South Africa - Inkaba.org

Petrophysical evaluation of the Albian Age Gas Bearing Sandstone
reservoirs of the O-M field, Orange Basin South Africa
Opuwari, Mimonitu 1 , Paul Carey 2 , Escordia de Poquioma 3
1.
Department of Earth Sciences, University of the Western Cape - Cape Town, SA -
mopuwari@uwc.ac.za, opusabem@gmail.com
2.
Department of Earth Sciences, University of the Western Cape - Cape Town, SA - pcarey@uwc.ac.za, paulcarey@live.com
3.
ABSTRACT
Petrophysical Evaluation of the Albian age gas bearing sandstone reservoirs of the O-M field,Orange basin has been
performed. The main goal of the research is to evaluate the reservoir potential of the field through the integration and
comparison of results from core analysis,production data and petrography studies for the evaluation and correction of
key petrophysical parameters from wireline logs which could be used to generate an effective reservoir model.A total of
ten (10) wells were evaluated and twenty eight sandstone reservoirs were encountered of which twenty four are gas
bearing and four are wet within the Albian age depth interval of 2800 to 3500m. six lithofacies (A1,A2,A3,A4,A5 and
A6 ) were grouped according to textural and structural and grain size .This study identifies the different rock types that
comprise reservoir and non reservoirs. Pore throat was estimated with the application of the winland’s method for
assessment of reservoir rock quality on bases of pore throat radius.Results from the winland method presents five
petrofacies (mega porous,macro porous,meso porous,micro porous and nanno porous).The best petrofacies is the mega
porous rock type which corresponds to lithofacies A1 and A2. The nanno porous rock type corresponds to lithofacies
A6 and was classified as non reservoir rock. The volume of clay model from log was taken from the gamma-ray model
corrected by the Steiber equation which was based on the level of agreement between the log data and the X-Ray
Diffraction clay data. The field average grain density of 2.67 g/cc was determined from core data and was used to
estimate porosity from the density log. Reservoir rock properties are generally good with reservoir average porosities
between 10 – 22 %, and permeability ranges from 6 to 1000mD. Log calculated water saturation models were calibrated
with capillary pressure and conventional core determined water saturations, and the Simandoux shaly sand model best
agree with capillary and conventional core water saturations and was used to determine field water saturations. The
study also revealed quartz as being the dominant mineral in addition to abundant chlorite as the major clay mineral. The
reservoirs evaluated in the field are characterized as normally pressured with an average reservoir pressure of 4800 psi
and temperature of 220 o F. An interpreted field aquifer gradient of 0.44 psi/ft (1.01 g/cc) and gas gradient of 0.09 psi/ft
(0.2 g/cc) were obtained from repeat formation test measurements.
KEYWORDS: Petrophysical, Gas Bearing Sandstone, O-M field, Orange Basin
68