deepwater hydrocarbon potentialof orange basin, south ... · soumen dasgupta*, moumita sengupta,...

Deepwater Hydrocarbon Potentialof Orange Basin, South Africa: An Untested Oil Play

Soumen Dasgupta*, Moumita Sengupta, Prem Kumar, Biswanath Ghosh, Dr. BNS Naidu andDr. SudhirMathur. Cairn India Limited, Gurgaon

[email protected]

Keywords

Late Cretaceous Orange river delta, untested deep water play

Summary

Orange Basin is located in South Atlantic Ocean offshorewest coast of South Africa. The basin is under exploredespecially in the deep water areas. This study demonstrateshuge undrilled exploration potential in different plays.This untested play in deeper water outboard area within thegrowth fault domain has very high hydrocarbon resourcepotential.

IntroductionOrange Basin, the largest of South Africa’s offshore basinsis extending from the Luedritz Arch in Namibia to theAgulhas Fracture Zone in the south of South Africacovering an area of about 160,000 km2. More than 7kms ofsediments derived from the Orange River and OlifantsRiver were dumped into the Orange basin throughout theCretaceous. Orange Basin is relatively underexplored withnearly 38 exploration and appraisal wells drilled all withinthe ~400m water depth line. Several petroleum systems areproven to be working in this basin. A-J-1 well drilled in thesynrift graben near to coast is the only oil discovery in thisbasin; Kudu field in north and Ibhubesi field in south arethe commercial gas discoveries so far. However the deepwater late cretaceous growth fault area remains untested.

Regional Geological UnderstandingOrange Basin is a typical rifted-volcanic passive marginand was created with breakup of the South American andAfrican continental plates during Late Jurassic/EarlyCretaceous, followed by rifting and drifting lead to theopening of the South Atlantic Ocean. Regional seismicprofiles and drilled well data indicate different play types inthe outboard in comparison to the inboard proven plays.Margin tilting as a response to the post rift thermalsubsidence coupled with progressive hinterland upliftmentfollowed by huge denudation of the hinterland andpreviously deposited inboard sequence sourced thesediments in the outboard. The latest 3D seismicinterpretation indicates at least two major extensionalepisodes in the outboard. First extension was related to thedestabilization of the Santonian(?)outer shelf which wasaggrading with little progradation in a stable margin, andlead to the gravity collapse like “tilted book shelf” of the

sequences deposited before this tectonic event, and faulteddown detaching over Albian prodeltaic shale. However thenext phase of upliftment during late cretaceous and earlyTertiary caused huge margin instability resulting indevelopment of series of growth faults and toe thrustsystem in the outboard which becomes younger withwesterly moving depocentre filled by substantial sedimentthickness by prograding Orange River delta in an unstablemargin. This substantial sediment thickness provided thenecessary and sufficient burial for the source rockdeposited during Cenomanian/Turonian which found to beimmature in the inboard wells. The immature CT sourcerock proves the source potential however gas prone in someof the inboard wells, but in key outboard wells with twohigh HI samples indicate the improvement of the sourcequality in the offshore. Integrating the depositional historysupported by the 3D and 2D seismic interpretation andincorporating the gross depositional maps petroleumsystem modeling was carried out and the results shows CTsource is oil matured and started expelling oil since KT inthe outboard and likely to feed the outboard structures thatformed before the migration from the source rock tookplace. Many structural leads have been identified in thenew 3D seismic in the outboard area. Integrating theregional interpretation, isopachs, seismic attributes some ofthese leads upgraded and matured to prospects based on theranking exercise considering the resource size andgeological risking.

Methodology

The Study area is covered with various vintages of 2Dseismic data of poor to moderate quality especially in thedeep water areas. The growth fault is so structurallycomplex that the 2D seismic couldn’t image well. CairnIndia acquired a 1981 sq km of full fold 3D seismic data inthe deep water area. This improved the subsurfaceimagesignificantly. The PSTM and PSDM processingappreciably brought out the several structural closureswhich are comparable to other proven growth faultdominated areas analogous to Niger delta or Gulf coastLusiana Frio.

11th Biennial International Conference & Exposition

Deep water under-exploredoil play, Orange basin, South Africa

Integrated approach has been used here to understand thedevelopment of the key tectonic events and there overallimplication to the petroleum system.The regional vintage 2D seismic data were tied with theinboard wells to constrain the stratigraphy. Mapping of themega sequences extended to the new 3D data to understandthe tectonic phases and its implication on the structuredevelopment from inboard to outboard. Regional well dataand literature helped to build up the stratigraphic conceptwhich then incorporated with the seismic facies andgeomorphologies to develop the stratigraphic framework.

Figure 1: Regional geo-seismic section of Orange Basin

Figure 2: Map showing the key tectonic elements ofOrange Basin

Figure 3: Tectono-stratigraphic event chart of deep waterOrange Basin

Figure 4: Flow chart depicting the methodology adapted toassess the hydrocarbon potential of the deep water orangebasin















Structures and Reservoirs prediction from 3D seismic:Outboard 3D seismic clearly definesthe large faultedantiformal structure in Cenomanian to Albian interval nearto the master break away fault extending much beyond the3D area. Cenomanian-Albian sequence tapers towardswest. Several faults cut through the structure with throwsranging from 20-80m. These are observable on varianceattribute extracted on mapped Cenomanian surface. RMSseismic amplitude extracted within window of +/-80mswith respect to Cenomanian surface shows there are brightamplitude package trending north south parallel to shelfedge which is interpreted as possible signature of shoreface sand deposits. Further deep water channelized featuresobserved in the attribute maps and in the seismic whichsuggests possible slope channels.

Figure 6: 3D PSTM section showing the structuralelements in the deep water orange basin

Figure 7: Depth structure map of Cenomanian surfaceshowing the major structural trend in outboard areas

Delta progradation commenced in Orange basin in Albianand well established by Cenomanian. Progdationalgeometry shows a change from strongly progradational toaggradation up to Santonian, before the destabilization tookplace.

Figure 8: Seismic attribute (windowed RMS 80ms +/-)showing the geomorphologies in the Cenomanian to Albiantime in the deep water areas

Regional understanding and the 3D seismic clearlyhighlights the two phases of extensional event in the deepwater. First extension during Santonian (?) destabilized theaggrading outer shelf and leads to the gravity collapse andfaulted down and detached over Albian prodeltaic shale.However the next phase of upliftment during latecretaceous and early Tertiary caused huge margininstability resulting in development of series of growthfaults and toe thrust system in the deep water whichbecomes younger with westerly moving depocentre filledby substantial sediment thickness by prograding OrangeRiver delta in an unstable margin. So the major Growthsequences developed in Campanian to Maastrichtian.

Figure 9: Time thickness map between Aptian toCenomanian interval showing the major depocenter in theinboard areas



















Figure 10: Time thickness map between Cenomanian toKT interval showing the major depocenter in the outboardareas clearly highlighting the shift in depocenter during theLate creatceous

This study also highlights the Fourmajor prospective areasin the deep water:1. Eastern deeper antiformal structure (close to the firstfault) which should have predominantly upper shoreface/shelf sands of possibly Cenomanian to Albian in age2. Western deeper toe-thrust area involving Albian –

Cenomanian sediment (deposited as fans in the deep water3. Shallow first growth areas (mid of the survey) whichpossibly in the shelf edge are of upper Campanian4. Western dominant growth area: possibly LST wedges of

Campanian to Maastrichtian

Petroleum System Analysis:

There are at least two petroleum source systems availablein the deep water area, Aptian marine source deposited in arestricted margin, and Cenomanian-Turonian marine shalesdeposited in open marine condition.Aptian marine shale is proven in this margin as it is thoughtto charge Ibhubesi& Kudu gas field. This sequence wasencountered in the inboard and offset wells with good TOCand HI values. Petroleum modelling suggest that in theoutboard area the matured source rock is presently in gaswindow however it might have expelled oil in lateCretaceous to Paleocene.Another source rock interval is Cenomanian-Turoniantransgressive shales encountered in various inboard wellshowever immature and mostly oil and gas prone. Sourcerock quality is inferred to get better in the outboard area.The huge thickness of Maastrichtian / Campanian deltaprovides the necessary burial for oil maturity for CT sourcein the deep water areas.Basin modelling studies suggestthat the generation from CT source postdates the lateCretaceous structuration. This structure can be charged bythese source rocks vertically and laterally through faults

and fractures.The immature CT source rock proves thesource potential however gas prone in some of the inboardwells, but in key outboard wells with two high HI samplesindicate the improvement of the source quality in theoffshore. Integrating the depositional history supported bythe 3D and 2D seismic interpretation and incorporating thegross depositional maps petroleum system modeling wascarried out and the results shows CT source is oil maturedand started expelling oil since KT in the outboard andlikely to feed the outboard structures that formed before themigration from the source rock took place.

Figure 11: Map showing present day maturity of theAptian source rock associated with Type II-III organicmatter

Figure 12: Map showing present day maturity of theCTsource rock associated with Type II-III organic matter























Conclusions

The current study reveals the deeper water Orange Basinhas huge potential for discovering large oil and gas pools.Cairn identified multiple plays in a gravity drivenextensional /compressional structural regime predating themigration from the deeper proven petroleum systems. Cairnintends to test these plays through the drill bit to unlock thevalue of this exciting petroleum province.

References

BROWN, L. F., BENSON, J. M. ET AL 1995. SequenceStratigraphy in Offshore South African Divergent Basins,An Atlas On Exploration for Cretaceous LowstandTraps by SOEKOR (Pty) Ltd. American Association ofPetroleum Geologists Studies in Geology, 41.

FRANCHETEAU, J. & LE PICHON, X. 1972. Marginalfracture zones as structural framework of continentalmargins in South Atlantic Ocean. American Association ofPetroleum Geologists Bulletin, 56, 991-1007.

JACQUIN, T. & DE GRACIANSKY, P. CH. 1988. Cyclicfluctuations of anoxia during Cretaceous time in the SouthAtlantic Ocean. Marine & Petroleum Geology,5, 359-369.

JUNOSLACER, E. H. A. 1999c. Geological aspects ofpetroleum systems and related exploration plays in SouthAfrica's Orange Basin.Southern Africa and the FalklandsMeeting, Petroleum Exploration Society of Great Britain,29 March 1999, London, Extended Abstracts.

D. Van Der Spuy 2003, Aptian source rocks in some SouthAfrican Cretaceous basinsGeological Society, London,Special Publications 2003, v.207;

Acknowledgement:

The authors wish to thank Cairn India Management forproviding opportunity to publish the work in SPG, Jaipur.A special thanks to PetroSA JV partner to allow for thepublishing this paper.


deepwater hydrocarbon potentialof orange basin, south ... · soumen dasgupta*, moumita sengupta,...

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