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