Diagenesis and reservoir quality of the lower Cretaceous sandstone in Orange Basin, South Africa.
O.A. Fadipe, P.F. Carey, A. AkinluaDepartment of Earth Sciences, University of the Western Cape,
Private Bag X17, Bellville, Cape Town. South Africa.
Introduction:The economic success of any prospectultimately depends on reservoir systemperformance. The reservoircharacterization of the Lower Cretaceoussandstone Orange basin wasinvestigated using a multi-mineral andgeochemical analysis approach coupledwith pore water geochemistry. Thesesandstones are siliciclastic in nature andthe sediments within the Orange basinmay be as old as the Jurassic, but the
3369
3370
3371
3372
3373
3374
3375
3376
3377
7.2 7.3 7.4 7.5 7.6 7.7 7.8
pH values
Dep
th(m
)
pH
3445
3450
3455
3460
3465
3470
3475
0 2 4 6 8 10
pH values
Dep
th(m
)
pH
Conclusions:Detrital frame work constituents of theAlbian sandstones are dominated by
Results:
UNIVERSITY of the
WESTERN CAPE
may be as old as the Jurassic, but theoldest recorded sediment date backHauterivian. The sediments within thebasin are continental in origin & aredeposited together with volcanicsediments.
Methodology:
KCh
Ch
Micaplagioclase
Q
Mica
Q
Oil show
Ch
30.00
40.00
50.00
60.00
70.00
80.00
CIA
A-G1
A-W1moderate to high degree of weathering
low degree of weathering
1.50
2.00
2.50
3.00
K2O
(wt%
)
A-G1
A-W1
Quartz-rich
Quartz-intermediate
Albian sandstones are dominated byquartz, feldspar and subordinate rockfragments and micas. The relativetiming of diagenetic processes in thetwo studied wells (A-G1 & A-W1) aresuggested to be mica and K-feldsparweathering and alteration to kaolinite,pore filling and alteration of kaolinite toillite especially in A-G1. This wasfollowed by the ductile deformation ofdetrital chlorite and the formation ofquartz overgrowths while themovement of formation waters was stillrelatively unrestricted and porosity andpermeability much higher. Kaolinitelocally developed as vermiform andaccelerated the minor porosity lossdue to pore occlusion. Kaolinite is aby-product of feldspar leaching in thepresence of acidic fluid producedduring the maturation of organic matter
A-G1 A-W1
Two wells were selected for thisstudy, after proper understandingof the Geology of Orange Basin,different wire line logs wereacquired and loaded into the petrelworkstation where necessaryediting & quality control wereperformed on the logs. Thereservoir intervals were identified& samples were picked at thedepths of interest for petrographic& geochemical analysis whichincludes SEM and HR-TEM, XRF& pore water geochemistry.
Average sandstones Average Albian age sandstone( Pettijohn., 1987)
Major Oxides (%) Quartz arenite Lithic arenite Grey wacke Arkose Major Oxides (%)A-G1 A-W1SiO2 95.4 66.1 86.7 77.1 SiO2 63.14 53.81
Al2O3 1.10 8.10 13.50 8.70 Al2O3 10.08 8.55Fe2O3 0.40 3.80 1.60 1.50 Fe2O3 13.20 18.03MgO 0.10 2.40 2.10 0.50 MgO 1.59 1.97CaO 1.60 6.20 2.50 2.70 CaO 1.10 6.67Na2O 0.10 0.90 2.90 1.50 Na2O 1.26 1.42
K2O 0.20 1.30 2.00 2.80 K2O 3.57 1.43
TiO2 0.20 0.30 0.60 0.30 TiO2 2.12 3.07
P2O5 - 0.10 0.20 0.10 P2O5 0.15 0.19MnO - 0.10 0.10 0.20 MnO 0.08 0.19
Na2O/ K2O 0.50 0.69 1.45 0.53 Na2O/ K2O 0.43 1.50
SO3 - - 0.30 - SO3 0.16 0.68
0.00
10.00
20.00
30.00
0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00
SiO2 (wt%)
degree of weathering
average shale
0.00
0.50
1.00
0.00 2.00 4.00 6.00 8.00
Na2O (wt%)
Quartz-intermediate
Quartz-poor
-1.00
-0.50
0.00
0.50
1.00
1.50
-0.50 0.00 0.50 1.00 1.50
Log (SiO2/Al2O3)
Log
(N
a2O
/K2O
)
A-W1
A-G1
Litharenite
Greywacke
Subarkose
Sublitharenite
Arkose0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
0.00 0.20 0.40 0.60 0.80 1.00
Log (SiO2/Al2O3)
Log
(Fe2
O3/K
2O)
A-G1
A-W1
Fe-shale
Fe-sand
wacke
LithareniteSublitharenite
during the maturation of organic matterin the adjacent Hauterivian age sourcerock. The pore water geochemistryrevealed a combination of processesincluding late burial precipitation of Fe-chlorite, meteoric water infiltration inthe studied wells. This is characterizedby the alternate decrease and increasein pH values along the depths. Basedon Pettijohn et al., (1972) classificationmodified by Herron (1988), thereservoir sands were characterized aslitharenite and greywacke for A-G1 andA-W1 respectively with both having asignificant amount of iron.
Acknowledgement:This research was supported by InkabayeAfrica and PetroSA. The authors aregrateful to PASA for their authorization topublish.
References:Pettijohn, F.J.., Potter P.E., & Siever, R. (1987). Sand and Sandstone. SpringerVerlag, New York, 553p.Pettijohn, F.J, Edwin, P., Siever, R., (1972). Sand and Sandstone diagenesis: recent andancient- Google Book pp. 462. Available Online at http://books.google.co.za/books
