Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
45th Society of Petrophysicists and and Well Log Analysts (SPWLA) Symposium, Noordwijk, The Netherlands, June 6-9, 2004
for Integrated Reservoir Characterization:
Shinichi Sakurai, Bob Loucks, Charlie Kerans, Fred Wang, and Mario Alfredo Mahrx Rojano
The Poza Rica field in eastern Mexico has produced more than 1 billon barrels of oil since 1930 and is the world's largest deepwater, carbonate-slope oil field. The pay zone consists of gravity-flow deposits that were sourced from the Tuxpan platform. Core description from more than 30 wells allows definition of sequence stratigraphic units and rock types. Core-based rock types include debris flow-facies, turbidites, lime muds and shale.
Challenges for petrophysical evaluation are working with old data, presence of both original and expanded gas caps, and estimating rock type from wireline log responses.
More than a half of the 580 wells available for this study were drilled before 1960 and have old GNT logs. Careful GNT normalization carried out by PEMEX was used to estimate porosity along with sonic logs, by using a basic porosity transform. Presence of gas caps made porosity log responses very complicated. Careful review eliminated wells with questionable logs (mostly very old wells) and allowed assigning missing codes in GNT logs in the gas cap. Reasonable porosity was estimated by selecting proper logs and by averaging GNT- and sonic-based porosity values, if applicable.
"Rock Type" based on core description was examined for each cored well, and wireline- log-based "Rock Type" was assigned in the un-cored intervals manually by using an interactive graphical package. For assigning rock type for uncored wells, all the wireline logs, perforation, and production data were displayed along with similar data from the nearby cored wells.
Each "Rock Type" has unique porosity, permeability, and irreducible water saturation relationships. Porosity-permeability relationship for each "Rock Type" was defined to estimate permeability. Irreducible water saturation based on PEMEX database and capillary pressure results against porosity for "Rock Type" groups were used to estimate initial water saturation in the field.
The 3-D petrophysical model was constructed within the stratigraphic framework by using core data and wireline log data.