The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System

By Thomas S. Ahlbrandt

U.S. Geological Survey Bulletin 2202–F

U.S. Department of the Interior U.S. Geological Survey

U.S. Department of the Interior Gale A. Norton, Secretary

U.S. Geological Survey Charles G. Groat, Director

Version 1.0, 2001

This publication is only available online at: http://geology.cr.usgs.gov/pub/bulletins/b2202-f/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government

Manuscript approved for publication May 8, 2001 Published in the Central Region, Denver, Colorado Graphics by Susan M. Walden, Margarita V. Zyrianova Photocomposition by William Sowers Edited by L.M. Carter

Contents

Foreword ...... 1 Abstract...... 1 Introduction ...... 2 Acknowledgments...... 2 Province Geology...... 2 Province Boundary...... 2 Geographic Setting...... 4 Political and Exploration History ...... 5 Geologic Setting...... 7 Total Petroleum System ...... 11 Petroleum Occurrence ...... 11 Source Rock ...... 13 Overburden Rock ...... 17 Reservoir Rock ...... 17 Seal Rock ...... 21 Trap Style ...... 21 Petroleum Assessment ...... 21 References Cited ...... 27

Figures

1–3. Maps showing: 1. Sirte-Zelten Total Petroleum System, assessment units, and oil and gas field centerpoints ...... 3 2. Structural elements of Sirte Basin ...... 4 3. Central Mediterranean Sea, USGS-defined Pelagian province, and offshore areas of Sirte Basin...... 5 4. Northeast-southwest offshore regional cross section ...... 6 5. East-west onshore structural cross section...... 8 6. Pre-Hercynian subcrop map in Sirte Basin area ...... 8 7. Stratigraphic section in central Sirte Basin...... 9 8. Stratigraphic section in eastern Sirte Basin...... 10 9. Schematic cross section showing general structural position, stratigraphic age, and lithology of most traps within onshore Sirte Basin...... 11 10. Diagram showing burial history curve for deepest portion of Sirte Trough...... 12 11. Map showing postulated petroleum migration pathways for oil sourced from Upper Cretaceous shales...... 13

III

12. Map showing postulated pre-Upper Cretaceous source areas and migration pathways of nonmarine source rocks in eastern Sirte Basin...... 14 13. Oil geochemistry plots for 81 oil samples from 55 fields in Sirte Basin ...... 15 14. Events chart for Sirte-Zelten Total Petroleum System ...... 16 15. Map showing tectonic elements and oil fields in eastern Sirte Basin...... 18 16. Stratigraphic cross section in Calanscio-Hameimat Trough ...... 19 17. East-west structural cross section across western margin of Messla High ...... 20 18. Schematic structural cross section from Sarir Trough across Messla High and into Hameimat Trough...... 20 19. Stratigraphic cross section through Zelten Formation bioherms...... 22 20. Cumulative plot of volume of known oil versus field discovery year for Southeast Sirte Clastics Assessment Unit ...... 23 21–23. Histograms showing: 21. Volumetric assessment results for geologically risked assessment units in Sirte-Zelten Total Petroleum System in Sirte Basin ...... 25 22. Volumetric assessment results for geologically risked and unrisked assessment units in Sirte-Zelten Total Petroleum System in Sirte Basin...... 25 23. Comparison of U.S. Geological Survey World Energy Assessment Team (2000) mean risked estimates for oil, natural gas, and natural gas liquids versus Masters and others’ (1994) mean estimates ...... 26

Tables

1. Sirte-Zelten, Total Petroleum System 204301—assessment results summary...... 24

IV

The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System

By Thomas S. Ahlbrandt

Foreword (3) generation, migration, and accumulation of hydrocarbons; and (4) preservation of hydrocarbons. A numeric code identifies each region, province, total petro- This report was prepared as part of the World Energy Project leum system, and assessment unit; these codes are uniform of the U.S. Geological Survey. For this project, the world was throughout the project and will identify the same type of entity in divided into eight regions and 937 geologic provinces. Of these, any of the publications. The code is as follows: parts of 128 geologic provinces were assessed for undiscovered Example petroleum resources. The primary documentation for these assessments is in U.S. Geological Survey World Energy Assess- Region, single digit 3 ment Team (2000). The petroleum geology of these priority and Province, three digits to the right of region code 3162 boutique provinces is described in this series of reports. Seventy- Total petroleum system, two digits to the right six “priority” provinces (exclusive of the United States and cho- of province code 316205 sen for their high ranking) and 52 “boutique” provinces (exclu- Assessment unit, two digits to the right of petroleum sive of the United States) were selected for appraisal of oil and system code 31620504 gas resources. Boutique provinces were chosen for their antici- The codes for the regions and provinces are listed in U.S. pated petroleum richness or special regional economic or strate- Geological Survey World Energy Assessment Team (2000). gic importance. Oil and gas reserves quoted in this report are derived from The purpose of the World Energy Project is to assess the Petroconsultants’ Petroleum Exploration and Production data- quantities of oil, gas, and natural gas liquids that have the poten- base (Petroconsultants, 1996) and other area reports from Petro- tial to be added to reserves within the next 30 years. These vol- consultants, Inc., unless otherwise noted. umes either reside in undiscovered fields whose sizes exceed the Figure(s) in this report that show boundaries of the total stated minimum-field-size cutoff value for the assessment unit petroleum system(s), assessment units, and pods of active source (variable, but must be at least 1 million barrels of oil equivalent) rocks were compiled using geographic information system (GIS) or occur as reserve growth of fields already discovered. software. Political boundaries and cartographic representations The total petroleum system constitutes the basic geologic were taken, with permission, from Environmental Systems unit of the oil and gas assessment. The total petroleum system Research Institute’s ArcWorld 1:3 million digital coverage includes all genetically related petroleum that occurs in shows (1992), have no political significance, and are displayed for gen- and accumulations (discovered and undiscovered) that (1) has eral reference only. Oil and gas field centerpoints shown in these been generated by a pod or by closely related pods of mature illustrations are reproduced, with permission, from Petroconsult- source rock, and (2) exists within a limited mappable geologic ants (1996). space, along with the other essential mappable geologic elements (reservoir, seal, and overburden rocks) that control the funda- mental processes of generation, expulsion, migration, entrap- Abstract ment, and preservation of petroleum. The minimum petroleum system is that part of a total petroleum system encompassing The Sirte (Sirt) Basin province ranks 13th among the discovered shows and accumulations along with the geologic world’s petroleum provinces, having known reserves of 43.1 bil- space in which the various essential elements have been proved lion barrels of oil equivalent (36.7 billion barrels of oil, 37.7 tril- by these discoveries. lion cubic feet of gas, 0.1 billion barrels of natural gas liquids). It An assessment unit is a mappable part of a total petroleum includes an area about the size of the Williston Basin of the north- system in which discovered and undiscovered fields constitute a ern United States and southern Canada (≈490,000 square kilome- single, relatively homogeneous population such that the chosen ters). The province contains one dominant total petroleum methodology of resource assessment based on estimation of the system, the Sirte-Zelten, based on geochemical data. The Upper number and sizes of undiscovered fields is applicable. A total Cretaceous Sirte Shale is the primary hydrocarbon source bed. petroleum system may equate to a single assessment unit, or it Reservoirs range in rock type and age from fractured Precam- may be subdivided into two or more assessment units if each unit brian basement, clastic reservoirs in the Cambrian-Ordovician is sufficiently homogeneous in terms of geology, exploration con- Gargaf sandstones, and Lower Cretaceous Nubian (Sarir) Sand- siderations, and risk to assess individually. stone to Paleocene Zelten Formation and Eocene carbonates A graphical depiction of the elements of a total petroleum commonly in the form of bioherms. More than 23 large oil fields system is provided in the form of an events chart that shows the (>100 million barrels of oil equivalent) and 16 giant oil fields times of (1) deposition of essential rock units; (2) trap formation; (>500 million barrels of oil equivalent) occur in the province. Abstract 1

Production from both clastic and carbonate onshore reservoirs is bathymetric contour. The thickness of sediments in the province associated with well-defined horst blocks related to a triple junc- increases from about 1 kilometer (km) near the Nubian (also tion with three arms—an eastern Sarir arm, a northern Sirte arm, known as Tibesti) Uplift on the south to as much as 7 km offshore and a southwestern Tibesti arm. Stratigraphic traps in combina- in the northern Gulf of Sirte. The onshore area is relatively well tion with these horsts in the Sarir arm are shown as giant fields explored for structures, which are dominated by regionally exten- (for example, Messla and Sarir fields in the southeastern portion sive horsts and grabens (fig. 2). Hydrocarbon resources are of the province). Significant potential is identified in areas mar- approximately equally divided between carbonate and clastic res- ginal to the horsts, in the deeper grabens and in the offshore area. ervoirs (pre-Tertiary, dominantly clastic; Tertiary, dominantly Four assessment units are defined in the Sirte Basin prov- carbonate reservoirs). The prospective area in the province cov- ince, two reflecting established clastic and carbonate reservoir ers about 230,000 km2 (Montgomery, 1994; Hallett and El Ghoul, areas and two defined as hypothetical units. Of the latter, one is 1996; MacGregor and Moody, 1998). The offshore portion (figs. offshore in water depths greater than 200 meters, and the other is 3, 4) is far less explored and its petroleum potential is largely onshore where clastic units, mainly of Mesozoic age, may be res- unknown. ervoirs for laterally migrating hydrocarbons that were generated Offshore, geologic relations in the Sirte Basin province indi- in the deep-graben areas. cate a potential for major reserves to be added by the dominant The Sirte Basin reflects significant rifting in the Early Cre- Mesozoic system, but Paleozoic and Cenozoic petroleum systems taceous and syn-rift sedimentary filling during Cretaceous may be proven to exist as well. Speculative hydrocarbon systems through Eocene time, and post-rift deposition in the Oligocene are also postulated for the eastern part of the province, including and Miocene. Multiple reservoirs are charged largely by verti- Lower Cretaceous and Triassic source rocks (Mansour and cally migrating hydrocarbons along horst block faults from Magairhy, 1996; Burwood, 1997; Ambrose, 2000), as well as Upper Cretaceous source rocks that occupy structurally low posi- Eocene and Silurian source rocks in the deeper grabens and off- tions in the grabens. Evaporites in the middle Eocene, mostly shore areas (Hallett and El Ghoul, 1996). post-rift, provide an excellent seal for the Sirte-Zelten hydrocar- Recent petroleum geochemistry data confirm the domi- bon system. The offshore part of the Sirte Basin is complex, with nance of the Upper Cretaceous Sirte Shale (equivalent to the subduction occurring to the northeast of the province boundary, upper part of the Rakb Group; see fig. 7) as the source of hydro- which is drawn at the 2,000-meter isobath. Possible petroleum carbons in the Sirte Basin (Ghori and Mohammed, 1996; Baric systems may be present in the deep offshore grabens on the Sirte and others, 1996). Recent exploration suggests additional poten- Rise such as those involving Silurian and Eocene rocks; however, tial in the grabens, particularly in the southern part of the Zallah potential of these systems remains speculative and was not Graben (Abu Tumayam), the Marada Graben (Maradah), and the assessed. Sirte Graben (Ajdabiya, Kalash, or Sirt), as discussed by Hallett and El Ghoul (1996, fig. 5). Hydrocarbons generated by Eocene source rocks, particularly in the offshore, offer speculative Introduction potential as well.

The Sirte Basin province ranks 13th among the world’s petroleum provinces, exclusive of the U.S. provinces, with 43.1 Acknowledgments billion barrels of oil equivalent (BBOE) of known petroleum vol- ume, and it ranks 15th if U.S. petroleum provinces are included The Sirte Basin was studied utilizing both published data (Klett and others, 1997). The Sirte Basin province is considered a and proprietary databases, including the 1996 Petroconsultants “priority” province by the World Energy Assessment Team as file, the 1998 Geomark oil geochemical file, and other industry described in the Foreword. Sixteen giant (>500 million barrels of sources. References listed in this publication reflect the most rel- oil equivalent (MMBOE)) fields occur in the province; reservoirs evant or current information in the author’s opinion, and also range in age from Precambrian to Miocene. Exploration has include items resulting from searches of the GEOREF database focused on structural highs, principally the horst blocks such as as well as other pertinent geologic information. Basic geologic the Waddan, Az Zahrah, and Zaltan platforms (also variously and petroleum information was utilized from USGS Open-File known as Jebel Uddan, Beda, Dahra, Al Hufra, and Zeltan plat- Reports 97-470A (Persits and others, 1997) and 97-463 (Klett forms) (figs. 1, 2). These platforms are dominated by carbonate and others, 1997), Digital Data Series 60 (U.S. Geological Sur- and bioherm Tertiary reservoirs of Tertiary age. In the eastern vey World Energy Assessment Team, 2000), and UNESCO Sirte Basin, significant stratigraphic clastic traps superimposed 1:5,000,000 tectonic and geologic maps of Africa. on structural highs principally occur in Mesozoic clastic reser- voirs such as at Sarir and Messla fields (fig. 1). The Sirte Basin province is characterized by one dominant Province Geology petroleum system, the Sirte-Zelten, which is subdivided into four assessment units (fig. 1). The Sirte Basin (also referred to as “Sirt Province Boundary Basin”) is a late Mesozoic and Tertiary continental rift, triple junction, in northern Africa that borders a relatively stable Paleo- The south and southeast boundaries of the Sirte Basin prov- zoic craton and cratonic sag basins along its southern margins ince (2043) are drawn at the Precambrian-Paleozoic contact along (fig. 2). The province extends offshore into the Mediterranean the Nubian Uplift and its northeast-trending extension, termed the Sea, with the northern boundary drawn at the 2,000 meter (m) Southern Shelf (figs. 1, 2) or also referred to as the Northeast 2 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System

15°E 20°E25°E

Crete 35°

Pelagian Basin 2048

Mediterranean Basin 2070

? ?

?

? Offshore Sirte Hypothetical

20430103 Cyrenaica Uplift

?

Cyrenaica Basin 2041

Central Sirte Carbonates 20430102 30° Mabruk (017-A/B/C) Bahi (032-A) 032-G-001 Dahra West (032-B/II) Sahl (006-O-004) Hamra Basin Amal (012-U) 2047 Dahra East-Hofra (032-F/Y/011-A/4O) Amal(012-B/E/N/R) Raguba (020-E) 006-HHH-001 LP003C-A-002 Lehib-Dor Marada (006-UU/013-T) Augila-Nafoora (102-D/051-A/G) Zenad(011-VVV) Sorra (006-PP) Intisar (103-A) NC098-A-001 Ghani (011-RRR) Nasser (006-C/4I/4K) 006-4D-001 Intisar (103-D) Bu Attifel (100-A) 011-4I-001 Gialo (059-YY) Zella (NC074B-A) Kotla (047-C/A) Jebel (006-P) Gialo (059-E) 082-UU Eteila (013-G/HH) Beda (047-B) Waha North (059-A) Waha South (059-A) Masrab (059-P) Samah (059-L) Defa (059-B/071-Q) Southeast Sirte Clastics Fezzan Uplift Messla (065-HH/080-DD) 2046 Sarir (065-L) Sarir North (065-C) 20430101 Sarir (065-C) Sirte Basin 20432043

Southeast Sirte Hypothetical 20430104 Murzuk Basin 2045

25°

Tibesti Arch/ Alma Arch

0 100 KILOMETERS Nubian Uplift 2044

EXPLANATION Hydrography ASSESSMENT DATA Shoreline Assessment units boundary and name Geologic province name and boundary Total petroleum system boundary Country boundary Gas field centerpoint Pod of active source rocks boundary— Oil field centerpoint Ticks indicate side of their presence; Annotated Fields with magnitude > 100,000 mbo or mmscfg queried where extent uncertain

Figure 1. Sirte-Zelten Total Petroleum System showing boundaries of the total and minimum petroleum system, pods of ac- tive (thermally mature) source rock, four assessment units, and oil and gas field centerpoints (named fields exceed 100 mil- lion barrels of oil equivalent). Note the 200 m bathymetric isobath forms boundary between Offshore Sirte Hypothetical Assessment Unit (20430103) and Central Sirte Carbonates Assessment Unit (20430102). The 2,000 m bathymetric isobath forms northern boundary of province, total petroleum system, and Offshore Sirte Hypothetical Assessment Unit 20430103. Projection: Robinson. Central meridian: 0. Province Geology 3

15°E 20°E 25°E30°E

Mediterranean Sea Darnah Gulf of Al Jabal Sirte Benghazi Al Akhdar Alexandria

W Suluq a d A Low H d z a u Z M n Z CYRENAICA n a a ( a ( G J h r A ll r a Z ° e a j SHELF r a d a E G Y P T 30 a b h h d a l a Cairo b e ( h t S e l T (A a b U a n i i n g l y r d r H t i ( a e f A d u , e T l a f K t r r J n ) o Sarir Study Area a a a ) u T h n l g a a H EXPLANATION P r , h l o D m s a a u h a m t a ) f g h e o h T h ) i r r r m Structural lows m a P o ) u a l WESTERN P a g t l t h T a f r Aswad o o tf u Tertiary volcanics o r g SHELF Platform r m Messlah h m High Al Bayda Cambrian-Ordovician rocks Zeltan Sari r Trou Platform Platform gh Precambrian rocks S outh Depre Inferred edge Calanscio ssion Trough 0250 KILOMETERS 25° SOUTHERN SHELF Cyrenaica Silver

Cyrenaica Sirte Arm ane L I B Y A Terr Sarir Arm

Tibesti m Arm 20° Tibesti Arm R Upper Cretaceous-middle Eocene E

right-lateral stress shear system C H A D G

I

S U D A N N I G E R

Figure 2. Structural elements of Sirte Basin. Troughs and grabens, platforms and horsts are synonymous terms. Individual horsts and gra- bens possess multiple names. For example, the Sirte (Sirt) Trough is also known as the Kalash or Ajdabiya Trough, as noted (modified from Ambrose, 2000). Barbs show direction of relative movement on faults.

Tibesti Arch/Alma Arch uplift by some authors (for example, Alternative basin outlines have been drawn (for example, Mont- Futyan and Jawzi, 1996). The Cyrenaica Shelf (also referred to as gomery, 1994; Futyan and Jawzi, 1996, Selley, 1997); however, a platform, including both basin and uplift) forms the eastern and the Sirte Basin boundary outline as just described was drawn northeastern border. The western border, generally called the based upon surface geologic and subsurface tectonic maps pre- Western Shelf (fig. 2), is a combination of the Nubian Uplift and pared at a scale of 1:5,000,000 by UNESCO and shown on sup- a northwest-trending extension called the Fezzan Uplift (Tripoli- porting maps such as the Africa geologic map prepared for the As Sawda Arch); the latter feature intersects the Nafusa (Talem- World Energy Project (Persits and others, 1997). zane-Gefara) Arch, an east-west-trending arch along the north- west margin of the province (Persits and others, 1997; fig. 3). The Geographic Setting northern margin is the 2,000 m bathymetric contour (isobath) in the Gulf of Sirte (figs. 1–3). Offshore the province is separated The Sirte Basin is a triple-junction continental rift along the from the Pelagian Basin petroleum province (2048) by the Medina northern margin of Africa (fig. 2). It is bordered on the north by Bank (fig. 3). To the west is the Hamra Basin, to the south the Mur- the Gulf of Sirte (Sidra) in the Mediterranean Sea. Although the zuk Basin, and to the east the Cyrenaica Basin (2041) (fig. 1). Nubian Uplift rises to 3,000 m south of the Sirte Basin, much of 4 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System

E ° 22

HELLENIC ARC CYRENAICA UPLIFT A' E ° 20

MEDITERRANEAN RIDGE ral high boundaries are

E FORE-ARC SCRAPING ZONE

GH Jongsma and others, 1985; Klett ,

TROU CYRENAICA RIDG

SIRTE E ° 18 LOWER SIRTE SLOPE

PE O A

SL IONIAN ABYSSAL BASIN RISE SIRTE UPPER SIRTE MARADA TROUGH E °

16 A-SIRTE RIDGE FORE-ARC SCRAPING ZONE URAT MIS

RAGUSA- MEDINA BANK 100 KILOMETERS MALTA Y PLATEAU BANK MELITA A VALLE LIBYA E MALTA °

BASIN MISURAT 0

14 RABEN G A F CALTANISSETTA MALTA GRABEN MEDINA GRABEN ARRA J

SICILY ISIS HORST MALTA-MEDINA CHANNEL

NEFUSA ARCH

iassic salt iassic Tr of of

limit TEM

eastern NTELLERIA Approximate GRABEN T SYS PA

MPEDUSA

PLATEAU gian E LA ° Sea 12 Pela PLATEAU

XTENSIONAL FAUL ADVENTURE

E MAGHREBIAN TREND MAGHREBIAN HTART-TRIPOLITANIA BASIN

of AS Gulf

GEFARA Hammamet ' is shown in figure 4. ult

GAFSA- of Country boundary Province boundary Province Structural high boundary Axis of structural high Location of cross section Main magmatic areas Fa A–A EXPLANATION Gulf

Gabes E AXIS

° 10 NORTH-SOUTH TUNISIA EMZANE- Central Mediterranean Sea, showing USGS-defined Pelagian province and offshore areas of Sirte Basin, the Rise. Structu TAL GEFARA ARCH GEFARA ° ° ° ° ° 30 38 36 34 32 based on seismic data and drawn 1.0 1.8 second two-way travel time intervals (modified from Finetti, 1982; Bishop, 1988; in press). Section Figure 3. the land area in the basin is characterized by desert steppes and Political and Exploration History includes eolian deposits of the Kalanshiyu and Rabyanah Sand Seas of the Sahara Desert. In a relatively narrow, northern coastal Libya became an independent nation in 1951; however, it strip, some land areas are as much as 47 m below sea level. The has a complex early history dating back to 10,000 years before Sirte Basin is roughly the size of the Williston Basin in North the present (B.P.) when Neolithic cultures domesticated cattle 2 America (≈490,000 km ). Libya is the fourth largest country in and cultivated crops in the coastal zone. Until about 4,000 B.P., Africa and the 15th largest country in the world. nomadic cultures thrived in what is now the Libyan Desert (part Province Geology 5

' 320 240 160 80 0 NE 0 5 10 15 20 A + + + + + + + + + + + + + + + + + + + + + + + + + + +

HELLENIC ARC HELLENIC TROUGH HELLENIC E °

E C E Rise to Hellenic Arc (modified from Finetti, 1982). E24

E

° R

G E22 MS-33

° LOCATION MAP LOCATION E20

° 18 ° ° 36 34

MOHO P-Q leocene Eocene

Messinian Oligocene RECONSTRUCTED ON SEISMIC REFLECTION LINE MS-33 RECONSTRUCTED Pa Cretaceous Middle-lower Miocene Middle-lower Middle Upper Jurassic BASALTIC BASEMENT BASALTIC EXPLANATION Melange Pliocene-Quaternary Messinian Miocene to Paleocene Middle lower Mesozoic Crystalline metamorphic basement Basaltic basement and volcanics Extrusives + + + ssic ra iassic Tr

Ju BOUGUER GRAVITY BOUGUER Cretaceous LOWER SIRTE MARGIN SIRTE LOWER IONIAN ABYSSAL BASIN FORE - ARC SCRAPING ZONE INTERMEDIATE CRUST TYPE CRUST INTERMEDIATE SIRTE RISE SIRTE SIRTE SIRTE Northeast-southwest offshore regional cross section showing structural, tectonic, and crustal relationships in Sirte TROUGH 0 0 5

80 10 15 20 A

320 240 160 DEPTH, IN KILOMETERS IN DEPTH, SW Figure 4.

6 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System

of the Sahara) in what was until then a savanna environment. TCF natural gas, 0.1 billion barrels of natural gas liquids About 4,000 B.P., either migration occurred or the population (BBNGL); Klett and others, 1997), an amount that constitutes 1.7 was absorbed into the Berber tribe. Phoenicians, Greeks, percent of the world’s known oil reserves. Exploration has Romans, Muslims, the Turks of the Ottoman Empire, and finally focused on structural highs (horsts) with little exploration of the Italians subsequently occupied this area prior to its indepen- intervening grabens such as those shown in figure 6. Recent drill- dence. Libya was liberated from the Italians and Germans in ing has demonstrated potential in the grabens; for example, such World War II, and Britain acted as the country’s administrative shows as observed in well A1-119 on the Al Brayqah Ridge overseer from 1943 to 1949. Following a period of transition within the Ajdabiya (Sirte) Trough (Graben) (Hallett and El under the United Nations, a Libyan government was formed in Ghoul, 1996) provide a strong indication that future drilling in the 1951. Concerned about domination by foreign interests, Libya deep trough areas may result in the discovery of several billion passed basic minerals laws in 1953 and 1955; multiple conces- barrels of additional oil reserves. Even though the Sirte Basin is sions were then granted to Esso, Mobil, Texas Gulf, and others, the most explored province in Libya, significant potential resulting in major oil discoveries by 1959. By 1969, production remains particularly in the grabens and offshore areas. from the Sirte Basin exceeded production from Saudi Arabia (3 The prospective area of the Sirte Basin occupies about million barrels of oil per day (MMBOPD), which has now 230,000 km2, with a wildcat drilling density of one new field decreased to 1.5 MMBOPD (Yergin, 1991; Petroconsultants, wildcat per 145 km2 (Hallett and El Ghoul, 1996). The overall 1996; EIA, 1997). Libya nationalized its oil industry in 1973, and drilling density of the basin is 3.3 wells per 100 km2 (MacGregor some U.S. oil companies began withdrawing from Libya in 1982, and Moody, 1998), with an average field depth of 2,100 m. By following a 1981 U.S. trade embargo. By 1986, the remaining comparison, the northern North Sea is nearly three times more U.S. companies were ordered to cease activities in Libya. In intensely explored (nine wells per 100 km2) to average depths of 1992, the United Nations sanctioned Libya in response to the 3,000 m. MacGregor and Moody (1998) believed that the petro- 1988 bombing of a Pan American flight over Scotland. Addi- leum discoveries in the future for the Sirte Basin lie in refocusing tional sanctions applied by the U.S. Sanctions Act of 1996 were exploration to subtle forms of traps such as hanging wall closure, relaxed in 1999. Today, U.S. and other companies have com- relay ramps, and stratigraphic traps; such subtle traps, they menced reentry into Libya, and a number are currently active, led pointed out, are represented in the North Sea but have yet to be by AGIP (Italy), OeMV(Austria), Veba (Germany), TOTAL developed extensively in the Sirte Basin. Recent indications of (France), Nimir (Saudi), WOC (NOC, Conoco, Marathon, hydrocarbons within grabens suggest that these areas have poten- Amerada Hess), ETAP (Tunisia), and others. tial as well as clastic reservoirs beneath the carbonate reservoirs The first reported occurrence of petroleum in the Sirte Basin in the Central Sirte Basin (Hallett and El Ghoul, 1996). The off- was observed in a coastal water well drilled by Italian colonists, shore area beyond 200 m depths is largely unexplored, but it has Libya having become an Italian colony following a 1911–1912 both significant hydrocarbon potential and significant exploration Turkish-Italian war in which Italy controlled part of the country risk. (and ultimately the entire country in 1923). The Italian govern- ment embarked on geologic investigations of the area and pro- duced a geologic map in 1934. Shows of natural gas were Geologic Setting observed in the late 1930’s, but World War II interrupted explora- tion efforts. Competitive bidding for petroleum concessions was The Sirte Basin province is considered to be a holotype of a subsequently permitted by two mineral laws passed in 1953 and continental rift (extensional) area and is referred to as part of the 1955, and exploration by Esso, Mobil, Texas Gulf, and others Tethyan rift system (Futyan and Jawzi, 1996; Guiraud and Bos- commenced with seismic, magnetic, and gravity data being col- worth, 1997). The structural weakness of the area is exemplified lected. From 1956 to 1961 giant oil fields were discovered, by alternating periods of uplift and subsidence originating in late including Amal, Sarir, and Raguba (Montgomery, 1994, fig. 1). Precambrian time, commencing with the Pan African orogeny Lewis (1990) documented the interesting discovery history of that consolidated a number of proto-continental fragments into an these fields including the notation that the Sarir field was nearly early Gondwanaland (Kroner, 1993). bypassed because oil was not anticipated in the Nubian Forma- Early Paleozoic history of the Sirte Basin reflects a relatively tion. That formation was subsequently shown to be a prolific res- undisturbed Paleozoic cratonic sag basin with intermittent peri- ervoir with initial production rates of 20,000 barrels of oil per day ods of arching in the Paleozoic(?) (for example, formation of a (BOPD). By 1961, Libya was exporting oil, and by 1966 it had regional uplift referred to as the Sirte Arch; Bellini and Massa, become the seventh largest oil-producing nation. For a period in 1980; Van Houten, 1980; Anketell, 1996). The timing of this the 1960’s, Libya exceeded Saudi Arabia in petroleum exports uplift is debatable; historically it is considered to be a mid-Pale- (Yergin, 1991). ozoic event, but it could have formed in the Mesozoic preceding According to Hallett and El Ghoul (1996), 9,850 wells have an Early Cretaceous rifting event. been drilled in the Sirte Basin, and of those 1,578 were wildcats. Rifting commenced in the Early Cretaceous, peaked in the They reported 100 billion barrels of oil (BBO), 50 trillion cubic Late Cretaceous, and terminated in early Tertiary time, resulting feet (TCF) of associated gas, and 20 TCF of non-associated gas in in the triple junction (Sirte, Tibesti, and Sarir arms; see inset, fig. place. Clifford (1984) reported 27.9 billion barrels of reserves 2) within the basin (Harding, 1984; Gras and Thusu, 1998; (recoverable oil). Based on data from Petroconsultants (1996), Ambrose, 2000). According to Anketell (1996), the Early Creta- the USGS reported that known petroleum reserves in the Sirte ceous rifting reflected east-west sinistral shear zones (strike-slip) Basin province (2043) were 43.1 billion BBOE (36.7 BBO, 37.7 that strongly controlled clastic deposition in the Sarir arm, but Province Geology 7

West East A1-63 (Projected) B1-3 A1-44 E1-17X1 N1 L1-11 E1-11 A1-11 G1 F1-92 A1 T1 L1-20C6 C10MM1-6 3T-16 3C1-6 I1-12 B3 J1-31 E1-31 D1-31 Feet 2000 AL HUFRAH ZALTAN AMAL L. E FIELD FIELD FIELD oce ne Feet Pale L. Eocene Sea Level oce ne Sea Level U. C Jabal retac M. eo Eoc Al Aswad us ene ne 2000 Pal U oce 2000 eozoic & . C Tertiary . E basement re M tac (Undifferent.) rocks eo L. Eocene 4000 us Paleo ne 4000 cene oce Dahra ale Hun Graben Pal. & P bas Ridge Manzila us em ceo 6000 en eta 6000 t r r cks oc . C ro ks U nt me Waddan Uplift se 8000 ba l. & 8000 Pa Maragh Low Az Zahrah-Al Hufrah Maradah Trough Cyrenaica 10000 Zallah Trough Platform 10000 Ar Rakb High Platform

Zaltan 12000 Platform INDEX MAP 14000 o o

19 23 Ajdabiya Trough 32 o 16000 Gulf of Sirte

Cyre 18000

naic

o 30 20000 Pla tform 22000

o h 28 28o

Southern Shelf

100 KM 0 100 KILOMETERS

Figure 5. East-west onshore structural cross section across Sirte Basin (modified from Roohi, 1996a). Note name changes for troughs and platforms relative to figure 1 and nearly vertical displacement on faults (heavy dashed lines).

15°E 17°E 19°E 21°E 23°E 25°E

33°

Al Bayda Darnah

Misratah CYRENAICA UPLIFT MEDITERRANEAN SEA Tubruq DEVONIAN Binghazi CARBONIFEROUS Al Burdi SILURIAN Suluq

DEVONIAN 31 Sirt ° PERMIAN Ajdabiya H U N G WADDAN Al Brayqah UPLIFT AJDABIYA TROUG DEVONIAN R A AZ ZAHRAH B M E AL HUFRAH ARA Al Jaghbub N PLATFORM D AH H Waddan ZALLAH Hun Maradah Awjilah Jalu N TR 29° TROUGH E B O Jalu S VICIAN A UGH R IL RDO HAMEIMAT TROUGH U G ZALTAN H R Zallah A IA L T BRIAN - O T AL BAYDA PLATFORM EN N O M CAM K E PLATFORM S L BA A . SILURIAN IC T D DEVON NI IAN Al Fuqaha A R GR . H D D - O G N R Samnu U A N O ABU R O N IA T - TUMAYAM IA H N R R A BASIN B S IA B S M 27° U R A C M Q B E L SIL A M R A A P C R URIAN U C D SIRTE BASIN PRE-HERCYNIAN SUBCROP MAP DEVONIAN CARBONIFEROUS 03060 KM Tazirbu Waw al Kabir Figure 6. Pre-Hercynian subcrop map in Sirte Basin area (modified from Hallett and El Ghoul, 1996).

8 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System

Ambrose (2000) alternatively proposed that dextral shear forces of the basin (fig. 5; Hallett and El Ghoul, 1996). The Sarir arm dominated this period of deformation in the Sarir arm. Dextral (eastern limb) of the triple junction in the Hameimat and Sarir shear forces dominated Late Cretaceous tectonism, as discussed Troughs is thought to have an Early Cretaceous origin in which by Gras (1996), Guiraud and Bosworth (1997), and Ambrose clastic reservoirs are juxtaposed against basin structures (El- (2000). The Late Cretaceous rifting event is characterized by the Alami, 1996a; Gras and Thusu, 1998; Ambrose, 2000). The syn- formation of a series of northwest-trending horsts and grabens depositional clastics of the Sarir Formation occupy structural that step progressively downward to the east; the Sirte Trough lows in the Sarir arm, and stratigraphic traps are important com- (variously known as the Ajdabiya Trough, the Abu Attifel Gra- ponents of the petroleum system of this area. These Cretaceous ben, the Hameitat, Kalash, or Sirt trough or graben (Finetti, 1982; clastic reservoirs are treated as a separate assessment unit within Montgomery, 1994; Hallett and El Ghoul, 1996; Roohi, 1996a, b; the total petroleum system. To the west, Late Cretaceous and Mansour and Magairhy, 1996)) represents the deepest portion of lower Tertiary carbonate reservoirs, commonly reefs or bioherms the basin (figs. 2, 3, 4, 5). These horsts and grabens extend from onshore areas northward into a complex offshore terrane that includes the Ionian Abyssal Plain to the northeast (fig. 3). This Marada Fm. plain is underlain by oceanic crust that is being subducted to the Mio. north and east beneath the Hellenic arc (Westaway, 1996, fig. 4). Diba Fm. IV POST-RIFT The Pelagian province to the west, particularly the pull-apart Olig. Arida Fm. Ls. Mbr Augila Fm. basins of the Sabratah Basin and extending along the South Up. Sh. Mbr. Cyrenaica Fault Zone (SCFZ) and the Cyrenaica Platform to the Gialo Fm. east, is strongly influenced by extensional dextral strike-slip Mid. faulting (Anketell, 1996; Guiraud and Bosworth, 1997; fig. 2). To Mesdar Ls. Eocene the south, the Nubian Uplift is the stable continental basement for Regressive Hon Evap. Gir Fm. Seal

this rifted basin. Lower Although the timing of formation of the Sirte Arch is uncer- Facha Dol. ans. Trans. tain, sufficient uplift of the area now known as the Sirte Basin Tr Kheir Fm. took place to cause erosion of pre-Cretaceous sediments over a Harash wide area (fig. 6). An argument favoring the deformation to be of Fm. Zelten

Mesozoic age rather than Paleozoic age may be that it was related Regressive Upper to the west-to-east migration of a mantle plume across North Fm. Jabal Zelten Gr. Khalifa Fm. aleocene Africa that was followed by Cretaceous rifting (Guiraud and Bos- ans. P Tr Dahra Fm. worth, 1997). The youngest rifting in North Africa is now east of III Rabia Sh. the Sirte Basin in the Red Sea where rifting is active today. Beda Fm. Source Thalith Mbr. Syn-rift clastics of the Sarir Sandstone (Nubian Sandstone MEGACYCLES equivalent) in the eastern part of the Sirte Basin province accu- Hagfa mulated in and across a series of east-west-trending horsts and Sh. SYN-RIFT ressive grabens in the Sarir arm during Middle and Late Jurassic and upper Satal Fm. Lower unit

Early Cretaceous time (fig. 2). This depositional period was fol- Transg lowed by rifting along the northeastern arm of a triple junction lower Kalash that resulted in the formation of a series of northwest-southeast- Maa. unit Ls. oriented horsts and grabens (Sirte arm) beginning in the Late Cre- Regr. Sirte Sh. taceous and extending into the early Tertiary. Transpressional Rakb Group Sen. Rechmaf Fm.

forces elevated platforms (horsts) in the Tibesti and Sirte arms to ressive higher structural elevations than in the nearby Trias/Ghadames Tur. Lidam Fm. province to the west in Algeria or the Cyrenaica Platform (Shelf) Transg Cen. Argub Carbonates Bahi Fm. to the east (figs. 1, 2, 5). Nubian Ss. The Sirte Basin is asymmetric, deepening to the east as Regr. Regress. Regressive shown in figure 5. The relative relief on the juxtaposed horst and graben blocks increases to the east coincident with significant thinning of sediments across the province. Erosion associated with the Sirte Arch resulted in truncation of the Paleozoic PRE-RIFT sequence in the Hamra Basin and western Cyrenaica Platform, but these deposits were preserved offshore (Hallett and El Ghoul, III Hofra Fm. 1996; fig. 6, this report). Cambrian-Ordovician sediments were

Pre-Upper Cretaceous Upper Cretaceous Precambrian also preserved over much of northern Libya prior to the formation of the Sirte Arch (El-Hawat and others, 1996). Progressive erosion of younger sediments and subsequent episodes of block faulting Figure 7. Stratigraphic section in central Sirte Basin (Sirte and Tibesti resulted in placing these mostly clastic Paleozoic and Mesozoic arms) (modified from Barr and Weeger, 1972; Montgomery, 1994). Primary reservoirs in a structural high position with respect to thermally hydrocarbon source and seal rock intervals are shown. Reservoirs are mature Cretaceous source rocks that occupied the deeper portion indicated by dots. Province Geology 9

SARIR TROUGH-CALANSCIO TROUGH HAMEIMAT TROUGH

U. EOCENE U. EOCENE BARITONIAN

M. EOCENE M. EOCENE SEAL LUTETIAN MIDDLE EOCENE

L. EOCENE L. EOCENE YPRESIAN LOWER KHEIR FORMATION KHEIR FORMATION ZELTEN ZELTEN UPPER UPPER LANDENIAN KHALIFA SHALEKHALIFA KHALIFA SHALE

RABIA SH. MEMEBER THALITH MEMBER MONTIAN BEDA FM. BEDA FM. BEDA

LEOCENE THALITH MEMBER PA

ANIAN HAGFA SHALE HAGFA SHALE D LOWER MIDDLE KALASH LS. KALASH LS. MAASTRICHTIAN SIRTE SHALE SIRTE SHALE SOURCE

CAMPANIAN TAGRIFET LS. TAGRIFET LS. RESERVOIR RACHMAT FORMATION RACHMAT FORMATION SANTONIAN/ CONIACIAN

UPPER CRETACEOUS ETEL FORMATION ETEL FORMATION TURONIAN

SALT SALT CENOMANIAN Upper Sarir Sandstone Upper Sarir Sandstone MAJOR Middle Sarir Variegated Shale RESERVOIR Sandstone Middle Sarir Red Shale Sandstone MIDDLE JURASSIC SARIR SANDSTONE SARIR SANDSTONE SARIR SANDSTONE LOWER CRETACEOUS LOWER Lower Sarir Sandstone Low Red Shale PRE SARIR er Sa BASEMENT rir Sandstone

Secondary oil reservoir Main oil reservoir Figure 8. Stratigraphic section in eastern Sirte Basin (Sarir arm). The Sarir Sandstone is equivalent of the Nubian Sandstone of figure 7. Modified from Ambrose (2000). on the northwest-southeast-trending horsts, are the dominant Zeltan, Beda, Dahra, and Zalten Platforms, figs. 2, 5). These res- reservoirs in the remaining two arms (the Tibesti and Sirte arms) ervoirs were in turn charged by vertical migration along faults of the triple junction. during the peak petroleum generation period of the early Tertiary; The complex tectonic history of the Sirte Basin resulted in the conditions favoring concentration of petroleum in reservoirs multiple reservoirs and conditions that favored hydrocarbon gen- of various ages along horst boundaries are demonstrated in figure eration, migration, and accumulation, principally on or adjacent 5. The so-called transfer zones of Harding (1984), Knytl and oth- to the horst blocks. Carbonate reservoirs including bioherms ers (1996), and Van Dijk and Eabadi (1996) provide opportunities mostly Paleocene and Eocene age (some Oligocene), were also for the development of stratigraphic or a combination of strati- concentrated on the structural highs in the central Sirte Basin, graphic and structural traps orthogonal to the trends of the horsts principally along the southern portion of the province in the Az and grabens. Stratigraphic nomenclature used in this report is Zahrah, Al Bayda, and Al Janamah Platforms (also known as the shown in figures 7 (Sarir arm) and 8 (Tibesti and Sirte arms). 10 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System

Total Petroleum System fields have 14.5 BBOE known reserves versus 10.6 BBOE known reserves for carbonates as of 1994 (Montgomery, 1994). However, as shown by Harding (1984), the carbonates of the Petroleum Occurrence Paleocene Zelten Formation, containing an estimated 8.5 BBO of ultimate recoverable reserves (33 percent of total EUR), are There is one dominant total petroleum system in the Sirte the single largest reservoirs. The Sirte-Zelten Total Petroleum Basin, here named the Sirte-Zelten Total Petroleum System. The System is divided into four assessment units (fig. 1), as will be naming of the total petroleum system follows the convention of discussed later. Magoon and Dow (1994) whereby the principal source rock is the The Sirte Basin is an example of a dominantly vertically Upper Cretaceous (Senonian/Campanian) Sirte Shale (also migrated petroleum system as shown by Harding (1984), wherein referred to as “Sirt Shale”) of the Rakb Group (figs. 7, 8, 9; El- Upper Cretaceous oil charges multiple reservoirs along fault Alami, 1996a, b). Some authors have distinguished an Upper zones adjacent to horsts and grabens (Price, 1980, fig. 9). Sirte Shale (Campanian) and a Lower Sirte Shale (Turonian), sep- Guiraud and Bosworth (1997) demonstrated the importance of arated by a Tagrifet Limestone (Coniacian/Santonian); all are right-lateral wrench fault systems of Senonian age, and pointed considered source rocks (Mansour and Magairhy, 1996). out that the periodic rejuvenation of these systems in the Sirte The reservoirs in the total petroleum system are nearly Basin was particularly important to vertical migration of petro- equally divided between clastic and carbonate rocks: clastic leum. Refinements of a dominantly vertical horst and graben

MIOCENE

OLIGOCENE INTERIOR SAG

EOCENE

PALE PALEOC*ENE * OCENE CAMBRIAN- ORDOVICIAN ?

GRABEN FILL * UPPER CRETACEOUS

UPPER * CRETACEOUS

CAMBRIAN- ORDOVICIAN ? PRECAMBRIAN CAMBRIAN- ORDOVICIAN ?

PREGRABEN EXPLANATION Shale Micrite Chalk Dolomite Sandstone Crystalline basement Quartzite Hydrocarbon accumulation Limestone Main hydrocarbon source * Unconformity Figure 9. Schematic cross section showing general structural position, stratigraphic age, and lithology of most traps within the onshore Sirte Basin (modified from Harding, 1984). Note Upper Cretaceous source rocks charging multiple reservoirs on platforms (horsts). Barbs show direction of relative movement on faults.

Total Petroleum System 11

tion Forma

Oligocene- Miocene Top Gialo Kheir Zelten Kalash Sirt Shale Tagr ifet Rachmat Etel USS VS MSS Basement Lithology and others, 1996). USS, upper Sarir Sandstone;

k

a

e

p

n

o

i

t

Gas window

a

r

e

n

e

g

l

i O Oil window IN Ma AGE,

Burial history curve for deepest portion of Sirte Trough (the Hameimat Trough), southeastern Basin (modified from Bender 144 123 103 82 62 41 21 0

0 5 1 2 3 4 DEPTH, IN KILOMETERS IN DEPTH, Figure 10. VS, variegated shale; MSS, middle Sarir Sandstone.

12 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System model were provided by Baird and others (1996), who argued for Source Rock normal listric extensional and growth faults in the Sirte Basin as opposed to the more nearly vertical faulting described by Harding Published studies support the interpretation that the Sirte (1984). The structural history is also complicated by the develop- Shale (Upper Cretaceous, Campanian/Turonian) is the dominant ment of transfer or relay fault zones (transtensional and transpres- source rock in the Sirte Basin petroleum province (Parsons and sional areas) that produced additional migration routes to others, 1980; Gumati and Schamel, 1988; Montgomery, 1994; El- reservoirs occupying horst blocks (Knytl and others, 1996; Van Alami, 1996b; Ghori and Mohammed, 1996; Mansour and Dijk and Eabadi, 1996). Magairhy, 1996; Macgregor and Moody, 1998; Ambrose, 2000). A contrasting view is offered by Pratsch (1991), who The thickness of the Sirte Shale ranges from a few hundred believed that the Sirte Basin was an example of lateral migration meters to as much as 900 m in the troughs. These rocks are within and vertically stacked hydrocarbon systems isolated from each the oil-generating window between depths of 2,700 and 3,400 m other. However, the oils in the various age reservoirs have been in the central and eastern Sirte Basin (Futyan and Jawzi, 1996; shown to be genetically linked (for example, El-Alami, 1996b; fig. 1). In figure 1, the outlines of the active pods of thermally Gumati and Schamel, 1988; Gumati and others, 1996). Internal mature Upper Cretaceous source rock are drawn reflecting depth seals occur between the major reservoirs, so if one were consid- of burial of at least 2,865 m (9,400 feet) (Mikbel, 1979; Goudarzi, ering the oils at a reservoir or play level (regardless of the source 1980; Gumati and Kanes, 1985; Ibrahim, 1996), where condi- of the oil) then Pratsch’s concept is understandable. At the larger tions are considered favorable for oil generation (time tempera- petroleum system level, on the other hand, the Sirte Basin is con- ture index (TTI)=15, vitrinite reflectance (Ro)=0.7; Gumati and sidered to be an example of a composite petroleum system in Schamel, 1988; Montgomery, 1994; fig. 10). Parsons and others which petroleum sourced from Upper Cretaceous shales has (1980) reported total organic content (TOC) values ranging from migrated into multiple age reservoirs, dominantly vertically 0.5 to 4.0 percent with an average of 1.9 percent TOC for 129 along normal, transfer, relay, or wrench faults adjacent to horsts. samples from the Upper Cretaceous source rocks. Baric and oth- (See Harding, 1984; Baird and others, 1996; Van Dijk and ers (1996) presented organic geochemical data for a 500 m sec- Eabadi, 1996; and Guiraud and Bosworth, 1997.) tion of Sirte Shale in wells on the Zelten Platform (southern Sirte 22°E

29°

KALASH Abu- TROUGH Attifel HAMEIMAT ? TROUGH

Gialo

Mesrab 'EEI'-8O 'A' pool 'Z' pool KKI-80 5A1-59 CALANSCIO HHI-80 01-80 HIGH UUI-80 MESSLA CALANSCIO TI-80 Messla 28° TROUGH III1-59 HIGH

EXPLANATION Sarir-L-Field JJI-65 VVI-65 UUI-65 SARIR Basement high TROUGH Sarir- ? C-North Oil field Sarir-C- Upper Cretaceous Main source area RACHMAT TROUGH Migration pathway ? Fault—Block on downthrown side

0203010 40 KILOMETERS

Figure 11. Postulated petroleum migration pathways for oil sourced from Upper Cretaceous shales, which also provide a regional seal in Sarir arm area of eastern Sirte Basin. Modified from Ambrose (2000).

Total Petroleum System 13

Abu- KALASH Attifel TROUGH Hameimat Trough

Gialo

Mesrab 'EE'-80

'A'-80 GGG1-59 'Z'-80 KKI-80 Calanscio 'O'-80 HHI-80 UUI-80 TI-80 Messla Messla CALANSCIO High ? TROUGH Sarir-L UU1-65 JJ1-65 VV1-65 Field S ARIR ? TRO ? ? ? UGH Sarir-C-North RACHMAT TROUGH ? Sarir-C-Main EXPLANATION ? Basement high ? Oil field Variegated shale source area Triassic source area Migration pathway Fault—Block on 0203010 40 KILOMETERS downthrown side

Figure 12. Postulated pre-Upper Cretaceous source areas and migration pathways of minor nonmarine source rocks in eastern Sirte Basin. Modified from Ambrose (2000).

Basin), in which total organic carbon (TOC) exceeds 0.5 percent El-Alami (1996b) concluded that the mudstones within the and ranges as high as 3.5 percent; and hydrogen indices (HI) gen- Nubian (Sarir) Formation (particularly in the Lower Cretaceous erally exceed 300 milligrams of hydrocarbon per gram of organic part) are mature enough to generate oil in the deep parts of the carbon (mg HC/g org.C) and reach as much as 600 mg HC/ g org. Sirte Basin, but “their richness does not appear to be sufficient to C). Organic matter is dominantly Type II, and the shales are oil cause generation of large amounts of hydrocarbons” (p. 347). prone. El-Alami (1996b) documented Upper Cretaceous source Ambrose (2000) considered the Upper Cretaceous “Tethyan” rocks ranging from 0.10 to 7.86 percent TOC in the Etel and shale system to be the dominant source rock, with particular Rachmat Formations (Cenomanian-Turonian) and recorded an emphasis on the Sirte Shale as being the primary source rock in average of 1.28 percent TOC for the Sirte Shale in the Abu Attifel the eastern Sirte Basin. He further concluded that potential Trias- (Sirte Graben) in the eastern Sirte Basin. sic source rocks are of limited distribution but as yet are not well known (compare figs. 11 and 12). Several other potential source rocks of ages other than Late Geochemical data based on analyses of 81 oils from Libya Cretaceous have been considered; Burwood (1997), for example, (GeoMark, 1998) indicate that the Sirte Shale is the dominant identified source potential in four mudstones within deeper parts source rock in the Sirte Basin province. Sirte oils are low sulfur of the Sirte Basin including the Sirte Trough. These potential and of relatively high gravity; for example, median oil gravity is source rocks occur in the Sirte Shale (Upper Cretaceous), a lower 36°, ranging from 30° to 43° API, and median sulfur content is part of the Nubian Formation (Triassic–Lower Cretaceous), the 0.3 percent (Petroconsultants, 1996; GeoMark, 1998; fig. 13). Harash Formation (Paleocene), and the Antelat Formation The oil generally has low gas/oil ratios (median 300 ft3/ barrel, (Eocene, Gir Formation equivalent) (figs. 7, 8). Silurian age U.S. Geological Survey World Energy Assessment Team, 2000). source rocks, the Tanezzuft Formation, may be potential in Alge- Although as many as four oil families can be identified, all ria where present in deeper areas off the flank of the ancestral but three of the 81 samples analyzed by GeoMark (1998) can be Sirte Arch, and in the offshore area where the Silurian may be linked to a single oil family (Jerry Clayton, written commun., present (Futyan and Jawzi, 1996; Klitzsch, 1996, Gumati and 1998) based on (1) pristane/phytane ratios (>1.0) to δC13 aro- others, 1996; fig. 6). matic content, (2) sulfur content, (3) API gravity, and (4) δC13 14 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System

Sirte Basin Oils

2.5 A

2

1.5 Series1 1

0.5

0 PRISTANE / PHYTANE RATIO 1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 SAMPLES

5 B 4.5

4

3.5

3

2.5 Series1 2

1.5 PERCENT SULFUR 1

0.5

0 1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 SIRTE BASIN OILS

60 C

50

40

30 Series1

20

10 API GRAVITY, IN DEGREES 0 1 4 7 61 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 64 67 70 73 76 79 SIRTE BASIN OILS

Figure 13. Oil geochemistry plots utilizing data from GeoMark (1998) for 81 oil samples from 55 fields in Sirte Basin. A, Pristane/phytane ratios; B, Sulfur percentages; C, API gravity.

Total Petroleum System 15

- ACCUMULATION

PETROLEUM

IGRATION M brian-Ordovician sandstones; SYSTEM EVENTS esian; G, Gialo Formation car- ian; S, Sirte Shale Member of Rakb

GEOLOGIC TIME ROCK UNIT RESERVOIR ROCK GENERATION- SCALE

OVERBURDEN ROCK PRESERVATION CRITICAL MOMENT SOURCE ROCK SEAL ROCK TRAP FORMATION

0 Plio Neogene Mio M Post Rift A 24 Olig G ? Paleogene Eoc H 50 Pal Z 65 S Syn-Rift L R 100 Cretaceous E N 150 144 L M Jurassic 200 E 206 L Tr iassic E M 250 248 L Permian E

290 L 300 Pennsylvanian M Pre-Rift 323 E Hercynian Erosion L Mississipian 350 E 363 L Devonian M 400 E 408 ? L Silurian TZ 439 E 450 L Ordovician M G E 500 510 L M Cambrian

550 E Sirte-Zelten Total Petroleum System events chart. Significant rock units noted on chart include GH, Gargaf/Hofra Formation—Cam 570 tures Frac Precambrian 600 Figure 14. Group, primary source rock—Campanian; Z, Zelten Group carbonates—Paleocene; H, Hon Evaporite Member of Gir Formation—Eocene-Ypr bonates—middle Eocene–Lutetian; A, Arida Formation—Oligocene; M, Marada Formation—Miocene. TZ, Tanezzuft Shale—Silurian; N, Nubian or Sarir Sandstone—Jurassic–Early Cretaceous; R, Rakb Group—Cretaceous, Turonian-Campan

16 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System

saturates vs. δC13 aromatic characteristics (fig. 13). El-Alami deposited within the rift basins, and some formed significant (1996b), Jerry Clayton (written commun., 1998), and Ambrose stratigraphic traps adjacent to horst blocks such as Messla and (2000), concluding that the Upper Cretaceous Sirte Shale is by far Sarir fields (Ambrose, 2000; figs. 15–18). the dominant source of oil for the Sirte Basin province, also rec- Northwest-southeast-trending horst and graben structures ognized that there may be contributions from other Upper Creta- formed in the Sirte arm of the triple junction from the Late Cre- ceous source rocks in a composite total petroleum system, the taceous to the end of the Paleocene in the central and western Sirte-Zelten. (See Glossary in U.S. Geological Survey World Sirte Basin, with the younger trends extending offshore (fig. 2; Energy Assessment Team, 2000.) Van der Meer and Cloetingh, 1996; Guiraud and Bosworth, The remaining three oils analyzed by GeoMark (1998) 1997). Overburden was largely deposited during the post-rift sed- appear to be younger (Tertiary, most probably reflecting a Pale- imentation stage (Oligocene and younger). Pre-rift and early syn- ocene/Eocene source); however, they may also reflect biodegra- rift deposition was largely clastic whereas later syn-rift deposi- dation of the Upper Cretaceous source (Jerry Clayton, written tion was dominated by carbonate deposition. According to Hal- commun., 1998). lett and El Ghoul (1996) (1) the deepest troughs, such as the Sirte Tertiary petroleum systems may possibly exist in the deeper (also called Ajdabiya Graben, fig. 2), have more than 7,000 m of part of the Sirte Basin (for example, the Sirte or Ajdabiya graben sediment, of which 5,500 m is of Tertiary age (fig. 5); and (2) 80 of Baird and others, 1996, figs. 1, 6) and in offshore areas. A dis- percent of the drilling in the province has been on platform tinctly different oil—lower gravity (<25° API), high sulfur (as (horst) areas at depths less than 3,000 m. much as 4.5 percent), and pristane/phytane ratios <1.0—is found in lower Tertiary reservoirs, particularly Eocene sequences (for example, Kalash Limestone, Gir or Kheir Formations, figs. 7, 8). Reservoir Rock Tertiary carbonates may contain indigenous hydrocarbon sources particularly in Eocene rocks (Gir Formation) that could poten- The distribution and quality of reservoirs in the Sirte Basin tially have contributed about 1 billion barrels of recoverable oil are most directly related to major tectonic events in the region, reserves in the Pelagian Basin (Bouris, Ashtart, Sidi el Itayem specifically reservoirs related to pre-rift, syn-rift, and post-rift fields, fig. 3) to the northwest of the Sirte Basin (Bishop, sequences (figs. 7, 8). The current configuration of the Sirte 1985,1988; Rodgers and others, 1990). However, few data are Basin, deepening from west to east and also offshore, signifi- available on these potential source rocks in the Sirte Basin. cantly influenced reservoir development. The basin is unusual in Abugares (1996) concluded that hydrocarbons in the Gir Forma- that oil is produced from Precambrian (fractured basement), tion are actually sourced by Sirte Shales. El-Alami (1996b) and Cambrian-Ordovician, Triassic through Lower Cretaceous, Pale- MacGregor and Moody (1998) suggested a secondary contribu- ocene, and Eocene rocks. Charging of multiple reservoirs from tion, where mature, from the Paleocene Heria Shales, which are Upper Cretaceous source rocks along faults, principally along depositionally similar to the Sirte Shale; they also recognized the horst blocks, is illustrated in figure 9. Carbonate reservoirs, possibility of other Tertiary-age source rocks, including the Kheir mostly of Tertiary age, contain 42 percent of the petroleum; and and Harash Formations (Paleocene/Eocene, figs. 7, 8). clastics, mostly of pre-Tertiary age, contain 58 percent (Harding, As shown on the events chart (fig. 14), hydrocarbon genera- 1984; Petroconsultants, 1996). Carbonates of the Paleocene tion commenced about 50 million years ago (Ma) in the deeper Zelten Group contain 33 percent of the known petroleum; this is basins, about 40 Ma in many other areas, and may continue to the the single largest reservoir interval. However, the clastics of present day. Geothermal gradients generally range from 1° F / Early Cretaceous age (Nubian or Sarir Sandstone) contain 28 100 ft to 1.8° F / 100 ft; in general the horsts and grabens have percent, and clastics of Cambrian-Ordovician age (Gargaf, roughly equivalent thermal regimes relative to the primary source Hofra or Amal Group) contain 29 percent of known petroleum rock (Gumati and Schamel, 1988). Along the southwest and west volume (fig. 9). margins of the province are extrusive igneous deposits (fig. 2) Montgomery (1994) divided the stratigraphy of the Sirte whose presence tends to diminish the hydrocarbon potential of Basin into four megacycles (fig. 7). The oldest, of Paleozoic age, these areas (Busrewil and others, 1996; Hallett and El Ghoul, consists mostly of nonmarine clastics and some volcanic rocks, 1996). bounded by a Precambrian unconformity below and the Hercyn- ian unconformity of Late Permian and Early Triassic age above; local thicknesses are as much as 1,500 m. Cambrian-Ordovician Overburden Rock sandstones, variously called Gargaf (western Sirte Basin), and Hofra Formation or Amal Group (central Sirte Basin; fig. 7) are As shown on the events chart (fig. 14), the Upper Cretaceous important reservoirs. These reservoirs produce oil in 23 fields, at Sirte Shale is the source rock deposited generally in grabens that least 5 of which are giant accumulations—Amal, Raguba, had formed during a period of active rifting in the Sirte and Nafoora, Samah/Bel Hadan, Waha fields (fig. 1). Following dep- Tibesti arms of the rift system (fig. 2). During the Cretaceous, osition of the Cambrian-Ordovician, uplift occurred and much of there were apparently two distinct rifting stages related to differ- the Paleozoic section was eroded from the central Sirte area ent limbs of a triple junction that were active at different times. In (Massa and Delort, 1984; Anketell, 1996, fig. 6). However, Cam- the eastern part of the Sirte Basin, the development of a series of brian-Ordovician reservoirs are preserved particularly in fault east-west-trending horsts and grabens in the Sarir arm (fig. 2) was blocks related to subsequent rifting, and possibly in some of the accompanied by syn-rift deposition of Early Cretaceous clastics offshore areas as well. The Cambrian-Ordovician reservoirs are called the Nubia or Sarir Sandstone. These clastic materials were commonly tightly cemented orthoquartzites (Roberts, 1970; Total Petroleum System 17

21°E 22°E

300

ABU ATTIFEL FIELD 29° QUADEEM FIELD

2100 2400 A' R3-82 1800 600 GIALO FIELD HAMEIMAT TROUGH KALASH TROUGH 1500 MESRAB FIELD 1200 300 900 300

'EE' Pool 600 'A' Pool IN 300 G 'Z' Pool R MA A N 5A1-59 GGG1-59 ER 'KK' Pool 600 RTH O E 900 N CALANSCIO HIGH A S 'HH' Pool

T 'O' Pool N CALANSCIO I E 'UU' Pool 300

G R ?900 ?1200 N 'T' Pool R

?600 III 1-59 A M 28° 300 TROUGH M A MESSLA FIELD MESSLA R G N HIGH I R N

E

T S HERN MARGIN RACHMAT E SOUT 300 TROUGH W SARIR-L FIELD VV1 Pool 300 UU1 Pool JJ1 Pool 900 300 12001500 SARIR-C-NORTH 600 SARIR TROUGH 900 600 300 C280-65 Y1-65 EXPLANATION SARIR-C-MAIN Sarir zero edge 300 300 m interval isopach Sarir Sandstone Pre-Sarir basement Oil field Fault—Block on down- 0 50 KILOMETERS thrown side 27° Figure 15. Tectonic elements and oil fields in eastern Sirte Basin. Sarir Sandstone isopachs are shown in main depocenters (modified from Ambrose, 2000). Cross section A–A' shown in figure 16. Barr and Weegar, 1972; Brennan, 1992). Production is enhanced both alluvial and eolian deposits, although Ambrose (2000) iden- by fracturing as noted by Roberts (1970) at Amal field and Bren- tified significant reservoirs in the Upper Sarir Sandstone that are nan (1992) at Raguba field. In a few areas, Silurian, Devonian, fan delta deposits. El-Hawat and others (1996) divided the and Carboniferous clastics are preserved and are potential reser- Nubian Sandstone into three members that reflect the interfinger- voirs (fig. 6). ing of alluvial deposits with Tethyan marine sediments as the The Nubian (Sarir) deposits, ranging from Triassic/Late Sirte rift system began to develop in Neocomian-Barremian time Jurassic? to Early Cretaceous in age, constitute the second major (El-Hawat, 1996; Ambrose, 2000). East-west-trending sinistral megacycle in the Sirte Basin province (Montgomery, 1994; El- shear zones (strike-slip) strongly controlled deposition of the Hawat and others, 1996; this report, figs. 7, 8) and are the produc- Nubian sequence according to Anketell (1996) and Abdulghader ing interval in at least 72 fields in the eastern Sirte Basin (Petro- (1996), and Ambrose (2000) documented the sedimentological consultants, 1996). These clastic strata are the primary reservoirs pattern and heterogeneities within the Nubian sequence that may in several giant fields including Messla, Sarir, Amal, and Abu have enhanced hydrocarbon potential of the Sarir Sandstone in Attifel fields (fig. 1). The reservoirs are continental sandstones, the eastern Sirte Basin. 18 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System

WEST (bend in section) EAST CALANSCIO AREA SARIR-C Field HAMEIMAT TROUGH 5A1-59 JJ1-65 C280-65 R3-82 A GR SLS GRSLSGR GR LLD A'

EXPLANATION Upper Sarir Sandstone (USS) Variegated shale Middle Sarir Sandstone (MSS) Scale Red shale (RS) Lower Sarir Sandstone (LSS) 150 m Basement Unconformity

Figure 16. Stratigraphic cross section A–A' in Calanscio-Hameimat Trough (line of section shown in fig. 15). Note subdivisions (members) within the Sarir Sandstone and the combined structural-stratigraphic trap at Sarir-C field (modified from Ambrose, 2000).

The Nubian (Sarir) Formation is highly variable in thickness these terminations, such as in the Upper Sarir fan delta deposits. (maximum as much as 2,500 m); that variation reflects the infill- Abdulghader (1996) documented average porosity of 27.5 per- ing of low areas (grabens) that partially extend across some Cre- cent for the main Sarir reservoir. Lewis (1990) documented aver- taceous horsts and formed prolific stratigraphic/structural traps age porosity of 18–19 percent, and average permeability of 200– such as Sarir field (Sanford, 1970; Lewis, 1990) and Messla field 300 millidarcies at Sarir field. (Clifford and others, 1980; Koscec and Gherryo, 1996; Ambrose, The third megacycle, largely composed of marine deposits, 2000; this report, figs. 15–18). Lewis (1990) suggested that Sarir of Late Cretaceous and early Tertiary ages, encompasses the field is a complex of individual fields, Sarir C alone containing entire Sirte Basin, and depositionally followed the early nonma- 6.5 BBO EUR. A cross section from Messla field by Ambrose rine graben fill of the Sarir (Nubian) sediments in the eastern Sirte (2000), shown in figures 15–18, portrays the major stratigraphic Basin. As summarized in Montgomery (1994), two major Upper truncation that occurs in the Sarir Sandstone across the structural Cretaceous transgressive and regressive cycles and two Pale- high and the opportunity for additional exploration targets near ocene cycles of sedimentation characterize this later syn-rift fill

Total Petroleum System 19

WEST EAST (Messla Field) 002-65 001-65 UU1-65 JJ1-65 VV1-65 HH11-65 Top Tagrifet Rachmat Fm. Etel Fm. Tethyan USS Top Sarir SS. shales Red shale MSS RS LSS Tar mat Tar mat Variegated Basement shale LSS MSS MSS 0 m

150 m Red shale

300 m Logs used: GR(LHS), LLD(RHS)

Tethyan Shale sequence EAST Upper Sarir Sandstone (USS) VV1-65 Variegated shale WEST JJ1-65 UU1-65 HH11-65 Middle Sarir Sandstone (MSS) 002-65 Producing well Red shale (RS) 001-65 Lower Sarir Sandstone (LSS) Dry hole 0 10 20 KILOMETERS Sarir Sandstone Basement Unconformity Figure 17. East-west structural cross section across western margin of the Messla High (modified from Ambrose, 2000). Note that faulting caused erosion of different members of the Sarir Sandstone along the highs.

SW NE SARIR TROUGH MESSLA HIGH HAMEIMAT TROUGH Approx 2000 m

0 20 KILOMETERS USS Truncation Play

UPPER CRETACEOUS

RY Messla Field Top Sarir Sandstone TIA TER UU1 Field USS 2500 m S USS EOU TAC Variegated shale CRE ER tone UPP nds ir Sa MSS Sar Top MSS Basement LSS Red shale USS Red shale MSS Thin silty variegated shale 3000 m

LSS Red shale USS = Upper Sarir Sandstone Unnamed Pre-Sarir MSS = Middle Sarir Sandstone (? Triassic) LSS = Lower Sarir Sandstone

Figure 18. Schematic structural cross section from Sarir Trough across Messla High and into Hameimat Trough showing hydrocarbon habitat of the Sarir Sandstone (modified from Ambrose, 2000). Note truncation and stratigraphic trap potential in various members of the Sarir Sandstone. Scale is approximate.

20 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System sequence (fig. 7). Sedimentation was largely controlled by north- because no commercial shows of hydrocarbons have been found west-southeast-oriented horst and graben structures. Erosion in rocks younger than Eocene. along the Sirte Arch in the central Sirte Basin has removed evi- In general, clastic reservoirs are dominantly of Early Creta- dence of the older sediments, including the Lower Cretaceous ceous and older (pre-rift, early syn-rift) ages in the eastern Sirte clastics, although the hydrocarbon potential is largely undevel- Basin, whereas carbonate reservoirs are dominantly uppermost oped in structurally low areas within the central Sirte Basin. Cretaceous, Paleocene, and Eocene (syn-rift or late syn-rift) else- The first Upper Cretaceous depositional cycle consists, in where in the Sirte Basin province. The assessment units, dis- ascending order, of (1) a transgressive sequence comprising the cussed later, were delineated using these criteria. Bahi Formation; (2) carbonates, commonly represented by dolo- mites of the Lidam Formation; and (3) a regressive sequence of carbonates, evaporites, and shales that now characterize the Etel Seal Rock Formation (a potential source rock interval, figs. 7, 8). A second transgression resulted in the deposition of shallow marine sedi- Although carbonates and shales dominate much of the Ter- ments (Rachmat Formation) followed by deposition of deep tiary sequence, the evaporite and salt deposits of the Eocene Gir marine sediments (Sirte Shale). Shallower water sequences in the Formation (Hon Evaporites Member; Abugares, 1996; fig. 7) are younger Cretaceous Kalash Limestone are in turn overlain by of the utmost importance to petroleum accumulation in the Sirte shales (Hagfa Shale, similar lithologically to the Sirte Shale) and Basin province because they form essential seals for the hydro- carbonates (Beda Formation) of Paleocene age. The Beda Forma- carbons migrating out of the Upper Cretaceous into reservoirs of tion is a significant reservoir, commonly consisting of calcilutite, many ages along the faulted horsts and grabens (Harding, 1984; calcarenites, oolites, and skeletal debris along the south margin of Montgomery, 1994; Abugares, 1996; figs. 7, 9). The formation the Sirte Basin; and significant production has taken place in shows rapid changes in thickness, reaching a maximum of 1,305 fields such as Dahra and Hofra and in many fields on the Az m in the Marada Trough. It contains dolomite, anhydrite, and Zahrah (Dahra and Beda) and Zelten (Al Janamah) platforms halite, but also shows abrupt lithologic changes. In some areas, (Bebout and Poindexter, 1975; Brady and others, 1980; this halite forms 35 percent of the total section, as in the Zallah report, figs. 2, 5). Trough (Abugares, 1996), but the salt component decreases dra- Continued sedimentation during Paleocene time resulted in matically around the onshore margins of the Sirte Basin, which deposition of the Zelten Formation, which is notable for its car- might help to explain the absence of significant oil fields near the bonate build-ups and reefs that grew on the structurally elevated outcrop belt and the high risk of drilling in the offshore area. horsts such as the Az Zahrah (Beda and Dahra) Platform and the Zelten (Al Janamah) Platform. As previously discussed, the Trap Style Zelten Formation is the single largest reservoir interval in the Sirte Basin province. Carbonates, mostly Upper Cretaceous, Shown on the oil and gas field map for the Sirte Basin (fig. 1) Paleocene, and Eocene, are the dominant reservoirs in 150 fields are the 237 fields that were considered in this study. Of these encompassed in the area shown as assessment unit 20430102 in fields, 221 are oil fields and 16 are gas fields (Petroconsultants, figure 1. Carbonate deposition in the late syn-rift sequences took 1996). Known petroleum volumes of 43.1 BBOE rank the Sirte place particularly during Paleocene time on the platforms along Basin province as the 13th in the world, exclusive of the U.S. (15th the south margin of the Sirte Basin. Carbonates are important res- if U.S. provinces are included). It is dominantly an oil province ervoirs in many giant oil fields such as Intistar, Beda, Defa, Waha, with 36.7 BBO, 37.7 TCFG (6.3 BBOE), and 0.1 BB NGL (Klett Haram, Zelten (Nasser), Hofra, and Nafoora (Belazi, 1989; and others, 1997). Twenty-three fields are major oil fields (>100 Roohi, 1996b; figs. 1, 19). MMBOE), and 16 fields rank as giant oil fields (>500 MMBOE). The upper Paleocene regression of this megacycle is repre- The dominant trap style is structural (84 percent), with the sented by the deposition of shallow marine carbonates (Harash remainder considered stratigraphic or a combination of the two and Kheir Formations) followed by transgressive deposits of the (Clifford and others, 1980). As examples of combined traps, Facha Dolomite (fig. 7). These were ultimately overlain and sub- bioherm developments in the Paleocene Zelten Group are found sequently sealed in a final regression of this megacycle by the on horst blocks, and clastic stratigraphic traps such as at Sarir or evaporites of the Gir Formation (Hon Evaporites Member). The Messla field are superimposed on structures (figs. 15–18). Simi- carbonates in the upper part of the Gir Formation are the strati- larly, bioherm development in Paleocene sediments of the Zelten graphically youngest potential reservoirs, containing about 1 per- Group in the Zelten field (Bebout and Poindexter, 1975), which cent of the proven hydrocarbon reserves in the Sirte Basin. occurs on a major horst, is shown to represent the carbonate Stratigraphic traps within the Eocene strata, including nummu- assessment unit (20430102). An example of such a carbonate litic banks and dolomite zones in the Facha Dolomite (fig. 7) as field is shown for Idris field in the northeastern Sirte Basin (Terry well as possible indigenous, high sulfur (3 percent), heavy oil and Williams, 1969; this report, fig. 19). shows in tests near Beda-Haram, between Intisar and Amal fields, and in the eastern Sirte Basin (Ghori and Mohammed, 1996, fig. 1) indicate that the Eocene sequence may be an exploration target in the deeper portions of onshore grabens and offshore Petroleum Assessment (Abugares, 1996). The final megacycle represents the post-rift fill of Oligocene The discovery history of the Sirte Basin started relatively and younger age and is of minor potential for hydrocarbons late (as discussed earlier) because Libya did not really become an Petroleum Assessment 21

P A L E O C E N E N E C O E L A P SERIES E N E C O E

Landenian Land. Landenian

STAGE Ypresian

Upper Middle Lower Upper Kheir Marl Lower Kheir Marl Lower Zelten Carbonates Heira Shale Heira Heira Carbonate Heira Non-reef well 9000 ft (2743 m)

n

e

t l out of section

e

Z 6560 ft (2000 m) .

L ing reservoirs in carbonate assessment unit (modified < 20%

Ο A15-103 < 20%

Ο 20-25%

Ο 20-25%

Limestone zones Bio-facies Porosity zones Porosity Ο

Reef Limestone

A22-103 A7-103 Ο

R

E

B

M

E M

< 20%

F

> 25%

Ο

E

Ο

E

ition Zone ition

R EXPLANATION

L

A

R

ity Trans ity

O

C

oros P ALGAL FORAMINIFERAL MEMBER Sealing evaporites Shale and marl limestone Argillaceous > 25%

> 25%

Coralline Biomicrite Ο Ο

< 20% 20-25%

Ο Ο Ο 20-25% > 25%

Ο

A9-103 A12-103 A20-103 A18-103

%

< 20 <

GIR FORMATION Ο out of section 1000 METERS 5740 ft (1750 m) 9000 ft (2743 m) Non-reef well HORIZONTAL SCALE HORIZONTAL Stratigraphic cross section through Zelten Formation (Paleocene) bioherms, Idris “A” field, northeastern Sirte Basin, represent 0 2700 2800 2900 3000 3100 3200 3300 VERTICAL EXAGGERATION =1X2.5 EXAGGERATION VERTICAL VERTICAL SCALE ft m Figure 19. from Terry and Williams, 1969). 9000 11000 10000

22 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System independent country until 1951. Giant fields were found between Some minor calcarenite Middle Cretaceous production is also 1956 and 1961, including Amal, Sarir, Raguba, and Zelten plat- included in these fields. form fields (Carmalt and St. John, 1986; Brennan, 1992; fig. 1). Offshore Sirte Hypothetical (20430103) extends from water Three distinct discovery segments are seen on cumulative oil depths of 200 m to 2,000 m and does not contain any established plots—a steep segment from 1956 to 1961, an intermediate seg- fields, although hydrocarbon shows have been encountered. Both ment from 1962 to 1970, and a nearly flat segment since 1970 positive and negative petroleum factors are recognized for this (fig. 20). The discovery process has been interrupted by political area. On the positive side is the likely presence of thermally events, including sanctions against Libya that prohibited U.S. mature Upper Cretaceous and possibly Silurian and Eocene company involvement in exploration in Libya since 1981. Mont- source rocks, and the likelihood of both carbonate and clastic res- gomery (1994) and MacGregor and Moody (1998) have con- ervoirs being present. On the negative side, tectonic complexity is tended that the Sirte Basin has a significant future potential, citing considerable, including a subduction zone and numerous extru- the fact that the North Sea is three times more heavily explored sive magmatic features that may have compromised preservation than the Sirte Basin as a comparison. of the total petroleum system (figs. 4, 5). To address such circum- Four assessment units within the Sirte-Zelten total petro- stances (and uncertainties), the world petroleum assessment leum system (two established, two hypothetical) were defined for (U.S. Geological Survey World Energy Assessment Team, 2000) the present study; they are as follows: takes into account three geologic risk elements pertaining to rock characteristics (source, reservoir, and seal), migration and trap- Southeast Sirte Clastics (20430101) is an established unit ping conditions, and timing of geologic events as well as an addi- and includes Cretaceous and older (Cambrian-Ordovician, tional risk involving accessibility for exploration. Two of the Precambrian) fields in the onshore area of the eastern Sirte Basin three geologic risk factors are applied (1.0 indicating no risk) in (fig. 1). this assessment unit: (1) rocks (0.8 or an 80 percent chance of Central Sirte Carbonates (20430102) is an established unit success), and (2) timing of geologic events (0.6 or a 60 percent and contains fields producing from carbonates of the Upper chance of success). Cretaceous, Paleocene, and Eocene platforms, mostly onshore, Southeast Sirte Hypothetical (20430104): Although there with potential for production to water depths of about 200 m have been more than 65 wildcat tests in the southeastern Sirte (fig.1). Offshore production has been established in the water Basin, none had hydrocarbon shows, according to Petroconsult- depth range of 200 m or less in the Gulf of Sirte (figs. 1, 2). ants (1996). This area is therefore also considered to be a

Southeast Sirte Clastics, Assessment Unit 20430101

25,000

20,000

15,000

10,000

5,000

CUMULATIVE KNOWN OIL VOLUME, IN MMBO 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 FIELD DISCOVERY YEAR Figure 20. Cumulative plot of volume of known oil (produced and remaining) versus field discovery year for Southeast Sirte Clastics Assessment Unit (from U.S. Geological Survey World Energy Assessment Team, 2000). Petroleum Assessment 23 field size assessed (MMBO or BCFG). Prob.,

the amount tabulated. Other fractiles are defined Results shown are fully risked estimates. For gas 000000 Gas (BCFG) NGL (MMBNGL) Undiscovered resources 00000000 2,467 5,618 18,740 8,366 71 166 576 251 53 0 13 222 60 0 1 13 4 840 1,488 3,633 6,946 3,841 88 217 419 231 Mean F95 F50 F5 Mean F95 F50 F5 Mean 56,1443, Oil (MMBO) 2,600 6,148 12,352 6,854 1,683 4,318 15,679 6,618 100 258 946 397 F95 F50 F5 1.00 1.00 0.48 1.00 890 2,2931.00 4,130 1,711 2,380 3,7850.48 6,144 195 3,8400.50 0 6720.50 3,956 1,879 0 9,252 808 0 1,254 18,1981.00 9,918 382 12 701.00 159 2,600 824 40 0 6,148 383 253 113 12,352 0 766 6,854 14,121 48 0 413 4,151 4,199 13 9,936 34,419 0 222 14,984 0 60 171 629 0 424 183 1,521 1 648 13 4 51,7113,78 50708242 1060 89030 2,293 4,13010 2,38060 195 0 67230 1,879 0 808 1,254 0 382 12 0 2,467 40 0 5,618 11,252 113 0 6,076 0 48 6,633 71 1,909 0 166 7,488 0 2,290 347 0 182 0 400 0 115 229 69 Total: Sirte-Zelten Total Petroleum System Sirte-Zelten, Total Petrolem System 204301—assessment results summary. Total Total Total Total Total Oil fields Oil fields Oil fields Oil fields Oil fields type (0-1) Code Gas fields Gas fields Gas fields Gas fields Gas fields and field MFS Prob. Table 1. fields, all liquids are included under the NGL (natural gas liquids) category. F95 represents a 95 percent chance of at least similarly. Fractiles are additive under the assumption of perfect positive correlation. Shading indicates not applicable] probability (including both geologic and accessibility probabilities) of at least one field equal to or greater than the MFS. [MMBO, million barrels of oil. BCFG, billion cubic feet gas. MMBNGL, natural gas liquids. MFS, minimum 20430102 Central Sirte Carbonates Assessment Unit 20430103 Offshore Sirte Hypothetical Assessment Unit 20430104 Southeast Sirte Hypothetical Assessment Unit 204301 20430101 Southeast Sirte Clastics Assessment Unit

24 The Sirte Basin Province of Libya—Sirte-Zelten Total Petroleum System 4,000.0 Mean oil 3,500.0 Mean gas 3,000.0 Mean NGL

2,500.0

2,000.0

1,500.0

1,000.0

500.0 MILLION BARRELS OF OIL EQUIVALENT

0.0 20430101 20430102 20430103 20430104 Geologic Risk 1.00 Geologic Risk 1.00 Geologic Risk 0.48 Geologic Risk 0.50

ASSESSMENT UNITS GEOLOGICALLY RISKED

Figure 21. Volumetric assessment results for geologically risked assessment units in Sirte-Zelten Total Petroleum System in Sirte Basin province 2043.

4,000.0 Mean oil 3,500.0 Mean gas 3,000.0 Mean NGL

2,500.0

2,000.0

1,500.0

1,000.0

500.0 MILLION BARRELS OF OIL EQUIVALENT

0.0 20430101 20430102 20430103 20430103 20430104 20430104 Unrisked Unrisked Unrisked Risk Factor 0.48 Unrisked Risk Factor 0.50

ASSESSMENT UNITS GEOLOGICALLY RISKED AND UNRISKED Figure 22. Volumetric assessment results for geologically risked and unrisked assessment units in Sirte-Zelten Total Petroleum System in Sirte Basin.

Petroleum Assessment 25 14,000 Mean oil Mean gas Mean NGL 12,000 Total

10,000

13,583 8,000

6,000 9,600 9,999

4,000 6,854

MILLION BARRELS OF OIL EQUIVALENT 3,983 2,000 2,497 648

0 1994 ASSESSMENT 2000 ASSESSMENT Figure 23. Comparison of U.S. Geological Survey World Energy Assessment Team (2000) mean risked estimates for oil, natural gas, and natural gas liquids versus Masters and others’ (1994) mean estimates, Sirte Basin province 2043. hypothetical assessment unit. The Southeast Sirte Hypothetical method calculated a median estimate of 12.2 BBOE, and the frac- (20430104) represents the possibility of hydrocarbon charging tal method calculated an estimate of 47.5 BBOE. The median dominantly shallow clastic reservoirs along major fault systems. value of Masters and others’ (1994) assessment, which was a Potential reservoirs include Campanian marine bar sandstones geologically based Delphi estimate, was 10.7 BBOE and a mean (Hammuda, 1980; Montgomery, 1994).The entire risk (0.5 or a of 11.5 BBOE. 50 percent chance of success) for this assessment unit is taken in The results of the current resource estimates are shown in the risk element involving the sourcing of petroleum for reservoir table 1 and in figures 21 and 22. These estimates are derived from rocks, because reservoirs and geologic structures appear to be geologic inputs to the Monte Carlo probabilistic model developed abundant. Lateral migration would be required to charge reser- for the World Energy Project (Charpentier and Klett, 2000). voirs in this assessment unit. Risking structure and other methodological considerations are Field growth has occurred in the Sirte Basin, as documented provided in chapters within the recent world assessment (U.S. by Montgomery (1994); for example, Mabruk field has recently Geological Survey World Energy Assessment Team, 2000). added more than 1 billion barrels of new reserves (fig. 1). For esti- Following is a summary of the unrisked assessments for the mation of undiscovered field sizes, a Lower U.S. 48-field-growth two established assessment units and the risked assessments for model was used, as described by Schmoker and Crovelli (1998). the two hypothetical assessment units in the Sirte Basin province Proprietary reserve data from North Africa presented to us from (figs. 21, 22). The mean geologically risked (GR) estimates com- World Energy consortium members are consistent with the pare favorably with the estimated mean of Masters and others Lower U.S. 48-field-growth algorithm as defined by Schmoker (1994). The current mean resource estimates are 11.63 BBOE and Crovelli (1998). (unrisked) and 8.98 BBOE (risked; fig. 23). The last assessment reported by the U.S. Geological Survey Compared to Masters and others (1994), less natural gas is for undiscovered oil and natural gas was given by Masters and estimated for the Sirte Basin as a result of the present study. This others (1994), who reported mean undiscovered petroleum reduction is in large part due to reduced amounts of gas associ- resources for Libya as 7.1 BBO, with a 95 percent to 5 percent ated with Sirte Basin oils calculated on the basis of very low gas/ fractile range of 3.5 to 13.3 BBO; mean undiscovered gas volume oil ratios (GOR median used in this assessment is 300; U.S. Geo- of 23.9 TCF, with a 95 percent to 5 percent fractile range of 8.9 to logical Survey World Energy Assessment Team, 2000), reflecting 48.9 TCF of gas; and mean undiscovered 0.4 BBNGL. Ahlbrandt data available to us from Petroconsultants (1996) and GeoMark and others (1998) tested several methods for calculating undis- (1998). The geologically risked numbers reflect levels of uncer- covered resources in the Sirte Basin. The discovery process tainty for the two hypothetical assessment units.

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