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The AAPG/Datapages Combined Publications Database

Journal of Petroleum Geology

Abstract

Journal of Petroleum Geology, vol. 27 ( 3 ), July 2004 , pp 299 - 312
Copyright copyrght.jpg (4253 bytes) 2004 by Scientific Press Ltd. All rights reserved.

GEOCHEMICAL SIGNIFICANCE OF SEEPAGE OILS AND BITUMINOUS SANDSTONES IN THE ALBERTINE GRABEN, UGANDA

Dou Lirong1,2,*, Cheng Dingsheng2, Wang Jianjun2, Ernest N. T. Rubondo3, Robert Kasande3, Abdul Byakagaba3 and Frank Mugisha3


1Research Centre, China National Oil and Gas E & D Corp., CNPC, PO Box 910, Xueyuan Road 20, 100083, Beijing, P. R. China.

2Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, P. R. China.

3 Petroleum Exploration and Production Department (PEPD), PO Box 9, Entebbe, The Republic of Uganda.

*corresponding author, email: [email protected]

Geochemical analysis of surface samples comprising three seepage oils, two bituminous sandstones and five shales collected from the Ugandan portion of the Albertine graben (a Cenozoic rift basin in Eastern Africa) indicates that the basin may have some petroleum potential. The analyses show that seepage oils from the Kibiro and Paraa localities were derived from Types I or II1 source rocks, deposited in semi-deep or deep-water lacustrine environments containing abundant freshwater algae. However, the seepage oil from Kibuku is derived from a Type III source rock.

A comparison of the seepage oils and the bituminous sandstones suggests that there are at least two sets of mature or threshold-mature source rocks in the Albertine graben – one in the Upper Tertiary and the other in the underlying (pre-Upper Tertiary) succession. According to molecular indicators of maturity, the Kibiro seepage oil was generated during peak maturation, whereas the Paraa and Kibuku seepage oils are of lower or threshold maturity.

INTRODUCTION

Although Uganda has a long history of surface petroleum exploration, no commercial oilfields have yet been found there. The most prospective area in the country is the Albertine graben (Fig. 1); this is 570km long and about 45km wide, and extends into the Democratic Republic of Congo. Previous geological and geophysical studies have concluded that mature source rocks are present in the graben, and the presence of an active petroleum system is indicated by the occurrence of oil seeps. Wayland (1925) identified oil and gas shows at 52 locations within and around Lake Albert. At the present day, surface seepages of petroleum occur at Paraa, Kibiro and Kibuku (see Fig. 2). The existence of these seepages indicates that not only are effective source rocks present, but also that they have generated and expelled oil and that migration has taken place.

In order to investigate the petroleum potential of the Albertine graben, CNODC geologists have recently participated in a joint research programme with PEPD Uganda. Three seepage oils, two bituminous sandstones and five shales were collected for organic geochemical analysis (Table 1). Sample locations are shown in Fig. 2.

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