Estimating the Ultimate Recoverable Reserves of the Paris Basin, France

J. Wendebourg1 and C. Lamiraux2

1 Institut français du pétrole, 1 et 4, avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex - France 2 Direction générale de l’énergie et des matière premières (DGEMP), 41, boulevard Vincent Auriol, 75703 Paris Cedex 13 - France e-mail: [email protected]

Résumé — Estimation des réserves ultimes récupérables du Bassin parisien, France — Le Bassin parisien est un bassin mature en exploration pétrolière. Pas moins de 800 puits exploratoires ont été forés pendant les 40 dernières années et 52 champs de pétrole ont été découverts : 33,4 Mt de pétrole ont été produits jusqu’en 2000. Le potentiel du Bassin parisien est estimé à l’aide de méthodes statistiques. La taille moyenne d’un champ est de 100 000 t avec une probabilité de 5 % de trouver un champ de plus de 2,5 Mt. Les réserves ultimes récupérables étaient de 15 Mt en 1986, 29 Mt en 1991 et 46 Mt en 1996. Aujourd’hui, il y a une probabilité de 5 % de trouver un champ de plus de 4 Mt dans les grès du Dogger ou du Keuper. Le taux de succès augmente de 1/66 dans la formation du Néocomien, le réservoir le moins profond, à 1/8 dans le Keuper, le réservoir le plus profond et le plus récemment foré. L’analyse présentée s’appuie sur des données de production et ne prend pas en compte une production supplémentaire éventuelle grâce à des méthodes de récupération assistée ou grâce à des découvertes basées sur de nouveaux concepts d’exploration, susceptibles d’augmenter les réserves de façon significative, bien au-delà des chiffres présentés comme cela est démontré par l’analyse de la courbe crémière.

Abstract — Estimating the Ultimate Recoverable Reserves of the Paris Basin, France — The Paris Basin is a mature basin from an exploration point of view. More than 800 exploration wells have been drilled over the last 40 years and 52 fields have been discovered from which 33.4 Mt of petroleum have been produced by the end 2000. Based on past production data, the future exploration and production potential of the Paris Basin is estimated using classical statistical methods. The mean size of a field in the Paris Basin is about 100 000 t with a 5% chance of a field whose size is greater than 2.5 Mt. The ultimate recoverable reserves based on the creaming curve analysis were about 15 Mt in 1986, 29 Mt in 1991 and 46 Mt in 1996. At present, there is a 5% chance of finding a field in the Dogger and Keuper formations that is bigger than 4 Mt. The drilling success ratio increases from 1/66 for the shallowest, the Cretaceous Neocomian formation to 1/8 in the deepest and latest to be drilled Triassic Keuper formation. The data analysis presented in the paper is based on known production data which do not take into account additional production gained by enhanced recovery methods or additional reserves added by discoveries based on new plays, that however may significantly increase reserves beyond the numbers presented here as the creaming curve analysis has shown. 622 Oil & Gas Science and Technology – Rev. IFP, Vol. 57 (2002), No. 6

INTRODUCTION future exploration potential, particularly in a mature basin such as the Paris Basin. The Paris Basin is one of the major petroleum production The Paris Basin is an intracratonic basin located in basins of continental Europe and the premier petroleum Northern France, with an extension of about 500 km east- province of France (Weaver et al., 1988). More than 800 west and 300 km north-south, approximately coinciding with exploration wells have been drilled over the last 40 years and the drainage basin of the Seine river (Fig. 1). It is a saucer- 52 fields have been discovered from which 33.4 Mt of shaped basin with a maximum thickness of sediments of petroleum have been produced by the end 2000. The Paris more than 3 km. The basin is slightly asymetric due to Basin is a good example of a mature petroleum basin where Tertiary erosion and uplift in its Eastern part (Fig. 2). The exploration is polyphase following technological progress stratigraphic column in the basin center includes some 150 m and favorable economics and where production is presently of Tertiary, about 1000 m of Cretaceous, some 1500 m of Jurassic and some 500 m of Permo-Triassic sediments in decline. In this paper, we will examine, based on past (Fig. 3). The fault pattern of the basin is governed by the Pre- production data, the future exploration and production mesozoic basement with NW-SE striking fault zones of potential of this mature basin using classical statistical Hercynian heritage. The principal oil-bearing formations are methods (Harbaugh et al., 1995). This approach does not try the Middle Jurassic (Bathonian) Dogger carbonates, the to develop new exploration plays based on new geophysical Upper Triassic Keuper sandstones, but also the Lower or geochemical data, or identify enhanced production Cretaceous Neocomian sandstones and the Upper Triassic possibilities of existing producing fields. It is based on the Rhaetian sandstones (Lamiraux and Mascle, 1998). The assumption that a sedimentary basin is a closed petroleum principal source rocks for the oils in the Paris Basin are the system and that past production data can be used to infer Lower Jurassic Toarcian black shales (schistes carton). They

0¡

Permian basin of Autun

0 100 km

Tertiary Cretaceous Jurassic Triassic Basement

Figure 1 Location map (from Perrodon and Zabek, 1991). J Wendebourg and C Lamiraux / Estimating the Ultimate Recoverable Reserves of the Paris Basin, France 623

W E

TriassicLiassic Dogger Eocene Oligocene Upper CretaceousLower CretaceousUpper Jurassic KeuperMuschelkalkBunterPermian PARIS

Basement 1 km ? 100 km 0 Carboniferous basin

Figure 2 Cross section (from Perrodon and Zabek, 1991).

70 STAGES TECTO. LITHO. Source- Oil and gas 0 rocks fields 60 CENO. 50 65 Ma Up 40 Cret. Low. 30 Malm. 20 Jura. Dogger 10

MESO. Lias Number of exploration wells Rhaetian 0 Keuper 1952 1954 1955 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 Trias Musch. Years Bunt. 225 Ma Perm. Figure 4 PALEO. Stephan. Total number of exploration wells drilled yearly in the Paris Carbo. Basin.

Westphal.

Figure 3 pause in the 1970s. In the 1980s, exploration experienced a revival with new discoveries in Triassic Keuper sediments, Chronostratigraphic log (from Lamiraux and Mascle, 1998). the most spectacular of which was the discovery of the Chaunoy field in 1982 which by the end 2000 has produced are presently in the oil window in the center of the basin almost 9.5 Mt of petroleum. Figures 7 and 8 show the (Gaulier and Burrus, 1994). Oil migration occurred vertically location and extension of all fields in the central part of the into the overlying Middle Jurassic carbonates and into the basin having produced from the Dogger and Keuper underlying Triassic sandstones, and then laterally within the reservoirs. Cumulative petroleum production as a function reservoir formations. of year of discovery reflects this evolution (Fig. 6). Petroleum exploration in the Paris Basin started in the Discoveries in the 1960s are fields that had produced more 1950s (Fig. 4) with a present-day maximum density of wells than 1 Mt of petroleum in 2000. After a low in the 1970s, in the central part just east of Paris (Fig. 5). First discoveries petroleum production picks up in the 1980s. In the 1990s, were made in the Cretaceous Neocomian sandstones and several discoveries of small to intermediate size have been in the Jurassic Dogger carbonates (Fig. 6). The first made. The production curve of the entire basin is dominated exploration peak was reached in the 1960s followed by a by the two biggest fields, Villeperdue and Chaunoy, both 624 Oil & Gas Science and Technology – Rev. IFP, Vol. 57 (2002), No. 6

600 650 700 750 150 150 MTL 1 RCQ 1 COM 1 NT 1 DPR 1 Montmorency JY 1 VIN 1 VS 1 Château-Thierry MTX 1D BEB 1 LVX 1 SMS 1 VRY 1 CAN 1 EI 1 AVE 1 MTR 1D VRY 2 Meaux MAP 1 VSO 1 Argenteuil CMT 1 MT 1 LUC 1 CPG 1 IDG 2D BUS 1 MTT 1 LMN 1D TRI 1 CS 44- CS 42 RZB 1D 33-32-35- 29-30-40 MV 101 VLL 1D MIL 1 CS 34 GLT 1 VLY 3 CSBE 2 CS 31 45 ME 1 CSBE 4 CSBE 3 CS 37 LFS 1D CS 26 VLY 1 IDG 1 ME 2 CS 41- HFE 6D BVE 1 28 49-43-46- HFE 5D VPS 101 CSBE 1 39-21-24 QV 7 18 FAB 3 FAB 13 CSBE 5 CS 19- HFE 1GD IDG 3D 16 9 50-36-38- FAB 11D 27-15-4 HFE 2 LSY 1 FAB 7 VLY 2 CS 5 SN 101 FAB 14D QV 1 CS 23-17- 2 PL 1 FAB 9D FAB 16 14-10-11- CS 4 FAB 10D FAB 1 FAB 15D FABE 1 13-8-12- CS 22 MEZ 1B PRN 1D 20-25-6 FAB 2 CS 7 Le Petit Morin HFE 7D CS 3 FAB 12D HM 1 VPU 171 FAB 6D FAB 8D CPV 1 VPU 143 FAB 4 MT 101 VPU 177H BYE 1 NSD 1 BTS 1 VPU 124 VPU 29D VPU 50 VPU 13 25D VPU 149 VPU 123 MV 1 22D 172H LEP 1 MT 2 145H FAB 5 PARIS CRY 1 MT 103 173H 141H SNC 1 VPU 1 37 83 169H 180H VGR 1 125 170H VPU 98 LCH 1D VPU 9 10 26D 179H SDN 102 VPU 74 BYL 2 175H 176H 81GD 38D 11 PMR 1 SDN 107 BLY 1 MT 102 12 84BD VPU 156 La Marne MAL 1 45D 82 VPU 47D 39D 41D 46D VPU 174H 43D 20 CFX 1 CGY 1 VPU 31 7 15 RB 101 35D 8 14 66BD 18 VPU 77 BYL 3 BRU 1 VPU 27D 17 SDN 105 SDN 1 VPU 21 36D VPU 86 VPU 3D 6 23 VPU 118 VPU P 1 VXP 1 VPU 4D 32D 24D 19 40 VPU 79 SDN 110D VPU 2D 28 SDN 104 Le Grand Morin 30D 48 133 VPU 87 BYL 1 CBG 1D VPU 178d CSM 1D VPU 5D SDN 103 VPU 33 34 44 42 VPU 72 SDN 101 MBN 1 LMS 1D VPU 57 LVV 1 SLZ 1 VPU 54 VPU 96 VPU 69 VPU 93 CPN 1 SDN 106 MR 101 VPU 16 VPU 62 LHB 101 MLN 2 MLN 7D JSL 1D LCY 1 Brie-Comte PZH 1 VPU 80 DML 1 VIX 3 MLN 1 CER 101 LES 1D SLP 1 CNT 2 -Robert CHO 1 GUE 1D LBT 1 VTN 1 CBS 1 FOX 1 LVI 1 MLN 3 CER 103 CNT 1 SMM 1 VGN 1D LBT 2 VNY 1 ESY 1 VIX 2 VIX 1 CHN 3 CER 102 SYP 2D CG 1 VGE 1 SYP 1 LNT 1 GRY 1 NSL 1 CHN 1 CER 1 CER 2D SYP 3D HDE 1 CER 3 SB 101 MA 101 MA 104 LPQ 1 LTR 2 COB 1D CNY 31 CHN 2 CQL 1 Sézanne LNT 2 GDB 1D CRM 1D LCC 1 MA 105 L' Yerres CHN 4 CER 4 BCN 101 MA 103 MA 1 EVRY CLV 1 CHB 101 LSB 1 STH 1 LFP 1 TOA 1 MA 102 LST 1 CNY 9 CHE 101 BC 1 CNY 15D MAT 1 MSG 2 GV 107 GVE 3 LHE 1 EGR 1D CNY 6 BIS 1 LCT 1 GV 105 GVE 2 CNY 10D CLF 1 RAC 2 LMR 1 GV 106 VSP 1D 22D JAY 1 LXE 1 25D CNT 2 LNB 1 MSG 1 VRC 1 GVE 1 BGN 1 CRB 1 19D CNY 14 GV 102 100 SGA 11D 18D GV 109 21D VCH 1 100 NDY 1 SGA 1 3 23D VUS 1 VUS 7 L'Aube GV 101 VLG 2 LTU 1D BNY 3D CNY 16D RAC 3 GVW 1 GV 104 SGA 9D CNY 1 CNY 5 RAC 1 VUS 3 2 SBR 101 SAU 1 SJS 2 GV 103 BBL 1D SGA 2D CNY 12D IVY 1D 9H SJS 1 VLN 1D VSD 1 MCY 1 BNY 1 8H CNY 4 HEU 1 5D 4 SCV 1 SJS 3D CNY 7D CHM 4 CHM 1 PSS 101 STL 1D AUV 101D SVY 5D HTE 1 TAL 1 6D VLT 1D BVD 1D CNY 11D BGX 101 NOZ 1 MLB 1 SVY 2D CNY 13D CHM 2D LDN 1 TAL 2 VLZ 1 CNY 17 CTB 101 LPS 1 Arcis-sur-Aube ITV 1 AUV 1 SVY 1 LE 102 NG 1 CHM 3 DOE 3 SAU 2 PSS 102 ITV 2 CH 21 BR 120 DOE 2 CH 102 CH 39 CNY 20D VY 101 BRM 2 DOE 5D SOU 101 ITV 9 VIB 1 LE 101 DOE 7D GL 101 MDV MELUN1 BR 106 ETC 1 Romilly-sur-Seine ITV 5 CH 3 DOE 6D DOE 4 CH 40 BDA 1 BR 104 CHM 5 HRM 1D VDL 1 CH 8 LGM 1 PE 2 CH 6 BRM 1 DOE 1 Nogent-sur- VSG 1 CLY 1 CH 18 VBS 101 DHU 1D CH 37 BR 127 12 CH 20 LGC 1 MOA 1 PE 1 LRM 1 CH 4 10 17 18D 7 CH 43 MAC 101 EB 101 Seine 4 STM 1 SMT 1D FDB 1 15 14 11 MCS 1 PE 3 FEB 1 SMT 2D 1 9 SMB RSS 1 HER 1D FR 101 VL 101 5 8 201 CHS 1 6 3 PVL 1 RLN 1 13 VDZ 1 16 ORV 1 AP 1 2 Fontainebleau La Seine MLG 1D SLU 1 PSJ 1 BOU 1 SLU 2 BDG 1 TSN 101 Montereau SRG 1 LMB 1D BOE 1 NY 101 L' Yonne VOV 1 GLB 1 TROYES CPY 1 126 L'Essonne TSN 1 EC 101 FRS 1 PSV 1 LPP 1 121 116 107 124 104 122 111 110 108 123 112 THF 1 118 102 125 109 106 BGU 2D LM 2 115 119 VM GN 3 120 103 FSY 1 GN 1 9 LRE 1 Nemours 101 POM 1 LM 1 5D EPI 1 LLY 1 3 6 SPN 1 BAR 1 GN 2 SAT 1 1D TAS 1 4D 7 La Seine PUI 1 BAR 2 CMU 1 BUG POM 2 SCY 1 8D AFV 4 GUI 1 VP 1 FNM 1 FLY 1 OBS 1 PO 1 MTP 1 SES 1 AFV 101 BOA 1 CHR 101 Sens RLF 1 AFV 1 VRS 1 AFV 3 LMA 1 AFV 5G GRN 1 CHR 1 La Vanne EST 1 BCH 1 BMT 2 CLD 1 CHX 1 LS 1 MPS 1B 50 50 600 650 700 750

Figure 5 Location map of exploration wells drilled into Middle Jurassic Dogger formation in central part of Paris Basin (includes development wells of some fields).

30 000 000 1 PRODUCTION HISTORY OF THE PARIS BASIN

25 000 000 Figure 9 displays petroleum production through time in the Paris Basin separated by producing formation. The data 20 000 000 consist of yearly production from production start to end 15 000 000 2000. Production is shown as liquid petroleum in all cases. In the Paris Basin, only 7% of the total production is gas of 10 000 000 which 63% come from a single field, the Trois-Fontaines

5 000 000 Cumulative field size (t) field producing from the Middle Triassic Muschelkalk 0 formation. 30% of gas production consists of coalbed 1955 1960 1965 1970 1975 1980 1985 1990 1995 methane from the Poisonnière field. Year of discovery Four main oil producing horizons are identified : the Figure 6 Cretaceous Neocomian sandstones, the Middle-Jurassic Field size versus discovery year in Paris Basin. Dogger sandstones, the Upper Triassic Rhaetian marine sandstones and the Upper Triassic Keuper Donnemarie and Chaunoy sandstones. The Dogger and Keuper sandstones are coming on production in the 1980s, and their subsequent by far the main producing horizons in the Paris Basin with decline, masking the additional production of all other fields. each about 40% of total production. J Wendebourg and C Lamiraux / Estimating the Ultimate Recoverable Reserves of the Paris Basin, France 625

550 600 650 700 750 800

Mourmelon L ' A L' Oise La Vesle is PONTOISE Château-Thierry La Marne ne 150 L' Ourcq 150 Meulan Montmorency Epernay Triel-s-Seine Meaux FONTAINE- Argenteuil CHALONS-EN Mantes-la-Jolie Poissy MONTLEVEE HAUTEFEUILLE AU-BRON (-1630) (-1695) (-1640) -CHAMPAGNE St Germain-en-Laye Le Petit Morin SOUDRON * PARIS La Marne COULOMMES MONTMIRAIL (-1678) (-1200) Le Grand Morin VERSAILLES Brie-Comte VILLEPERDUE MALNOUE * (-1620) -Robert (-1835) CERNEUX *(-1803) Sézanne Palaiseau LA BOCHETIERE Vitry-le-François Trappes (-1810) (-1745) Rambouillet SANCY-LES-PROVINS St-Dizier EVRY L' Yerres

100 100 (-1700) L' Aube VERT-LE-PETIT(-1450) VULAINES * MAROLLES-EN-HUREPOIX ST ELOI (-1430) (-1378) BRIE * PONT-SUR-SEINE (-1580) TROU-AUX-LOUPS Arcis-sur-Aube (-1710) ITTEVILLE MELUN CHARMOTTES * (-1670) Romilly-sur-Seine (-1458) Nogent-sur- Etampes CHAILLY * (-1629) Seine ST MARTIN-DE-BOSSENAY (-1580) VALENCE-EN-BRIE AVON-LA-PEZE (-1255) La Seine ST LUPIEN (-1230) L' Essonne Fontainebleau BOURDENAY Montereau TROYES L' Yonne GISY LES MARCHAIS (-1335) (-1435) La Seine Nemours VILLEMER * SUATTE (-1345) BAGNEAUX(-1090) Bar-sur-Aube Sens FLACY (-1097) Pithiviers (-1485) AUFFERVILLE La Vanne 50 50 550 600 650 700 750 800 0 50 km

Figure 7 Location map of oil fields of central part of Paris Basin that produce (underlined) or have produced from Middle Jurassic Dogger formation. Number next to name indicates subsea depth of top of reservoir (m). (*) indicates that field also produces from underlying Keuper formation.

550 600 650 700 750 800

Mourmelon L ' A L' Oise La Vesle isn Château-Thierry La Marne PONTOISE e 150 150 Meulan Montmorency L' Ourcq Epernay Triel-s-Seine Meaux CHALONS-EN Poissy Argenteuil -CHAMPAGNE Mantes-la-Jolie MESNIL (-2547) St Germain-en-Laye La Marne (-2160) Le Petit Morin PARIS ILE-DU-GORD (-2551) ST-LAZARELe Grand M VERSAILLES Brie-Comte (-2325) IVRY PEZARCHES orin (-1929) -Robert (-2470) CERNEUX * Sézanne Palaiseau CHAMPROSE (-2334) MALNOUE * (-2540) Vitry-le-François Trappes (-2202) LA VIGNOTTE (-2270)NESLES CHAMPOTRAN (-2385) Rambouillet (-2335) St-Dizier EVRY L' Yerres VILLARCEAUX CONQUILLIE (-2403) ST GERMAIN CHAUNOY (-2147) 100 100 (-2145) 100 (-2420) L' Aube VERT-LE-GRAND ** MAINCY RACHEE (-2140) (-1800) CHARMOTTES * (-2450) Arcis-sur-Aube (-2140) DONNEMARIE ** MELUN SIVRY Romilly-sur-Seine CHAILLY * (-2538) Etampes (-2136) BRIE * Nogent-sur- (-2110) BREMONDERIE (-2500) Seine La Seine L' EssonneFontainebleau Montereau TROYES L' Yonne Nemours La Seine MONTAPOT(-2033) Bar-sur-Aube FAY-LES-NEMOURS Sens e nn (-2030) BOULAY(-2020) a Pithiviers V La 50 50 550 600 650 700 750 800 0 50 km

Figure 8 Location map of oil fields of central part of Paris Basin that produce (underlined) or have produced from Upper Triassic Keuper formation. Number next to name indicates subsea depth of top of reservoir (m). (*) indicates that field also produces from overlying Dogger formation. 626 Oil & Gas Science and Technology – Rev. IFP, Vol. 57 (2002), No. 6

1 200 000 900 000

Keuper 800 000 1 000 000 Dogger 700 000 Neocom. Keuper

800000 600 000 Rhaetian 500 000 600 000 400 000 Dogger 400 000 300 000

200 000 200 000 Cumulative production (t) Oil production (metric tons) 100 000 0 0 1995 2000 2005 2010 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 Years Years

2 Figure 9 Neocomian: R = 2E + 60 EXP (Ð0.0642*year), R = 0.9917 Dogger: R = 2E + 86 EXP (Ð0.093*year), R2 = 0.9987 Yearly total production in Paris Basin. Rhetian: R = 9E + 69 EXP (Ð0.075*year), R2 = 0.9987 Keuper: R = 4E + 118 EXP (Ð0.2028*year), R2 = 0.9989

The Neocomian is the shallowest reservoir which also was Figure 10 discovered first. The Neocomian yields only small fields Projected production decline of existing fields in Paris Basin. where production is rarely greater than 100 000 t/year. Most fields producing from the Dogger formation have been discovered in the early years of exploration in the Paris Basin. Villeperdue is the biggest Dogger field with a yearly produced by the end 2000 (Table 1). Using the ultimate production of almost 1 Mt in the 1980s but also in strong production of each field, the field size distribution of a basin decline since the 1990s. The Upper Triassic Rhaetian fields can be plotted (Fig. 11) from which probabilities can be have all very small sizes. The Rhaetian play had been estimated for a given field size. In a petroleum producing discovered shortly before the slightly deeper Keuper play. sedimentary basin, such a distribution is usually lognormal The Keuper play is the most important play in the Paris Basin plotting along a straight line on lognormal paper (Harbaugh and has been explored since the early 1980s. The Keuper et al., 1995). This is also the case in the Paris Basin (Fig. 12). production is dominated by the Chaunoy field which has The mean size of a field in the Paris Basin is therefore about been particularly high (more than 700 000 t/year over a 100 000 t (50% probability) with a 5% chance of a field period of several years). whose size is greater than 2.5 Mt.

2 FIELD SIZE ESTIMATION

Decline curves have been extrapolated beyond 2000 to determine the ultimate cumulative production assuming no enhanced recovery. Integrating all production curves over the 0.2 next 10 years (Fig. 10), ultimate production is estimated to be 37.2 Mt of which nearly 90% have already been

TABLE 1

Reserves and production statistics of Paris Basin (Mt) 0.1

Ultimate Ultimate Relative frequency Production production recoverable until 2000 from existing reserves without fields* new plays

Neocomian 14.23 (13%)114.48 (12%)11 15.52 0 Dogger 14.31 (43%)1 16.37 (44%)1 20.24 1 10 100 1000 10 000 3 Rhaetian 11.68 (5%)11 12.12 (6%)11 12.84 Field size (10 t) Keuper 13.14 (39%)1 14.24 (38%)1 18.40 Total 33.41 (100%) 37.21 (100%) 47.00 Figure 11

* Using decline functions based on production data from 1995-2000 (Fig. 10). Field size distribution of Paris Basin as histogram. J Wendebourg and C Lamiraux / Estimating the Ultimate Recoverable Reserves of the Paris Basin, France 627

ultimate recoverable reserves of the basin (Fig. 13). In the Mean field size: 100 000 t Paris Basin, this value is not constant in time. It is about 10 000 (50% probability) 15 Mt in 1986, 29 Mt in 1991 and 46 Mt in 1996. This rather

t) empirical and graphical method introduces uncertainties with 3 respect to the extrapolated reserves but gives a good idea 1000 about the reserves that cannot be exceeded significantly except by discovering different plays in the future. For this reason, reserve estimates are different in the past. For 100 example, the Triassic discoveries in the early 1980s have significantly changed the form of the creaming curve and

Ultimate production (10 10 estimated reserves have trippled from 15 to 46 Mt.

2.2 By Year of Discovery 1 12 510 25 50 75 9095 98 99 Figure 14 displays the same data as Figure 12 with the Probability difference that field size is separated by year of discovery. Fields discovered in the 1960s have been separated from Figure 12 fields discovered between 1976 and 1986 and from those Field size distribution of Paris Basin on lognormal paper. discovered between 1986 and 1996. This analysis shows that fields discovered in the 1960s have also been biggest, with a mean size of 230 000 t and a 5% probability of more than 2.1 By Global Statistics 8 Mt. Later, this trend is towards smaller fields which is typical for petroleum-bearing sedimentary basins where the In petroleum-bearing basins that are in a mature exploration biggest fields are usually discovered first (Harbaugh et al., stage, a curve plotting cumulative number of exploration 1995). In the 1980s and 1990s, mean field size in the Paris wells against cumulative production (also called creaming Basin decreased to about 100 000 t. Between 1976-1986, the curve) tends asymptotically towards a single value, the slope of the lognormal distribution is significantly higher,

46 MMt (1996) 29 MMt (1991) 15 MMt (1996, pre-Trias) 1958-1964 1000 10 000 1976-1985 900 1986-1996

800 1997-2007 1000 t)

700 3

600

500 100

400 Reserves (10

300 drilled between 1958-1996 200 10

Cumulative number of exploration wells 100

0 012345678 1 Log cumulative production (t) 12 510 25 50 75 90959899 between 1958-1996 Probability

Figure 13 Figure 14 Total cumulative petroleum production in Paris Basin Field size distribution of Paris Basin separated by year of (creaming curve). discovery. 628 Oil & Gas Science and Technology – Rev. IFP, Vol. 57 (2002), No. 6 yielding a 5% probability of a field size of more than 3 Mt. explained by the steadily increasing knowledge of the This is due to the Chaunoy field which pulls the distribution petroleum system of the Paris Basin, the increasing quantity up to larger field sizes. The slope then falls back to its and quality of available data, and by the fact that the Keuper original value with a 5% probability of a field size bigger formation is geologically and geographically closest to the than 2 Mt. When extrapolating this trend over the following Toarcian source rocks and therefore expelled petroleum has 10 years (1996-2006), 95% of new discoveries will not be the shortest migration distance and smallest migration bigger than 900 000 t. resistance.

2.3 By Geological Formation TABLE 2 Exploration well statistics of Paris Basin Figure 15 repeats the previous procedure but this time separating field sizes by geological formation. The Dogger Fields Exploration wells Success discovered targeting formation has the best field size distribution but the slope of ratio its lognormal distribution is smaller than the one of the since 1958 formation Triassic Keuper formation. For both formations, there is a Neocomian 4 264 1/66 5% chance of finding a field that is bigger than 4 Mt. Dogger 17 640 1/37 However, this value is probably smaller for future years for Rhaetian 10 157 1/16 the same reasons as for the total Paris Basin. Keuper 20 160 1/8 Total 52 798 1/15

CONCLUSIONS Keuper 10 000 The relatively high sucess ratio of 1/8 and a nonnegligable Dogger probability to discover a field of several million tons of petroleum makes the Triassic Keuper formation a good target

t) for further exploration and potential additional reserves in the

3 Rhaetian 1000 Paris Basin. Its success ratio is not only the highest of all producing formations (it is 8.3 times higher than the Cretaceous Neocomian and 4.6 times higher than the Jurassic Dogger reservoirs) but it is also the most realistic. Despite of Reserves (10 100 its low value of 1/66, the success ratio of the Cretaceous Neocomian is still too optimistic because it does not include wells that have targeted underlying formations but also 10 encountered the Neocomian. In a similar way, the success ratios of the Jurassic Dogger and Triassic Rhaetian formations are overestimated. 12 510 25 50 75 90959899 The data analysis presented in this paper is based on Probability known production data which do not take into account Figure 15 additional production gained by enhanced recovery methods or additional reserves added by discoveries based on new Field size distribution of Paris Basin separated by reservoir formation. plays. A new play may indeed significantly increase reserves beyond the numbers obtained by our calculations as past experience in the Paris Basin has shown. Such new plays could be stratigraphic traps in pinch outs of the Keuper 3 SUCCESS RATIO sandstones or targets deeper than the Triassic Keuper formation such as Middle Mesozoic Muschelkalk or The success ratio is the ratio of the number of commercial Paleozoic Carboniferous reservoirs. discoveries over the number of exploration wells that have reached their target. In the Paris Basin, this ratio increases almost linearly from 1/66 for the shallowest, the Cretaceous ACKNOWLEDGEMENTS Neocomian formation that also had been drilled first, to 1/8 in the deepest and latest to be drilled Triassic Keuper The authors would like to thank J. Delmas from IFP formation (Table 2). The evolution of the success ratio can be and C. Ducreux from Beicip-Franlab for their help to J Wendebourg and C Lamiraux / Estimating the Ultimate Recoverable Reserves of the Paris Basin, France 629 prepare the data base and Y. Mathieu from IFP for his Lamiraux, C. and Mascle, A. (1998) Petroleum Exploration and instructive comments. Production in France. First Break, 16, 4, 109-116. Perrodon, A. and Zabek, J. (1991) Interior Cratonic Basins. Analog Basins: Paris Basin. AAPG Memoir, 51, 663-679. REFERENCES Weaver, O.D., Van Damme, A.M. and Calatayud, P. (1988) Oil and Gas Potential of France. Oil and Gas Journal, May 1988. Gaulier, J.M. and Burrus, J. (1994) Modeling Present and Past Thermal Regime in the Paris Basin: Petroleum Implications. EAGE Special Publications, 4, 61-73. Harbaugh, J.W., Davis, J.C. and Wendebourg, J. (1995) Computing Risk for Oil Prospects, Pergamon Press, Oxford. Final manuscript received in June 2002