assessment of undiscovered oil and gas resources of the ... · 2 assessment of undiscovered oil and...

Assessment of Undiscovered Oil and Gas Resources of the Tanzania Coastal Province, East Africa

By Michael E. Brownfield

Digital Data Series 69–GG

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

Chapter 12 ofGeologic Assessment of Undiscovered Hydrocarbon Resources of Sub-Saharan AfricaCompiled by Michael E. Brownfield

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ATLANTIC OCEAN

SOUTHATLANTIC

OCEAN

INDIAN OCEAN

INDIAN OCEAN

MEDITERRANEAN SEA

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TanzaniaCoastal

U.S. Department of the InteriorSALLY JEWELL, Secretary

U.S. Geological SurveySuzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:Brownfield, M.E., 2016, Assessment of undiscovered oil and gas resources of the Tanzania Coastal Province, East Africa, in Brownfield, M.E., compiler, Geologic assessment of undiscovered hydrocarbon resources of Sub-Saharan Africa: U.S. Geological Survey Digital Data Series 69–GG, chap. 12, 12 p., http://dx.doi.org/10.3133/ds69GG.

ISSN 2327-638X (online)

http://www.usgs.gov

http://www.usgs.gov/pubprod/

http://dx.doi.org/10.3133/ds69GG

iii

Contents

Abstract ...........................................................................................................................................................1Introduction.....................................................................................................................................................1Tectonic History and Geology of the Tanzania Coastal Province, East Africa ....................................4Petroleum Occurrence in the Tanzania Coastal Province, East Africa ................................................4

Source Rocks.........................................................................................................................................4Reservoirs, Traps, and Seals ...............................................................................................................4

Exploration ......................................................................................................................................................9Geologic Model ..............................................................................................................................................9Resource Summary .....................................................................................................................................11For Additional Information ..........................................................................................................................11Acknowledgments .......................................................................................................................................11References ....................................................................................................................................................12

Figures

1. Map showing locations of the Tanzania Coastal Province and Mesozoic- Cenozoic Reservoirs Assessment Unit along the central coast of East Africa ..................2

2. Map showing generalized geology of East Africa ..................................................................3 3. Map showing reconstruction of the early breakup of Gondwana during the

Early Jurassic ................................................................................................................................5 4. Stratigraphic columns of coastal Mozambique, the Mozambique Channel,

and coastal Tanzania, Morondava Basin, and Seychelles ....................................................6 5. Composite stratigraphic column of Tanzania coastal basins ................................................7 6. Cross section of the Rovuma Delta, southern Tanzania .........................................................8 7. Seismic cross section A–A‘ of the Ruvu and deep-marine basins and the

Zanzibar Channel, Tanzania ........................................................................................................9 8. Seismic cross sections B–B‘, Ruvu and coastal basins and the Dar es Salaam

platform, Tanzania, and C–C‘, Selous, Mandawa, and deep-marine basins, Tanzania........................................................................................................................................10

9. Events chart for the Mesozoic-Cenozoic Composite Total Petroleum System (727301) and the Mesozoic-Cenozoic Reservoirs Assessment Unit (72730101). ..............11

Table

1. Tanzania Coastal Province assessment results for undiscovered, technically recoverable oil, gas, and natural gas liquids .........................................................................12

iv

Abbreviations Used in This Reportkm2 square kilometers

mg HC/g TOC milligrams hydrocarbon per gram total organic carbon

AU assessment unit

HI hydrogen index

TOC total organic carbon

TPS total petroleum system

USGS U.S. Geological Survey

Assessment of Undiscovered Oil and Gas Resources of the Tanzania Coastal Province, East Africa

By Michael E. Brownfield

Most generated hydrocarbons migrated into Cretaceous and Paleogene reservoirs and traps. Hydrocarbon traps are mostly structural in the syn-rift rock units and both structural and stratigraphic in the post-rift rock units. The primary reservoir seals are Mesozoic and Cenozoic mudstone and shale. Rifted passive margin analog was used for assessment sizes and numbers because of similar source, reservoirs, and traps.

Using a geology-based assessment, the U.S. Geological Survey estimated mean volumes of undiscovered, technically recoverable conventional oil and gas resources for the Mesozoic-Cenozoic Reservoirs Assessment Unit in the Tanzania Coastal Province. The mean volumes are estimated at 2,806 million barrels of oil, 71,107 billion cubic feet of gas, and 2,212 million barrels of natural gas liquids. The estimated mean size of the largest oil field that is expected to be discovered is 594 million barrels of oil, and the estimated mean size of the expected largest gas field is 6,162 billion cubic feet of gas. For this assessment, a minimum undiscov-ered field size of 5 million barrels of oil equivalent was used. No attempt was made to estimate economically recoverable reserves.

IntroductionThe main objective of the U.S. Geological Survey’s

(USGS) National and Global Petroleum Assessment Project is to assess the potential for undiscovered, technically recover-able oil and natural gas resources of the United States and the world (U.S. Geological Survey World Conventional Resources Assessment Team, 2012). As part of this program, the USGS recently completed an assessment of the Tanzania Coastal Province (fig. 1), an area of approximately 250,373 square kilometers (km2) that covers parts of Kenya and Tanzania. The Tanzania Coastal Province contains rift, marginal-sag, and passive-margin rocks of Middle Jurassic to Holocene age. This assessment was based on data from oil and gas explora-tion wells, producing fields (IHS Energy, 2009), and published geologic reports. A geologic map of Tanzania is shown in figure 2.

AbstractThe main objective of the U.S. Geological Survey’s

National and Global Petroleum Assessment Project is to assess the potential for undiscovered, technically recoverable oil and natural gas resources of the United States and the world. As part of this program, the U.S. Geological Survey completed an assessment of the Tanzania Coastal Province, an area of approximately 250,373 square kilometers that covers parts of Kenya and Tanzania. The Tanzania Coastal Province contains rift, marginal-sag, and passive-margin rocks of Middle Jurassic to Holocene age. This assessment was based on data from oil and gas exploration wells, producing fields, and published geologic reports.

The Tanzania Coastal Province and associated assessment unit was assessed because of increased exploratory activity, recent discoveries, and increased interest in its future oil and gas potential. The assessment was geology based and used the total petroleum system concept. The geologic elements of a total petroleum system consist of hydrocarbon source rocks (source rock maturation and hydrocarbon generation and migration), reservoir rocks (quality and distribution), and traps for hydrocarbon accumulation. Using these geologic criteria, the U.S. Geological Survey defined the Mesozoic-Cenozoic Composite Total Petroleum System with one assessment unit, the Mesozoic-Cenozoic Reservoirs Assessment Unit, an area of approximately 169,965 square kilometers. At the time of the 2012 assessment, the Tanzania Coastal Province contained eight gas accumulations exceeding the minimum size of 5 million barrels of oil equivalent. This province is considered to be underexplored on the basis of its exploration effort.

Oil and gas were generated from Permian to Jurassic Karoo-age lacustrine and continental source rocks (rift stage) averaging 4.0 to 5.0 weight percent total organic carbon, from Early to Middle Jurassic restricted-marine rocks contain-ing as much as 9 weight percent total organic carbon, and from Cretaceous Type II source rocks containing as much as 12 weight percent organic carbon. Hydrocarbon generation most likely began in the latest Jurassic–earliest Cretaceous and continues today in the offshore parts of the assessment unit.

2 Assessment of Undiscovered Oil and Gas Resources of the Tanzania Coastal Province, East Africa

Figure 1. Locations of the Tanzania Coastal Province and Mesozoic-Cenozoic Reservoirs Assessment Unit along the central coast of East Africa.

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TanzaniaCoastal

7273

AFRICA

INDEX MAP

Base from U.S. Geological Survey digital data, 2002World Geodetic System 1984 (WGS 84)Prime Meridian, Greenwich, 0°

0 150 300 KILOMETERS

0 300 MILES150

Réunion

MAURITIUS

COMOROS

Mayotte

ZAMBIA

MALAWI

ZIMBABWE

LESOTHO

SWAZILAND

SOUTHAFRICA

KENYA

BURUNDI

RWANDA

70°E66°E62°E58°E54°E50°E46°E42°E38°E34°E30°E

2°S

6°S

10°S

14°S

18°S

22°S

26°S

30°S

34°S

TANZANIA

MADAGASCARMOZAMBIQUE

SEYCHELLES

INDIAN OCEAN

Mesozoic-Cenozoic Reservoirs Assessment Unit boundary

EXPLANATIONTanzania Coastal Province boundary

Gas field

Tanzania Coastal7273

Introduction 3

Figure 2. Generalized geology of East Africa. Modified after Ophir Energy Company (2011).

CongoBasin

TANZANIA

KalahariBasin

Beira

MAPUTO

Dar es Salaam

MOGADISHU

Somali OceanicBasin

Continent-oceanicboundary

(COB)

OgadenBasin

RovumaDeltaDavie Fracture Zone

Moz

ambiq

ue C

hann

el

Aswa Shear Zone

Cupa Fault Zone1,0

00

KarrooBasin

Continent-oceanicboundary

(COB)

MandawaBasin

MajungaBasin

MorondavaBasin

MozambiqueBasin

INDIAN OCEAN

MA

DA

GA

SCA

R

Mombasa

0 200 400 600 KILOMETERS

0 600 MILES200 400

EXPLANATION

Bathymetric contour, 1,000 meter line—Location approximate

Continental crust—Palo fill

Permian-Triassic–Early Jurassic—Karoo-age rock

Phanerozoic—Intracratonic basin

ContactFault or fault zone—Dashed where approximate. Arrows show relative motion

Post Early Jurassic—Passive margin basin

Tertiary—Volcanic province

Undifferentiated—Precambrian basement

Graben—Teeth on downthrown side

MOZAMBIQUE

Tsimiro

roFie

ld


The Tanzania Coastal Province and associated assessment unit were assessed because of increased energy exploration activity, recent discoveries, and increased interest in its future oil and gas potential. The assessment was geology based and used the total petroleum system concept. The geologic elements of a total petroleum system consist of hydrocar-bon source rocks (source-rock maturation and hydrocarbon generation and migration), reservoir rocks (quality and distribution), and traps for hydrocarbon accumulation. Using these geologic criteria, the USGS defined the Mesozoic-Cenozoic Composite Total Petroleum System (TPS) with one assessment unit, the Mesozoic-Cenozoic Reservoirs Assessment Unit (AU) (fig. 1), an area of approximately 169,965 km2.

Tectonic History and Geology of the Tanzania Coastal Province, East Africa

The Tanzania Coastal Province is directly related to the breakup of Gondwana (fig. 3) in the late Paleozoic and Mesozoic (Reeves and others, 2002). The province developed in four phases: (1) pre-rift stage that started during the Carboniferous, during which a mantle plume caused uplift, extension, microplate formation, rifting, and volcanism; (2) a syn-rift phase that started during the Permo-Triassic and continued into the Jurassic, forming grabens and half-grabens and depositing possible lacustrine and continental source rocks; (3) a syn-rift-drift phase that began in the Middle Jurassic and continued into the Paleogene, during which marine clastic and carbonate rocks were deposited, and marine source rocks were deposited during periods of restricted-marine conditions; and (4) a passive-margin phase that began in the late Paleogene and continues to the present. The stratigraphic section present in the Tanzania Coastal Province is generally the same as the section present in the coastal Morandava Basin and coastal Mozambique (figs. 2, 4; Rusk, Bertagne & Associates, 2003; Fugro Multi Client Services, 2010).

The opening of the Indian Ocean began in the Permian and continued into the Jurassic during the syn-rift phase. In the Middle Jurassic, Madagascar, India, and the Seychelles separated from Africa and formed a passive margin and a carbonate platform that was later covered by Upper Jurassic to Cretaceous marine clastic deposits (figs. 4, 5). During the middle Cretaceous, the rift-sag passive-margin basin again became the site of deposition of marine sediments. Deltaic progradation of the Rovuma (fig. 6) and Rufiji Deltas began in the Oligocene and continues today. These deltas are the sites of several recent large gas discoveries.

Petroleum Occurrence in the Tanzania Coastal Province, East Africa

Source Rocks

The Mesozoic-Cenozoic Composite TPS was defined to include Paleozoic to Paleogene source rocks and conventional reservoirs (figs. 4, 5). The Permian to Triassic section contains fluvial and lacustrine source rock in which Type I and Type III kerogen contain 1.0 to 6.7 weight percent total organic carbon (TOC); some samples have as much as 17.4 weight percent. Kagya (1996) reported that Triassic and Lower Jurassic source rocks contained TOC values ranging from 1.23 to 7.16 weight percent, and Nagati (2011) reported Jurassic source rock with TOC contents as much as 8.4 weight percent. The Early to Middle Jurassic section contains restricted marine Type II kerogen source rock and marginal marine, deltaic Types II and III kerogen source rocks, and Type I lacustrine source rock (Cope, 2000; Kejato, 2003; Rusk, Bertagne & Associates, 2003). The Early to Middle Jurassic lacustrine Type I and restricted marine Type II source rocks contain as much as 8.7 weight percent TOC (Cope, 2000). Type II and III kerogen source rocks of Cretaceous and Paleogene age have been identified in the Tanzania Coastal Province (Cope, 2000). The Cretaceous marine sources contain TOC contents as much as 7.4 weigh percent and hydrogen index (HI) values as much as 396 milligrams hydrocarbon per gram total organic carbon (mg HC/g TOC) and the Paleogene marine sources contain 12.1 weight percent and HI values as much as 688 mg HC/g TOC (Cope, 2000).

Generation of syn-rift Karoo age source rocks most likely began in the Middle Jurassic, whereas Middle Jurassic to Early Cretaceous syn-rift source rocks began in the Late Jurassic to Early Cretaceous (Cope, 2000). Generation of hydrocar-bons by early post-rift sources most likely began in the Late Cretaceous, whereas the drift and post-rift source rocks most likely began generating hydrocarbons in the Late Cretaceous. Hydrocarbon generation most likely continues today in the offshore parts of the assessment unit (Cope, 2000).

Reservoirs, Traps, and Seals

The Tanzania Coastal Province contains Mesozoic and Cenozoic clastic reservoirs. Triassic to Middle Jurassic syn-rift rocks contain possible alluvial fans, fan deltas, fluvial deltas, and lacustrine sandstones reservoirs (Cope, 2000). The Late Jurassic post-rift rocks contain reef and platform carbonate rocks that are potential reservoirs. The post-rift Cretaceous rocks contain regressive and transgressive marine sandstone, slope-turbidite channel and sandstone and basin-floor fan sandstone reservoirs (Cope, 2000; Ophir Energy Company, 2007, 2011). Upper Cretaceous and Cenozoic passive-margin rocks contain possible carbonate reservoirs and Maastrichtian

Petroleum Occurrence in the Tanzania Coastal Province, East Africa 5

Figure 3. Reconstruction of the early breakup of Gondwana during the Early Jurassic (200 Ma). Modified from Reeves and others (2002).

0 100 200 KILOMETERS

0 200 MILES100

HORN OF AFRICA

KENYA

TANZANIA

MADAGASCAR

INDIA

INDIA

SRI LANKA

EAST ANTARCTICA

SEYCHELLES

Mik

ume

Rift

Cambay Rift

Saurastra

Anza Rift

Narmada-Son

Basin

NORT

HM

OZA

MBI

QUE

Selous

Line of latitude or longitude— Oriented as on modern map

EXPLANATION

Precambrian crustal fragment

Rift-related rock



EXPLANATION

Sandstone

Sandstone and conglomerate

Shale

Sandy shale

Calcareous shaley marlstone

Limestone Reservoir rock, East Africa

Salt

Igneous rock—Extrusive

Igneous rock—Oceanic crust

Basement

Unconformity

Contact—Dashed where location approximate

Extent uncertain

R

Source rock

Potential reservoirOil fieldGas field

CoastalMozambique

Tectonicevents

Tectonicevents

Mozambique Channeland Coastal Tanzania

CoastalMorandava Basin

Envir. Envir.W W

TERT

IARY

CRET

ACEO

USJU

RASS

ICTR

IASS

ICPE

RMIA

NCA

RBON

-IF

EROU

SPR

ECAM

-BR

IAN

Basement

Neo

gene

Pale

ogen

eUp

per

Low

erUp

per

Uppe

rM

ML

UL

Mid

dle-

Uppe

rLo

wer

FluvialDeltaic

Deltaic

Marine

?

?

?

Cont

inen

tal w

ith m

inor

mar

gina

l mar

ine

Zone

s o

f re

serv

oir

pote

ntia

l

Mar

ine

Mar

ine

Cont

inen

tal w

ith m

inor

mar

gina

l mar

ine

Shallowmarine

Deltaic tocont.

Shal

low

mar

ine

to c

ontin

enta

l

Shal

low

mar

ine

lago

onal

toco

ntin

enta

l

Mar

gina

l sag

and

pas

sive

mar

gin

Rift,

mar

gina

l sag

and

drift

Mar

gina

l sag

and

drif

t

Rift

and

mar

gina

l sag

Recu

rren

t rift

ing

and

uplif

t

Recu

rren

t rift

ing

and

uplif

t

Seyc

helle

s-In

dia

brea

kup

Mad

agas

car-

Seyc

helle

s-In

dia

brea

kup

from

Afri

ca

Mad

agas

car-

Seyc

helle

s-In

dia

brea

kup

from

Afri

ca

Rest

ricte

d m

arin

e, o

pen

mar

ine,

and

mar

gina

l mar

ine

Seychelles

SeychellesEast Africa

Drift

Rift

Lithology

?

?

?

Lithology Lithology Lithology

?

?

?

?

?

W EEE

?Re

stric

ted

mar

ine,

shal

low

mar

ine,

and

ope

n m

arin

eSh

allo

w m

arin

e an

d op

en m

arin

eCo

ntin

enta

l with

min

or m

argi

nal m

arin

e

Envir.

R

R

R

R

R

R

R

R

Age

Rifti

ng a

ndm

argi

nal s

ag

Figure 4. Generalized stratigraphic columns for Coastal Mozambique, Mozambique Channel and Coastal Tanzania, Coastal Morondava Basin, and Seychelles along the east coast of Africa (see fig. 1). Carb., Carboniferous; cont., continental; Envir., environment; L, Lower; M, Middle; U, Upper; W, west; E, east. Modified from Rusk, Bertagne & Associates (2003) and Fugro Multi Client Services (2010).

Petroleum Occurrence in the Tanzania Coastal Province, East Africa 7


EXPLANATION

Sandstone

Carbonate rock

Shale

Salt

Precambrian basementContactUnconformityCarbonaceous shale and lignite

Miocene deltaic sands inZanzibar channel with lignitesand carbonaceous shale(gas shows)

Gas shows in Songosongo area,Zanzibar & Maflia wells

Sands in Zanzibar and Maflia wells

Marine shale

Upper Aptian Kihuluhulumarlstone and lower AlbianKigongo marlstone

Albian deltaic sand acting asreservoirs in Songosongo gasfields and Maflia channel?

Continental Makonde beds

Regressive Neocomian sand

Marine shales

Shallow marine limestonesreefal, oolitic & detritalprobably present over the whole coast

Evaporite sequence inMandawa salt basin

Lagoonal marine shales excellentsource potential in Mandawa-7

Continental sandstones withcarbonaceous shale and lignite

Composite Stratigraphic Column forTanzanian Coastal Basins

QuaternaryPliocene

upper

upper

Uppe

rUp

per

Mid

dle

Low

erLo

wer

Permo-Triassic

Precambrian

lower

lower

Maastrichtian

Campanian

Santonian

Turonian

Cenomanian

Aptian

Barremian

Hauterivian

Valanginian

Tithonian

Oxfordian

Kimmeridgian

BathonianBajocian

Aalenian

Pleinsbachian

Hettangian

Sinemurian

Toarcian

Callovian

Berriasian

Albian

Coniacian

Paleocene

Oligocene

middle

middle

Neo

com

ian

Seno

nian

Mio

cene

Eoce

ne

Terti

ary

Cret

aceo

usJu

rass

ic

Figure 5. Composite stratigraphic column of Tanzania coastal basins. Modified from Mbede and Dualeh (1997).



EXPLANATION

Neogene

Paleogene

Maastrichtian-Campanian

Santonian-Cenomanian

Lower Cretaceous

Jurassic

Salt

Reservoir and potential reservoir

Basement—Triassic

UnconformityContact

Gas discovies and prospects

Oil prospects

Fault

NeocomianSongo Songo and

Nyuni FieldsMiocene and Oligocene

Msimbati and Mnazi Bay Fields

Paleocene

Santonian Campanian

Cenomanian

Jurassicsalt?

NOT TO SCALE

Seafloor

INDIAN OCEAN

Figure 6. Schematic cross section of the Rovuma Delta, southern Tanzania. Modified from Ophir Energy Company PLC (2011). Not to scale.

and Paleocene turbidite reservoirs (slope channels, sandstones, and basin-floor fans). Volcanic rocks found within the Karoo-age and Cretaceous section may have degraded some of the reservoirs by thermal alteration of minerals in the reservoir rock units.

The Mesozoic-Cenozoic Reservoirs AU (fig. 1) contains reservoirs that mostly are associated with growth-fault-related structures, rotated fault blocks within the continental shelf, deep-water fans, turbidite units, and slope truncations along the present-day shelf and paleoshelf edge. For example, the Rovuma River delta (fig. 6) contains turbidite sandstone units, and deep-water fan and growth-fault related sandstone reservoirs (Ophir Energy Company, 2007, 2011; Law, 2011). The Triassic to Jurassic syn-rift section contains structural traps related to graben and half-grabens with facies trapping typical of rifts (Cope, 2000). Lower Cretaceous post-rift rocks (drift and passive margin) contain stratigraphic and structural traps in transgressive and regressive sandstones,

salt structures, drape anticlines, and flower structures (Ophir Energy Company, 2007, 2011). Upper Cretaceous and Paleogene post-rift rocks (drift and passive margin) contain both structural and stratigraphic traps in growth-fault-related structures, rotated fault blocks within the continental shelf, deep-water fans, turbidite channels and sandstones, and slope truncations along the present day shelf and paleoshelf edge (Cope, 2000; Ophir Energy Company, 2007). For example, the Rovuma River delta (fig. 6) contains turbidite sandstone units, deep-water fan, and growth-fault related sandstone reservoirs (Ophir Energy Company (2011).

The primary reservoir seals are Mesozoic and Cenozoic drift and marginal-marine mudstone and shale.

Three generalized seismic cross sections drawn from onshore to offshore along the central coast of Tanzania are shown in figures 7 and 8 (Kejato, 2003). Seismic profiles have imaged potential prospects in the offshore part of the Tanzania Coastal Province (Law, 2011; Ophir Energy, 2011).

Geologic Model 9


INDEX MAP

WEST EAST

FEETSEA LEVEL

2,000

4,000

6,000

8,000

10,000

12,000

Dry hole with gas show and identifier

Top Upper CretaceousTop Lower JurassicTop Upper PermianUndated seismic horizonSeismic reflectorFault—Arrow shows relative motion

Top Eocene

EXPLANATION

0 10 20 KILOMETERS

0 20 MILES

Shoreline location unknown

10

TangaFault

RuvuBasin

Makarawe-1

ZanzibarHigh

PembaHigh

DavieFracture

Deep Marine

Makarawe-1

Ruvu Basin - Zanzibar Channel - Deep Marine

Dar es Salaam

TANZANIA

A A'

Seafloor

INDIANOCEAN

A'A

Line of section

Figure 7. Generalized seismic cross section A–A‘ of the Ruvu and deep-marine basins and of the Zanzibar Channel, Tanzania. Modified from Kejato (2003). Not to scale.

ExplorationAt the time of this 2012 assessment, the province

contained 5 gas fields (IHS Energy, 2009) and 3 new gas discoveries and is considered to be underexplored on the basis of its exploration history. There have been some new discover-ies since this assessment. The producing gas fields and recent petroleum discoveries are limited to Cretaceous-Tertiary offshore marine rocks.

Exploration wells and discovered accumulations on the continental shelf and upper slope (IHS Energy, 2009) provide evidence for the existence of an active petroleum system containing Mesozoic source rocks, the migration of hydrocar-bons most likely since the Late Cretaceous, and the migration of the hydrocarbons into Cretaceous and Cenozoic reservoirs.

Geologic ModelThe geologic model developed for the assessment of

conventional oil and gas in the Tanzania Coastal Province and the Coastal Plain and Offshore Assessment Unit, east Africa is as follows:

1. Oil and gas was generated from Permian to Jurassic Karoo-age lacustrine and continental rocks (rift stage). The lacustrine rocks contain TOC values averaging 4.0 to 5.0 weight percent. Early to Middle and Jurassic restricted marine rocks contain as much as 9 weight percent TOC, and Cretaceous rocks contain Type II kerogen with as much as 12 weight percent TOC. Hydrocarbon generation of syn-rift Karoo-age source rocks most likely began in the Middle Jurassic, whereas generation of the Middle Jurassic to Early Cretaceous syn-rift source rocks began in the Late Jurassic to Early Cretaceous. Generation of early post-rift sources most likely began in the Late Cretaceous. The drift and post-rift source rocks most likely began generating hydrocarbons in the Late Cretaceous. Hydrocarbon generation most likely continues in the offshore parts of the assessment unit.

2. Hydrocarbons migrated into mostly Cretaceous and Paleogene reservoirs and traps.

3. Hydrocarbon traps are mostly structural within the syn-rift rock units and both structural and stratigraphic in the postrift-rock units.



INDEX MAP

WEST EAST

FEETSEA LEVEL

WEST EAST

FEETSEA LEVEL

2,000

2,000

4,000

6,000

6,000

10,000

4,000

6,000

8,00010,000

EXPLANATION

Well and identifier—Dry hole

Well and identifier—Oil show

Top Upper CretaceousTop Lower CretaceousTop Lower JurassicTop Upper PermianUndated seismic horizon

Fault—Arrow shows relative motion

Top Eocene

Salt

Seismic reflector

0 10 20 KILOMETERS

0 20 MILES 10

Ruvu Basin

Selous Basin

Masasi Spur

Mandawa Salt Sub-Basin

Dar Es Salaam Platform Coastal Basin

Kiwangwa-1

Liwale-1

Liwale-1

MBUO-1

MBUO-1

Mandawa-7

Kisarawe-1 Kimbiji Main-1

Kimbiji East-1

Tancan-1

Coastline

Coastline

LathamIsland

Major BoundingFault Zone

PuguUplift Coastal Tertiary

Fault Zone

100 kilometers gap

Ruvu Basin - Dar es Salaam - Deep Marine

Selous Basin - Mandawa Basin - Deep Marine

Dar es Salaam

Mandawa SaltBasin

TANZANIA

Dar es SalaamPlatform

Line of section

Line of section

B B'

C C'

Seafloor

Seafloor

INDIANOCEAN

INDIAN OCEAN

INDIAN OCEAN

B'

C'

C

B

Figure 8. Generalized seismic cross sections B–B‘, Ruvu and coastal basins and the Dar es Salaam Platform, Tanzania, and C–C‘, Selous, Mandawa, and deep-marine basins, Tanzania. Modified from Kejato (2003). Not to scale.

For Additional Information 11

4. The primary reservoir seals are Mesozoic and Cenozoic mudstone and shale.

5. The rifted passive margin analog (Charpentier and others, 2007) was used for assessment sizes and numbers because of similar source, reservoirs, and traps.

An events chart (fig. 9) for the Mesozoic-Cenozoic petroleum systems and the Mesozoic-Cenozoic Reservoirs AU summarizes the age of the source, seal, and reservoir rocks and the timing of trap development and generation, and migration of hydrocarbons.

Resource Summary

Using a geology-based assessment, the U.S. Geological Survey estimated mean volumes of undiscovered, technically recoverable conventional oil and gas resources for the Mesozoic-Cenozoic Reservoirs Assessment Unit in the Tanzania Coastal Province (table 1). The mean volumes are estimated at 2,806 million barrels of oil, 71,107 billion cubic feet of gas, and 2,212 million barrels of natural gas liquids. The estimated mean size of the largest oil field that is expected to be discovered is 594 million barrels of oil, and the estimated mean size of the expected largest gas field is 6,162 billion cubic feet of gas. For this assessment, a minimum undiscovered field size of 5 million barrels of oil equivalent was used. No attempt was made to estimate economically recoverable reserves.

Acknowledgments

The author wishes to thank Mary-Margaret Coates, Jennifer Eoff, Christopher Schenk, and David Scott for their suggestions, comments, and editorial reviews, which greatly improved the manuscript. The author thanks Wayne Husband for his numerous hours drafting many of the figures used in this manuscript, and Chris Anderson, who supplied the Geographic Information System files for this assessment.

For Additional Information

Assessment results are available at the USGS Central Energy Resources Science Center website: http://energy.cr.usgs.gov/oilgas/noga/ or contact Michael E. Brownfield, the assessing geologist ([email protected]).\\IGSKAHCMVSFS002\Pubs_Common\Jeff\den13_cmrm00_0129_ds_brownfield\dds_69_gg_ch12_figures\ch12_figures\ch12_figure09.ai

PETROLEUMSYSTEM EVENTS

Age in million years (Ma)

100

200

50

150

250

Paleogene

Cretaceous

Jurassic

Triassic

Permian

E

E

E

M

M

LE

L

L

L

Neogene

Pal

Eoc

Olig

Mio

Plio

PRES

ERVA

TIO

N

GEN

ERA

TIO

N-

TRA

P FO

RMA

TIO

N

OVE

RBU

RDEN

RO

CKS

RESE

RVO

IR R

OCK

S

SEA

L RO

CKS

SOU

RCE

ROCK

S

ROCK

UN

ITS

ACC

UM

ULA

TIO

NM

IGRA

TIO

N-

251

200

146

65

23

0

Figure 9. Events chart for the Mesozoic-Cenozoic Composite Total Petroleum System (727301) and the Mesozoic-Cenozoic Reservoirs Assessment Unit (72730101). Gray, rock units present; yellow, age range of reservoir rock; green, age ranges of source, seal, and overburden rocks and the timing of trap formation and generation, and migration and preservation of hydrocarbons; wavy line, unconformity. Divisions of geologic time conform to dates in U.S. Geological Survey Fact Sheet 2010–3059 (U.S. Geological Survey Geologic Names Committee, 2010). Ma, million years ago; Plio, Pliocene; Mio, Miocene; Olig, Oligocene; Eoc, Eocene; Pal, Paleocene, L, Late; E, Early; M, Middle; ?, uncertain.

http://energy.cr.usgs.gov/oilgas/noga/


References

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Cope, Michael, 2000, Tanzania’s Mafia deepwater basin indi-cates potential on new seismic data: Oil and Gas Journal, v. 98, no. 33. Available at http://www.ogj.com/articles/ print/volume-98/issue-33/exploration-development/ tanzanias-mafia-deepwater-basin-indicates-potential-on-new-seismic-data.html. Last accessed February 28, 2013.

Fugro Multi Client Services, 2010, New Seychelles, non-exclusive 2D seismic survey: Available at http://www.fugromcs.com.au/Fliers/Flier_Seychelles.pdf. Accessed March 6, 2012,

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Kejato, Kejo, 2003, The geology and petroleum potential of Tanzania: Dar es Salaam, Tanzania, Tanzania Petroleum Development Corporation, 29 p.

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Table 1. Tanzania Coastal Province assessment results for undiscovered, technically recoverable oil, gas, and natural gas liquids.

[Largest expected mean field size, in million barrels of oil and billion cubic feet of gas; MMBO, million barrels of oil; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids. Results shown are fully risked estimates. For gas accumulations, all liquids are included as natural gas liquids (NGL). Undiscovered gas resources are the sum of nonassociated and associated gas. F95 represents a 95-percent chance of at least the amount tabulated; other fractiles are defined similarly. Fractiles are additive under assumption of perfect positive correlation. AU, assessment unit; AU probability is the chance of at least one accumulation of minimum size within the AU. TPS, total petroleum system. Gray shading indicates not applicable]

Total Petroleum Systems (TPS) and Assessment Units (AU)

Field type

Largest expected

mean field size

Total undiscovered resourcesOil (MMBO) Gas (BCFG) NGL (MMBNGL)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Tanzania Coastal-Mesozoic-Cenozoic Composite TPS

Mesozoic-Cenozoic Reservoirs AU Oil 594 1,223 2,608 5,064 2,806 1,583 3,555 7,571 3933 42 95 208 106Gas 6,162 35,902 64,054 108,925 67,174 1,118 2,006 3,433 2,106

Total Conventional Resources 1,223 2,608 5,064 2,806 37,485 67,609 116,496 71,107 1,160 2,101 3,641 2,212

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ATLANTIC OCEAN

SOUTHATLANTIC

OCEAN

INDIAN OCEAN

INDIAN OCEAN

MEDITERRANEAN SEA

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TanzaniaCoastal

http://www.ogj.com/articles/print/volume-98/issue-33/exploration-development/tanzanias-mafia-deepwater-basin-indicates-potential-on-new-seismic-data.html

http://www.fugromcs.com.au/Fliers/Flier_Seychelles.pdf

http://www.petrocommunicators.com/moz/index.htm

http://pubs.usgs.gov/fs/2010/3059/

http://pubs.usgs.gov/fs/2012/3042/

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