Architecture of turbidite sandstone bodies in a rift-lake setting, Gabon Basin, offshore Gabon R.D.A. Smith Kroninklijke/Shell Exploratie en Producktie Laboratorium, Shell Research BV, Volmerlaan 6, 2288 GD Rijswijk, The Netherlands

lntroduction The Lower Cretaceous Ludna Formation, part of the lacustrine syn-rift fill of the Gabon Basin, contains both channelized and non-channelized reservoir-forming turbidite sandstone bodies. The basin formed part of the South Atlantic Rift, the zone of rifting between Brazil and Africa. It contains an alluvial-Jacustrine succession of Neocomian to Barremian age (Robert and Yapudjian 1990). Fades and faunal evidence indicate that the Ludna with the underlying Kissenda and overlying Melania formations were deposited in a relatively deep rift-lake. An absence of bioturbation suggests that bottarn waters were depleted in dissolved oxygen, as is the case in modern rift-lakes of East Africa below water depths of c. 200m.

In the Malembe Permit, offshore south Gabon, turbidite sandstones of the Ludna Formation form oil­bearing reservoirs in the Ludna Marine and Lucina West Marine field (Fig. 37.1). The formation comprises turbidite sandstone bodies contained in a background of fine-grained deep-lacustrine fades (Fig. 37.2). Reservoir facies are provided by clean sandstones, with or without dish structures and fluidization pipes (Fades Bl.l of Pickering et al. 1986). In the Lucina West Marine Field, reservoir sandstones occur in a laterally extensive tabular body (Zone 2) and an overlying channelized interval (Zone 1) (Figs 37.3 and 37.4). This chapter summarizes fades descriptions and interpretations from the Ludna West Marine Field. The evolution of depositional systems in the area is discussed by Smith (in press).

Facies associations in the Lucina Formation Five lithofades types were identified in the cored interval taken from weil LWM-2 (Fig. 37.3 and 37.5).

1. Clean sandstones Fades B of Mutti and Ried Lucchi (1975). Fades Bl.l of Pickering et al. 0989). Modalgrain sizes of this facies are of fine sand grade, whereas maximum grain size range from fine to coarse sand. Thicknesses range from as little as 20cm in single beds up to 7.5m in thick amalgamated bodies. Beds are non-graded or occasionally exhibit coarse-tail normal grading. The dominant structures are water escape features. Dish structure is most common, but thin water-escape pipes also occur. Very thin ( < 1 cm) to occasionally medium (lücm) thickness irttervals of parallel to ripple cross-lamination may occur at the tops of individual beds.

2. Muddy sandstones Fades Cl of Mutti and Ricci Lucchi 0975). Facies Cl.l of Pickering et al. 0986). Sparse to abundant 'floating' mudstone rip-up clasts are characteristic of this fades. It occurs in intervals ranging from 10 cm beds to 1.4 m thick amalgamated bodies.

3. Massive slltstones Slurried beds of Wood and Smith 0959) and Mutti and Ricd Lucchi 0975). Facies Dl.l of Pickering et al. (1986). Beds of this fades range in thickness from 20 to 80 cm. Grain sizes range from fine sand to clay. Mudstone rip­up clasts and plant fragments are abundant.

4. Thin-bedded Iaminated sandstones Fades D2-3 of Mutti and Ricci Lucchi 0975). Facies C2.3 of Pickering et al. (1986). Individual beds range in thickness from c. 1 to 20 cm, but amalgamated bodies reach 85 cm. This fades is characterized by near-horizontal and ripple cross-lamination.

5. Mudstones Facies D3 of Mutti and Ried Lucchi (1975). Fades groups D2 and E2 of Pickering et al. 0986). Mudstone intervals rangein thickness between 1 and 35 cm. Lamination in the form of very thin coarse silt laminae is present in some intervals.

Atlas of Deep Water Environments: Architectural style in turbidite systems. Edited by K.T. Pickering, R.N. Hiscott, N.H. Kenyon, F. Ricd Lucchi and R.D.A. Smith. Published in 1995 by Chapman & Hall, London. ISBN 0 412 56110 7.

Fades 1-3 represent the deposits of high concentration cohesionless to cohesive sediment gravity flows, whereas Fades 4 and 5 represent the deposits of dilute turbidity currents. These lithofades are indistinguishable from deep-water fades of numerous marine turbidite systems.

The lithofades occur in three assodations: 1. sub­lacustrine channel-fill (represented by Zone 1): 2. sandstone lobe (represented by Zone 2); 3. sandstone­poor basin-plain (represented by the interval between Zone 1 and Zone 2, and fine-grained sections of the overlying Lucina Formation and underlying Kissenda Formation).

Channel-fill facies association {Zone 1)

The Zone 1 channel-fill, 17-20 m thick, comprises dominantly Fades 1 clean sandstones (89%), with subsidiary muddy sandstones 00%) and mudstone 0 %). The Zone 1 interval seen in weil LWM-2 is characterized by a paudty of mudstones (only two very thin mudstone layers) wirhin the thick amalgamated sandstone body. A third, thicker, mudstone interval is seen at the top of the cored interval and represents the beginning of channel abandonment. The channel-fill has a blocky log signaeure with upward fining motifs towards the top reflecting channel abandonment. Comparable channel­fills have been described from the Precambrian Koogsfjord Formation of Norway (Pickering 1983) and the Permian Tanqua Karoo of South Africa (Wickens et al. 1992).

Sandstone lobe facies association {Zone 2)

This fades assodation is 21-24m thick and comprises dominantly clean sandstones (69%), with subsidiary muddy sandstones 02%) and interbedded massive siltstones (5%), thin-bedded Iaminated sandstones ( 4%) and mudstone (100/o). The Zone 2 interval in weil LWM-2 is characterized by a higher frequency of mudstones than seen in Zone 1, including 13 thin mudstone intervals. The log signaeure of the sandstone

N

I

Fig. 37.1. Location map for the Ludna Martine and Lucina West Marine fields, in the Malembe Permit, offshore south Gabon.

lobe fades association differs from that of the channel­fill fades assodation in having a more serrated character due to the presence of numerous thin non­reservoir intercalations.

Many comparable outcrop examples of tabular sandstone bodies have been described, for example in the Miocene of the Apennines (Ried Lucchi and Pignone 1979) andin the Permian Tanqua Karoo of South Africa (Wicke.ns et al. 1992).

Sandstone-poor basin-plain facies association

This is the dominant fades assodation of the lacustrine section and occurs in intervals up to several hundred metres thick, as the major component of the Kissenda and Ludna Formations. The fades assodation is dominared by thin-bedded Iaminated sandstones and

•

•

mudstones with some massive siltstones. Log character is characterized by generallyhigh gamma-ray and low microresistivity due to the mudstone-dominated nature

of this facies association, but with sporadic thin spikes reflecting the presence of turbidite sandstones.

Summary Turbidite reservoir sandstone bodies in the rift-lake deposits of south Gabon are highly comparable with those of marine basins in terrns of fades, facies proportians and scale. In the Lucina West Marine Field both channel-fill and laterally extensive tabular sandbodies are represented.

Acknowledgement This contribution is published with the kind permission of Shell Gabon.

Heferences Mutti, E. and Ried Lucchi, F. 1975. Turbidite fades and fades

associations. In: Examples of Turbidite Facies and Facies Associations jrom Selected Fonnations oj the Northern Apennines. Field Trip Guidebook A-11 , pp. 21-36. IX International Sedimentology Congress, Nice, France. lAS.

Pickering, K.T. 1983. Transitional submarine fan deposits from the late Precambrian Kongsfjord Formation submarine fan, NE Finnmark, N. Norway. Sedimentology, 30, 181-199.

Pickering, K.T., Stow, D.A.V., Watson, M. and Hiscott, R.N. 1986. Deep-water fades, processes and models: a review and classification scheme for modern and ancient sediments. Earth Science Reviews, 22, 75-174.

Ricci Lucchi, F. and Pignone, R. 1979. Ricostruzione geometrica parziale di un lobo di conoide sottomarina. Societa Geologica Italiana Memorie, 18, 125-133.

Robert, P. and Yapudjian, L. 1990. Early Cretaceous rift sediments of the Gabon-Conga Margin: Lithology and organic matter; tectonic and paleogeothermal evolution. Journal of Ajrican Earth Sciences, 10, 319-330.

Scholz, C.A., Rosendahl, B.R. and Scott, D.L. 1990. Development of coarsecgrained fades in lacustrine rift basins: examples from East Africa. Geology, 18, 140-144.

Smith, R.D.A. in press. Reservoir architecture of lacustrine turbidite systems. Lower Cretaceous, offshore Gabon. In: Lambiase,]. (ed.) Hydrocarbon Habitat of Rifts. Special Publication of the Geological Society, London.

Wickens, deV., Brink, G. and Rooyen, W.V. 1992. The Tanqua turbidite and deltaic complexes: depositional models, reservoir realities and the application of sequence stratigraphy. Fieldtrip Guidebook. AAPG Field Seminar.

Wood, A. and Smith, A.]. 1959. The sedimentation and sedimentary history of the Aberystwyth Grits (Upper Llandoverian). Quarterly Journal oj the Geological Society, 11, 163-195.

sw LWM Field

CJ

D

NE

LUM Field Fonrudlon

Ezanga Ftnmalion

Gamba Format1on ..................... Cardita

Formabon

Melanta Formation

Upper Luc1na Formabon

Lower Luona Formation

Klssenda FormatJon

LUM-3

Lithologies

Satt

fluv1al deep-water

Sandstones

Fine-gramed lacustrine facies

Age

c .0

~ Regoonal unconfornity . .... ................

c ... E ~ ~

c

" E 0 8 GI z

Fig. 37.2. Lower Cretaceous stratigraphy in the Lucina Marine and Lucina West

Marine fields.

m 1430

1440

1450

1460

1470

1480

0 GR 100

100 MSF~ooo I I

c::::J 1. Clean Sandstone c::::J 2. Muddy Sandstone

3. Massive siltstone 4. Thin-bedded Iaminated

sandstone - 5. Mudstone

ZONE 1

Channel-fill

ZONE2

Sandstone

Lobe

Fig. 37.3. Core-log calibration in well LWM-2. GR = gamma-ray log; MSFL = micro spherically focused log, a type of micro-resistivity log.

NW

LWM-3 GR MSFL

Melania Formation

--Lucina Formation

Zone2

Kissenda Formation

m 20

10 I V.E. = xs 0 0 0

0 100 200m

LWM-2 GR MSFL

Fig. 37.4. Correlation panel in the Lucina West Marine field showing laterally extensive Zone 2 body and channelized Zone 1 interval.

SE

LWM-1 LWM-14 GR MSFL GR MSFL

N e LWM-3

l e LWM 2

LWM 1 . e LWM4

0 1 kill

Fig. 37.5. Core photographs of the Zone 1 (a) and Zone 2 (b) Sandstones of the Lucina Formation in well LWM-2.

UE 0

H

40

ll

lO

10

" 10

•10

" 111

1\

100

CO

IIE-

4 C

OII

C •'

CO

IIE

-~

coo~-

4 C

OR

E·-

S

~URE

10

10

10 ., ·0

U[T

RE