3D source-rock modelling of the Espirito Santo Basin, Brazil

Jarbas Guzzo 1, Ute Mann 2, Maarten Felix 2, Monika Majewska-Bill 2

1 Geochemistry, Petrobras/Cenpes, Rio de Janeiro, Brazil. 2 Basin Modelling, SINTEF Petroleum Research, Trondheim, Norway

guzzo@petrobras.com.br; ute.mann@sintef.no

References1. Bralower, T. J. and Thierstein, H. R. 1984. Low productivity in slow deep-water circulation in Mid-Cretaceous oceans. Geology,

12: 614-618.2. Brownfield M.E., Charpentier R.R., Geology and Total Petroleum Systems of the Gulf of Guinea Province of West Africa. U.S.

Geological Survey Bulletin 2207-C.3. Franca R. L., Del Rey A. C, Tagliari C. V., Brandão J. R. and Fontanelli P. R., 2007. Stratigraphic Chart of Espirito Santo Basin.

Boletim de Geociências da Petrobras 15(2): 501-509.4. Isaksen, G. H. and K. H. I. Ledje, 2001. Source Rock Quality and Hydrocarbon Migration Pathways within the Greater Utsira

High Area, Viking Graben, Norwegian North Sea. AAPG Bulletin 85(5):861-883.5. Mann, U. and Zweigel, J., 2008. Modelling source rock distribution and quality variations: the organic facies modelling

approach. In: P.L. de Boer, G. Postma, C.J. van der Zwan, P.M. Burgess and P. Kukla (eds.): Analogue and Numerical Forward Modelling of Sedimentary Systems; from understanding to prediction. International association of sedimentologistsspecial publications, 40, p. 239-274.

6. Miller K.G., et al., 2005. The Phanerozoic record of global sea-level change. Science 310, 1293-1298.7. Tissot B., Demaison G., Masson P., Delteil J.R., and Combaz A., 1980. Paleoenvironment and petroleum potential of middle

Cretaceous black shales in Atlantic basins: AAPG Bulletin, v.64, no. 12, p.2051-2063.

Introduction and background

Geological setting The study area covers a 110 km x 332 km offshore portion of the Espirito Santo Basin (see location map). The studied section consists of a transgressive sequence that drowned the Albian carbonate platform with deltaic and prodeltaic sediments and canyon-fill turbidite sandstones (Urucutuca Fm.).

This study examines to which extent the local canyon-fill turbidite sediments overprint the possible influence of the Cenomanian/ Turonian Anoxic Event on the source rock properties of Top Albian to Top Turonian (99.0 – 89.0 Ma) sediments.

The process-based modelling software OF-Mod 3D - Organic Facies Modelling (Mann and Zweigel, 2008) was used to model Total Organic Carbon (TOC) and Hydrogen Index (HI) distribution throughout the area during the Cenomomanian -Turonian interval.

The data show: Distinct terrestrial input in all wells including ESS-86A ‘Campos

Basin type’ Terrestrial organic matter confirmed by biomarkers: high

hopanes/steranes ratio; high C29 steranes > 45 % and saturated hydrocarbons dominated by long-chain n-alkanes (e.g. ESS-151 well)

ESS-86 well (distal, Campos Basin type) shows the signature of the C/T anoxic event, higher TOC but only slightly higher HI

Depth

Total Organic Carbon (wt.%) Hydrogen Index (mg HC/g TOC)

Depth

Cenomanian/Turonian Anoxic Event

pristane/phytane ratio 4.11 hopanes/steranes ratio 8.3 20S/(20S+20R) 0.45 αββ/(αββ+ααα) 0.46 CPI = 1.18 (predominance of odd n-parafins TET24/26TRI=1.94 C29st=52%

NC14NC15 NC16 NC17

PRINC18

PHYNC19 NC20 NC21 NC22 NC23 NC24 NC25 NC26 NC27

NC28NC29

NC30NC31

NC32 NC33NC34 NC35

NC36 NC37 NC38 NC39

ESS151, 5052-5094 m

TR19 TR20 TR21 TR22 TR23 TR24 TR25A

TR25B TET2

4TR26

ATR26

B

TR28A

TR28B

TR29A

TR29B TS

TMTR30

ATR30

B H28 NOR25H

H29C29T

SDH30 M29

OLH30

NOR30H

M30H31S

H31RGAM

H32S H32R H33S H33R H34S H34R H35S H35R

m/z 191

DIA27S

DIA27R

DIA27S2 DIA27R2 C27S

BB_D29S

C27BBS C27R

C28S C28BBR C28B

BSC28R

C29S C29BBR

C29BBS

C29R

m/z 217

Palaeogeographic maps of the Cretaceous breakup of South America and Africa showing the extent of anoxia in this region. From U.S.Geological Survey Bulletin 2207-C, 2006 (after Tissot et al., 1980).

oxic

oxic

Geochemical information available from wells: Three key wells were selected for comparison between measured and modelled results. These wells were chosen because of (1) their geographical position within the investigated area and (2) data density including biomarker data.

ESS-40 - proximal, influenced by river input, ESS-151 - intermediate distance from shore, ESS-86A - distal, “Campos Basin type”.

Location map of the Espirito Santo Basin.

Stratigraphic chart of the Espirito Santo Basin (Franca et al., 2007)

I

II

II-III

III-II

III

IV-III

IV

Ro=0.62%

Ro=0.88%

Ro=1.1%

0

100

200

300

400

500

600

700

800

900

1000

350 370 390 410 430 450 470 490 510 530 550

Tmax (°C)

Hyd

roge

n In

dex

(mg

HC

/g T

OC

)

other wells

ESS 40

ESS 86 ESS 151

ESS 95 → Campos Basin

→ intermediate → proximal

→ distal ‘Campos Basin type’

Approx. VR (% Rm)

Kerogen type (Isaksen & Ledje, 2001)

OF-Mod considers basin-fill stratigraphy organic carbon source fractions: marine/autochthonous and terrestrial/allochthonous degradation of the organic matter in the water column and burial efficiency at sea floor oxic and anoxic conditions calibrated with analytical data from well samples

Stratigraphic input into OF-Mod Geometry Seismic depth maps: Top Albian (99Ma) and Top Turonian (89Ma) 110 x 332 km area of the Espirito Santo Basin

Age model Modelled interval: 99.0 – 89.0 Ma; 40 layers - i.e. 250 ky/layer on average

Reconstructed palaeowater depth top Turonian top Albian

Sand fraction model created based on: V-shale logs data from 29 wells (V-shale calculated from gamma ray), and a background model based on the water depth Results in isolated sand patches, which are reminiscent of actual turbidite sands

Sea level: Eustatic sea level curve of Miller et al. (2005)

(Deep water) preservation in the model is increased during the time period of the CTAE, to reflect the reduced oxygen conditions to a max. PF of 2% (Bralower & Thierstein, 1984).

pTOC constant over time High content of residual organic matter (SOC) - the highest

value in anoxia period (93.5 – 96Ma)

0

0,5

1

1,5

2

2,5

8990919293949596979899

Time (Ma)

PF (%

)

0

0,5

1

1,5

2

2,5

8990919293949596979899

Time (Ma)

TOM

(wt.%

)

pTOCSOC

Preservation variable with time: Terrestrial Organic Matter:

Organic Matter Properties: OF-Mod calculates the HI from the amount of marine, terrestrial and residual organic matter HI MOC: 550 (mg HC/gTOC) HI pTOM: 80 (mg HC/g TOC) HI SOM: 20 (mg HC/g TOC)

Average Water depth (m) period: 99-89 Ma

Average Sedimentation rate (cm/ka) period: 99-89 Ma

Average Sand Fraction (%) period: 99-89 Ma

The organic input into OF-Mod: Primary productivity – defined in OF-Mod as a function of distance

from shore:

PP at the coast: 10 gC/(m2a) to reflect high energy environment, low export production out of the photic/shallow water zone, and transport of deposited organic material away from shore

PP in the open ocean: 60 gC/(m2a)

Source rock modelling

degradation carbon flux

Fc water

depth (m)

epibenthic respiration

* * * *

* *

* *

* * * *

* * *

* * * * * * *

* *

* * * * *

* *

* * * * *

* * * * * *

BFM: erosion, bypass and

sedimentation

* * * * * * *

* * * * * * * * * upwelling PP = 250 - 300 g C·m-2·a-1 PP = 50 – 60 g C·m-2·a-

1 * PP = 100 - 250 g C·m-2·a-1

TOM

fluvial sediment and

nutrient supply

MOC (oxic) = Fc · BE · dilution

* * * * * * * *

* * * * * * * * * * * *

* *

MOC (anoxic) = PP · PF · dilution

* * * * * * *

* * *

* *

* * * * * * * * *

burial efficiency BE

* * *

* * * *

*

*

* * ABW * * *

primary productivity PP (g C·m-2·a-1) CO2 + H2O CH2O + O2

OMZ * * * *

W E

W E

W E

Particulate Terrestrial Org. Carbon (wt.%) Hydrogen Index (mg HC/g TOC)

Average Total Organic Carbon (wt.%) period: 99-89 Ma

1.

2.

3.

1.

1.

2.

3.

1.

2.

+ =

no marine organic matter, mainly SOC and pTOC

Modelled TOC and HI values (colours) compared to measured data (dark blue crosses) vs. depth for proximal (ESS-40) and intermediate (ESS-151) distance to shore wells. In both wells the data indicates a predominance of terrestrial organic matter.

Modelled total, terrestrial, and marine organic carbon and hydrogen index for distal well (ESS-86A). The distinct increase of TOC in the depth range between 4900 and 5025 m is caused by increased preservation and higher shale content.

W-E cross-sections showing the influence of the anoxia: enhanced TOC values but only slightly enhanced HI values. The wells are indicated by drilling symbol.

Source Rock Potential before CTAE Age: 99 – 98,75 Ma

Distal - Campos Basin type well:

Total Organic Carbon (wt.%) Hydrogen Index (mg HC/g TOC)

Proximal influenced by the southern river input well: Intermediate distance from shore well:

Maps showing trends of increasing TOC and HI distribution to the deep sea for the entire modelling interval. However, maps of source rock potential before and within the CTAE show that the general trend of increasing TOC and HI values is only due to the enhanced preservation conditions during the CTAE.

Total Organic Carbon (wt.%) Hydrogen Index (mg HC/g TOC)

Shale fraction (%) Soil/Residual Org. Carbon (wt.%) Marine Org. Carbon (wt.%) Total Organic Carbon (wt.%)

+

Total Organic Carbon (wt.%) Age: 89 Ma

Hydrogen Index (mg HC/g TOC) Age: 89 Ma

Conclusions

The model results in a good match between modelled and measured TOC and HI values

Increased preservation has been modelled during the CTAE to match the high TOC values in well ESS-86A and to produce a slight increase in HI

Influence of the turbiditic infill decreases and influence of the CTAE increases from NW to SE

CTAE event has minor impact in near shore area but more prominent in the distal area

W E

E W

E W

3.

2.

3.

Average Hydrogen Index (mg HC/g TOC) period: 99-89 Ma

Source Rock Potential in CTAE Age: 94,25 – 94 Ma

Results