J. geol. Soc. London, Vol. 140, 1983, pp. 423-430, 7 figs., 1 table. Printed in Northern Ireland.

Petroleum geochemistry of the Southern Santos Basin, offshore

M. J. Gibbons, A. K. Williams, N. Piggott & G. M. Williams

SUMMARY: Significant thicknesses of hydrocarbon source rocks (up to 1000m) were identified in 1-SPS-17, l-SCS-4A, -5 and -6, in the Southern Santos Basin. These marine of Cenomanianto ages generally containl-2.5% organic carbon. Pyrolysis maximum potential yields are moderate rather than good, averaging 3-4 kg hydrocarbons/tonne of rock over thehydrocarbon source intervals. Oil will be the principal product, although horizons of gas-prone kerogens are also present. Palaeotemperature indicators(spore colour and reflectance measurements) show that the basin is characterized by relatively low maturation gradients; calculated depths to the oil generation threshold (OGT) rangefrom c. 3600-4500111. The identified source rocks generally lie within f500 m of the OGT and the presence of 1000-1500 m of and later sediments therefore restricts the time available for significant generation and migration to, at most, about 10 Ma, i.e. from late Miocene onwards. However, a variety of basin models predict substantially greater maturities and, in some instances, suggest oil generation commenced in Palaeogene times. Direct measurements of evolved hydrocarbons give support to some of the modelling predictions by apparently demonstrating hydrocarbon evolution at depths less than the palaeotemperatureOGTs. The absence of significant hydrocarbon accumulations is probably a function of inadequate source richness and the short time available for generation. Migration from thick source beds bounded above and below by non-generative shales may also be a problem.

Geological setting Late4. -AlbianRegional subsidence, lime- stone depositionon inner The Santos Basin lies entirely offshore and has an area shelf. of about 160,000 km2 (Fig. 1). The landward margin of 5.Late -Coniacian Continuing regional subsi- this elongate -basin is bounded by a system of dence; deposition of argil- normal faults controlled by the structural grain of the laceous sediments on outer Precambrianbasement. Seaward,the prograding shelf and upper slope; ulti- sedimentary wedge thins over the volcanic Sao Paulo mate progradation of mar- Plateau. In theN, the Cab0 FrioArch divides the ginal marine sands. Santos Basin from the petroliferousCampos Basin, hiatus.6.Regional 7. -MaastrichtianProgradation of fluvio- and in the S the Torres Arch separates the Santos and deltaic clastic sediments, Pelotas basins. distally passing intomar- No data are available from below a thick sequence ginal marine sands and, of Neocomian , and these volcanics are effec- ultimately,into deeper tively economicbasement for most of the Santos marine argillaceous sedi- Basin. The basalts are usually overlain by a layer of ments. BarremianiAptian salt, whose original thickness may 8. Middle-late Renewedsubsidence;de- position of argillaceous have been c. 1000 m, and by a maximum of c. 9000 m sediments onouter shelf of Cretaceous to Quaternary sediments (Fig. 2). The and upper slope. geological development of the basin can be divided 9. EarlyPalaeocene Regional hiatus. chronologically into the following stages: 10. Palaeocene-OligoceneRenewed subsidence and deposition of marine argil- 1. NeocomianExtrusion of basalts associ- laceous sediments. ated with theopening of 11. Miocene Progradation of shelf lime- the South Atlantic. stones; distal argillaceous 2. ?Neocomian-AptianDeposition of fluvial and sedimentation continued. lagoonal clastic sediments Miocene-Recent12. Renewed subsidence and (cf. ). deposition of predominant- 3. Aptian Deposition of . ly argillaceous sediments.

0016-764918310500-0423$02.00 0 1983 The Geological Society

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Maturation Two types of maturation parameters were used- vitrinite reflectance andspore colour measurements. Data quality were variable due to problems of caving, reworking, bleaching or insufficient organic matter. Thedata show thatthe area is characterized by relatively low maturation gradients, with OGTs in the range 3600-4500 m (Fig. 4). In those wells with known , theOGT lies in sediments of late Cretaceousage. The relatively great depth of the regional OGT would appear to have adverse effect on the prospectivity of the SantosBasin. All potential source rocks are of to Coniacian age and generally occur within f500 m of the OGT. Thus, the presence of 1000-1500m of Miocene andlater sedimentsapparently restricts the time available for significant generation to,at most, about lOMa,i.e. from late Miocene onwards. The regional geothermal gradient of 29"Clkm is not significantly below the world-wide average for basins of this type of 30"CIkm. The high thermal conductivity of salt appears to have had only very local effects on maturity in the immediate vicinity of the 0 100 200krr various salt structures. SCALE Source rocks On the basis of the available geochemical data, the only possible sources of the observed hydrocarbon shows aremature deep-watermarine shales of FIG. 1. Regional setting of the Santos Basin Cenomanian to Coniacian ages. Appreciable thicknes- ses of shales with provensource potential occur in TheSantos Basin contains many large structural wells 1-SPS-17 (525 m), 1-SCS-4A (1000 m), 1-SCS-5 traps, most of which arethe result of halokinesis. (225 m) and 1-SCS-6 (550m) (Table 1, Fig. 4). No Characteristically, there is a transition from gentle salt other analysed sediments had significant hydrocarbon pillows aroundthe basin margin to salt diapirs and source potential. ridges in thedepocentre. Additionally, traps Characteristically,these shales contain l-2.5% occur along theinner basin margin and drape organiccarbon with occasional richer horizons. structures over highs occur in the S of the Pyrolysis maximum potential yields are moderate basin. Thereare potential sandstonereservoirs of rather than good, averaging 3-4kg of hydrocarbons1 non-marine to marginalmarine origin in the late tonne of rock over the source intervals. The organi- Cretaceoussequence. Late Aptian - earlyAlbian cally richest horizons give the highest yields, with a also have someporosity. Seals forthese maximum value in well 1-SCS-5 of 12.9 kgitonne. potentialreservoirs are provided by overlying mud- Pyrolysis gasioil generation indices (GOGI) indicate stones and laterally adjacent salt. that oilwill be the principal product, although 32 wells have been drilled in theentire basin to potentially gas-rich horizons are present.Measure- date, 6 by BP Development Brazil Limited ments on desorbed and extracted hydrocarbons show in the southern part of the basin (1-SPS-9, -12, -17, that detectable, though not necessarily significant, oil l-SCS4A, -5 and-9A; Fig. 3). No commercial generation is occurring in wells1-SPS-17 (below c. discoveries have been made, but oil andlor gas shows 3800 m), 1-SCS-4A (c. 3700 m), -5 (c. 4000 m) and -6 of various intensities have been recorded in Pennzoil (c. 3900 m). 1-RJS-72 and -81, Esso 1-SPS-6 and -7, Pecten SomeSouth Atlantic basins contain anoxic sedi- 1-SCS-6 (gas and 46" API condensate from an Albian ments of Cretaceous age with higher hydrocarbon ) and 1-SPS-18, and BP 1-SPS-9 (32-33" API sourcepotentials than the Santos Basin shales. oil from a Coniacian ) and -12. Consideration of the history of the opening of the

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NW

ALBIAN CARBONATE PLATFORM 1 PROGRADINGCONlAClAN DELTA SANDS i APT l BARR SALT M-L MAAS SHALESA FIG.2. Schematic cross-section of the Southern Santos Basin, Brazil. South Atlantic, based on the work of Campos et al. deep-waterconnection was established between the (1975), McCoy & Zimmerman (1977), Sclater et al. Cape and Argentine basins and the Indian Ocean, so (1977), Van Andel et al. (1977) andNatland (1978), that relatively free circulation and generally oxic provides a possible explanation for this observation. conditions developed. Surface waters from the North The SouthAtlantic began to open c. 125-130Ma Atlanticprobably reached the Angolaand Brazil ago in the Neocomian (Fig. 5A).Ocean basins basins from Middle or late Albian times, but there was developed initially in the S and latterlyin the N. no deep-waterconnection until the or Anoxic sediments weredeposited onthe Falkland Santonian. Plateau and in the Magallanes Basin during this time. Following cessation of evaporitedeposition, the At the earliest stage of rifting and drifting, volcanics restrictedktratified nature of the Angola and Brazil were being extruded in the Santos Basin. An epicon- basins led tothe development of anoxic sediments tinental sea may have existed through and to the E of throughout the Albian. After the Albian the pervasive the basin, but it is more likely that filled the rift anoxic conditions disappeared, possibly as a result of valley. Towards theend of this volcanic phase,the flushing of the basins by water from the S (? and N). Jiquiaevaporites were formed in the SantosBasin, However, anoxic conditions continued to occur inter- heralding the northwardsadvance of the South mittently until Santoniantimes, and the potential . source rocks of Cenomanian to Coniacian age in the By Aptian times (Fig. 5B) all the oceanic basins of Santos Basin weredeposited during this period of the South Atlantic had formed. In the N, the Angola fluctuating anoxicloxic conditions. The moderate qual- and Brazil basins wereisolated from theNorth ity of these source rocks compared to other basins in Atlantic, and the prevailing arid climate resulted in the region may be consequencea of therather extensive deposition. The climate of the isolated, marginal position of the Santos Basin. It may basins to the S of the Rio Grande and Walvis ridges have been better mixed and less prone to anoxicity was not arid and there was widespread deposition of than other basins. For example, during the Albian, the anoxic sedimentsas a result of stratification of the optimumperiod for anoxic deposition in the Brazil water column through restriction of circulation. Basin, limestones were being deposited in the Santos Fig. 6Arepresents the SouthAtlantic mid-way Basin. through the Cretaceous. During Albian-Cenomanian and Africa had separated sufficiently times, the FalklandPlateau cleared Africa anda by65 Ma (Fig. 6B) and the Rio Grande and Walvis

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4 FORMER BP ACREAGE 0 ( RELINQUISHED 1981)

FIG. 3. BP acreage and well locations, Southern Santos Basin.

Ridges had subsidedenough for both surface and 27-3l0C/km were calculated from the available data. deep-water circulation to occur throughout the Atlan- For wells where no temperature data were available, tic. The establishment of open-ocean oxygenated the mean value of 29”Cikm was applied. conditionscurtailed anoxic sedimentation along the Two types of model were applied: Waples’s (1980) Brazilian continental margin. modification of Lopatin’s method,and Tissot’s (ex- panded version in Tissot & Welte 1978). Basin modelling The principal aim of geochemical basin modelling is to Waples’s model determinethe timing andextent of hydrocarbon Lopatin (1971) developed a method for taking into generation. This study was restricted to the southern account time and temperature in kerogen maturation part of the Santos Basin (location of all current BP andhydrocarbon generation. Essentially kerogen acreage) and utilized stratigraphic information and a degradation is assumed to be a first order reaction limited amount of temperaturedata from 11 wells; (following Arrhenius’s law) with reaction rates doubl- (1-SPS-5, -8, -9, -11, -12, -17, 1-SCS-1, -4A, -5, -6 and ing every 10°C rise in temperature. Waples (1980) 1-PRS-1). Area1 well control was only moderate and, calibrated Lopatin’s time-temperature index (TTI) of combined with the lack of temperaturedata, made maturity with vitrinite reflectance to create a model to accurate volumetric assessment of basin hydrocarbon predict the thermal conditions under which hydrocar- potential unrealistic.Incomplete sets of temperature bons could be generated and preserved. data and mud circulation times made it impossible to The first step is to reconstruct the geological history of calculate true formation temperatures using Horner the section of interest through construction of a burial plots. However, overallgeothermal gradients of diagram with a superimposed temperature grid (10°C

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Om

KEY 1000 SOURCE ROCK INTERVAL

OILGENERATION THRESHOLD 2000 t OIL SHOW 3000 t GAS SHOW 4000 ..... SANDSTONE

SILTSTONE 5000 H___ SHALE

LIMESTONE

FIG.4. Well correlations in the Southern Santos Basin,with locations of source rocks, oil generation thresholds and hydrocarbon shows.

spacing). Through necessity, present thicknesses have of section have been recognized in the Santos Basin been extrapolated back through geological time since (of Santonian - early Campanian and mid-Palaeocene there were insufficient data to make adjustments for ages, respectively). Where applicable, small adjust- compaction, eustatic changes in sea level or variation mentswere madeto the subsidence curves through inpalaeo- water depth. The temperature grids were calculating thicknesses of lost sediments by projecting constructed by calculating present day geothermal sedimentation rates immediately prior to uplift. Fortu- gradients and assuming that the gradients and sea floor nately, no prolongedbreaks occurred and the rela- temperature (taken as 10°C) have remained constant tively continuous history of sedimentationmakes throughout the time covered by the reconstructions. the Santos Basin particularly suitable for modelling. Two regional unconformities involving probable loss Following construction of the burial diagrams,a

TABLE1: Cenomanian-Coniacian source rock data.

Average Pyrolysis yields Average Well TOC AV.P2(kglT) AV.oil GOGI 1-SCS-4A 1.2 2.8 - - (3700-4700 m) (0.9-2.0) (1.3-6.4) 1-SCS-6 2.7 0.32 3.2 2.2 (3950-4500(0.22-0.74) m)(1.0-5.3) (1.2-7.4) 1-scs-54.7 1.6 0.39 2.8 (3950-4165(0.29-0.5) m)(1.9-12.9) (0.5-3.6) 1-SPS-17 1.8 0.24 4.0 3.0 (3640-4125(0.15-0.3) m)(1.5-6.1) (1.5-2.1)

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60"s'

K€ V BATHYMETRICINTERVALS LAND AREAS 0o -zoo m FIG. 5. Schematicpalaeo-reconstruction of the 200 - 3000 m South Atlantic (after Sclater ef al. 1977, van Andel er al. 1977). A, ; B. Aptian. Key as for Fig. 6. FIG. 6. Schematicpalaeo-reconstruction of the :-:.:.::.: 4000- 5000 m South Atlantic (after Sclater et al. 1977. van Andel U et al. 1977). A. Albian-Cenomanian; B. end- m 5 5000 m Cretaceous.

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CRETACEOUS I TERTIARY IVa IHt lBr IAp Be 10"

30" -1

60" GY -2 aW r 3 az W I 90" S ' U -3 I-

IMMATURE 120" IN OIL WINDOW -4 m

-5

FIG.7. Well SCS-6. Burial diagram showing oil window for Cenomanian-Turonian source (Waples method) modified Lopatin method was applied to calculate a contrast, only in wells 1-SPS-9 and 1-SCS-4A have progressive TT1 for the horizons of interest in each Turonian to Coniacian source rocks achieved sufficient well. The TTI, or total maturity of a bed, is defined as depth of burial for significant hydrocarbon generation. the sum of the maturitiesacquired in each 10°C Such predictions are based on maturity calculations temperature interval. The TT1 can be correlated with only and take no account of source rock quality. geochemical maturityindicators such as vitrinite Thedependence of timing of generation on the reflectance andspore colours, and,therefore, geothermal gradient was tested by raising the calcu- accepted oil generation thresholds, i.e. latedgradient for well1-SCS-6 from 27"Cikm to 30"Cikm. Such 10% increase transferred the Cenoma- TT1 R,,% TA1 nian-Turonian source interval from the OGT to the 7 0.55 2.55 OGT OGM (i.e. significantly raised the oil window). Such 35 0.8 2.8 OGM an effect demonstrates the temperature sensitivity of 160 1.3 3.2 Oil floor the model and emphasizes the needfor accurate Thus 'oil-windows' can be addedto the burial geothermal data. diagrams, allowing prediction of timing of generation Tissot's model in those beds identified as source rocks. For example, the burial diagram for well 1-SCS-6 is shown in Fig. 7. 4 wells (1-SPS-17, 1-SCS-4A, -5 and -6)with For source rocks at the base of the Cenomanian, the established source rock potentialand whose burial burialdiagrams indicated that oil generation com- diagrams indicated adequate levels of maturity, were menced during the or Miocene ( in analysed by a development of a model of petroleum the case of 1-SPS-17). Maturity for peak oil generation generation first proposed by Tissot (see Tissot & (50% kerogenconversion) was attained by thelate Welte 1978). By assuming the source rock kerogen in Miocene in wells 1-SPS-9, -11, -17, 1-SCS-4A and 16, the Santos Basin is Type I1 (autochthonousmarine a consequence of rapidNeogene sedimentation. In organic matter deposited in a reducing environment),

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Generation Level Timing Well Level Conclusions 1-SPS-17 OGT 20 Ma (early Miocene) OGM Present day The differencesbetween locations of hydrocarbon 1-SCS-4A OGT 30 Ma (late Oligocene) generation thresholds predicted by the models and the OGM 12 Ma (late Miocene) evidence of petroleum generation, and those derived 1-SCS-5 OGT 15 Ma (mid-Miocene) from palaeotemperature indicators are disturbing, but OGM Present day not crucial in terms of prospectivity in this particular 1-SCS-6 OGT 20 Ma (early Miocene) basin. The fundamentalconstraint on the petroleum OGM 5 Ma () potential of the Santos Basin is the relatively low Although this approach has certain limitations, the organic richness of the source rock intervals. In generationthresholds listed are in partialagreement addition, the relatively shorttime available for with those obtained from the first approximation significant generation means that a sufficient volume burial diagram model. of hydrocarbons may not have been able to migrate out of the thick source rock sections to form Comparison with geochemical maturation commercial accumulations. The presence of non- indices source shales lying above and below the source rock The models predict maturities for the Santos Basin intervals is an additional factor that may be hindering wells substantially greaterthan those suggested by migration;conversely, the presence of sandstone vitrinite reflectance (Ro)and spore colours. The latter layers interfingering with the source rock sections indicatemarginal maturity for the source rocks with would probably enhance hydrocarbon movement, and values characteristic oi the OGT (i.e., R. = 0.55%), mature source bed sequences of this type should be whereas the modelspredict R. of c. 0.74.8% (i.e. more prospective. characteristic of the OGM). However, palaeotempera- ture methods are only indirect indications of maturity ACKNOWLEDGMENTS.We wish to acknowledge the following and must be calibratedthrough measurements of contributions: Dr H. A. Bockmeulen and Mr S. R. Andrews desorbed (cuttings gas) or extractedhydrocarbons. who completedadditional geochemical analyses for wells The few trustworthy analyses available suggest that 1-SPS-17, 1-SCS-5 and 1-SCS-6; DrG. Dungworth who hydrocarbon generation (not necessarily significant) is supervised the geochemical modelling program and Dr A. J. occurring at c. 3800 m in 1-SPS-17 (OGT at c. 4500 m, Fleet who wrotethe section on theopening of theSouth Atlantic (all GeochemistryBranch, BP Sunbury). We also according to palaeotemperature indicators), c. 3700 m wish to thank Pecten Do Brasil, Servilos de Petroles LTDA in l-SCS-4A (OGT at c. 4000 m), c. 4000 m in 1-SCS-5 for permission to publish geochemical results from well (OGT at c. 4250 m) and c. 3900 m in 1-SCS-6 (OGT at 1-SCS-6, andPetrobras, Rio deJaneiro, for permission to c. 4000 m). Since the directly measuredgeneration publish this paper. References CAMPOS,C. W. M,, MIURA,K., & REIS, L. A. N. 1975. The Atlantic. In: BOLLI,H. M,, RYAN, W. B. F., et al. Initial East Brazilian continental margin and petroleumpros- Rep.Deep Sea Drill Proj. 40, 1025-62. Washington (US pects. Proc.9thWorld Petrol. Congr. 2, 71-81. Government Printing Office). LOPATIN,N. V. 1971. Temperatureand geologic time asSCLATER, J. .G.,HELLINGER, S. CG TAPSCOTT,C. 1977. The factors in coalification (In Russian). Akad.Nauk SSSR palaeo-bathymetry of theAtlantic Ocean from the Izv.Geol. Ser. 3, 95-106. Jurassicpresent,the to J. Chicago,Geol. 85, 509-52. McCou,F. W. & ZIMMERMAN,H. B. 1977. A history of TISSOT,B. P. & WELTE,D. H. 1978. PetroleumFormation sedimentlithofacies in theSouth Atlantic ocean. In: and Occurrence. Springer-Verlag. 500-20. SUPKO,P. R., PERCH-NIELSEN, K., et al. Initial Rep. VAN ANDEL, T. H., THIEDE,J., SCLATER,J. G. & HAY, Deep Sea Drill.Proj. 39, 1047-79. Washington (US H. W. 1977. Depositional history of the South Atlantic Government Printing Office). Oceanduring the last 125 million years. J. Geol. NATLAND, J. H. 1978. Composition,provenance and dia- Chicago, 85, 651-98. genesis of Cretaceous clastic sediments drilled on the WAPLES,D. W. 1980. Timeand temperature in petroleum Atlantic continental rise off Southern Africa, DSDP Site formation: application of Lopatin’s method to petroleum 361-implications for the early circulation of the South exploration. Bull. Am. Assoc. Petrol. Geol. 64, 916-26. Received 30 July 1982. M. J. GIBBONS,A. K. WILLIAMS,N. PIGGOTT, BP Research Centre, Sunbury-on-Thames Middlesex TW16 7LN. G. M. WILLIAMS, BP PetroleumDevelopment Brazil Ltd.

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