo oil fields (Western Mediterranean)
Authors: Bárbara Carla Patricia , Cabello Patricia , Bouche Alexandre , Aarnes Ingrid , Gordillo Carlos , Ferrer Oriol , Roma Maria , Arbués Pau 
Published: September 2019

Abstract. Structural uncertainty is a key parameter affecting the
accuracy of the information contained in static and dynamic reservoir
models. However, quantifying and assessing its real impact on reservoir
property distribution, in-place volume estimates and dynamic simulation has
always been a challenge. Due to the limitation of the existing workflows and
time constraints, the exploration of all potential geological configurations
matching the interpreted data has been limited to a small number of
scenarios, making the future field development decisions uncertain. We present a case study in the Lubina and Montanazo mature oil fields
(Western Mediterranean) in which the structural uncertainty in the seismic
interpretation of faults and horizons has been captured using modern
reservoir modeling workflows. We model the fault and horizon uncertainty by
means of two workflows: the manually interpreted and the constant
uncertainty cases. In the manually interpreted case, the zones of ambiguity
in the position of horizons and faults are defined as locally varying
envelopes around the best interpretation, whose dimensions mainly vary
according to the frequency content of the seismic data, lateral variations
of amplitudes along reflectors, and how the reflectors terminate around
faults when fault reflections are not present in the seismic image. In the
constant case, the envelope dimensions are kept constant for each horizon
and each fault. Both faults and horizons are simulated within their
respective uncertainty envelopes as provided to the user. In all
simulations, conditioning to available well data is ensured. Stochastic
simulation was used to obtain 200 realizations for each uncertainty modeling
workflow. The realizations were compared in terms of gross rock volumes
above the oil–water contact considering three scenarios at the depths of the
contact. The results show that capturing the structural uncertainty in the picking of
horizons and faults in seismic data has a relevant impact on the volume
estimation. The models predict percentage differences in the mean gross rock
volume with respect to best-estimate interpretation up to 7 % higher and
12 % lower (P10 and P90). The manually interpreted uncertainty workflow
reports narrower gross rock volume predictions and more consistent results
from the simulated structural models than the constant case. This work has
also revealed that, for the Lubina and Montanazo fields, the fault
uncertainty associated with the major faults that bound the
reservoir laterally strongly affects the gross rock volume predicted. The multiple
realizations obtained are geologically consistent with the available data, and their differences in geometry and dimensions of the reservoir allow us
to improve the understanding of the reservoir structure. The uncertainty modeling workflows applied are easy to design and allow us to
update the models when required. This work demonstrates that knowledge of
the data and the sources of uncertainty is important to set up the workflows
correctly. Further studies can combine other sources of uncertainty in the
modeling process to improve the risk assessment.