Petroleum systems of the Taoudeni Basin, West Africa

Transcription

1 24 Pet.Sci.(28)5:24-3 DOI 1.17/s Petroleu systes of the Taoudeni Basin, West Africa Huang Zhilong *, Zhao Baoshun, Jiang Qingchun, Wang Songpo and Liu Bo Key Laboratory for Accuulation, China University of Petroleu, Ministry of Education, Beijing 12249, China Abstract: The Taoudeni Basin is a typical steady intracratonic basin. Based on the distribution of effective source rocks in the Taoudeni Basin, cobined with the structure characteristics of the basin and the distribution characteristics of beds, two petroleu systes are recognized in the basin: the infra- petroleu syste and the petroleu syste. Structural uplift and tiing of petroleu generation controlled the tiing of petroleu charging and preservation of hydrocarbon accuulations. Maturity, evolution history, and distribution of effective source rocks controlled hydrocarbon richness. The geological key factors and geological processes controlled the type of hydrocarbon accuulations. Key words: Taoudeni Basin, source--cap rock asseblage, petroleu syste, integrated analysis 1 Characteristics of geology 1.1 Stratigraphy and Depositional Environent The Taoudeni Basin is a typical steady intracratonic basin in Mauritania and Mali, west Africa (Macgregor, 1998) (Fig.1). Its sedient fill was doinated by arine clastic rocks which consisted of, siltstone and claystone. In addition, subordinate carbonates were also deposited and were ainly coposed of doloitic liestone and thickly bedded stroatolite. Three depositional cycles were developed in the Taoudeni Basin (Mann et al, 23; Makhous et al, 1997). They are infra- -carbonate (including liestone and stroatolite/doloite) depositional cycle, - -claystone depositional cycle and -Carboniferous transgressive claystone and carbonate depositional cycle. Two ain depocenters were present in the Taoudeni Basin during the infra- tie (Tong and Guan, 22; USGS, 2). They are the Makteir depocenter in the west and the Taoudeni depocenter in the east. Subsidence began fro the infra- tie and a few hundred eters of thick s were deposited. In the Middle infra- tie, the sedientary environent evolved into arine setting, which led to the deposition of a suit of very thick stroatolite and interbedded claystones (udstone and shale). The stroatolite was coposed of doloitic liestone. The claystones constituted source rocks. In the late infra- tie, water becae shallower with tectonic uplifting and deposited rocks were subject to weathering. *Corresponding author. eail: huang5288@163.co Received Noveber 13, 26 Nouakchott Dakar Freetown Mauritanides Rokelides Atar Kedougou 5k Reguibat Shield Rayes Baako Taoudeni Basin Leo Shield Tobouctou Volta Basin Fig. 1 Location ap of the Taoudeni Basin Gao Accra Dahoeyides Post-Paleozoic sedients Upper Proterozoic and Paleozoic rocks Panafrican and Caledonian-Hercynian fold belts Archean and lower Proterozoic baseent Pharusides

2 Pet.Sci.(28)5:24-3 Then the basin subsided again to receive sedients. The Upper overlies the infra- stroatolite unconforably. Subsequent frequent arine transgression and regression led to the deposition of a suit of interbedded and shale sequences. 1.2 Source rock The Taoudeni Basin is a typical steady intracratonic basin. Source rocks have siilar characteristics, but their thickness varies fro 2 to 5. The organic atter abundance of source rocks in different stratigraphic intervals varies significantly. The kerogens are pro-sapropelic type II. So far, there is little well data in the Taoudeni Basin. The Abolag-1 and the Ouasa-1 wells are the only wells drilled in the Mauritanian sector of the basin. The features shown by just two wells cannot truly reflect source rock distribution within the entire Taoudeni Basin. Source rock intervals in the two wells consist of red udstone, dust-brown udstone and black udstone. There are two periods for the developent of large-scale depressions in the Taoudeni Basin. They are the infra- depression and the Paleozoic depression. During the infra-, two depressions were developed: the Maqteir depression (or west depression) in the west and Taoudeni depression (or east depression) in the east. The Paleozoic depressions were superiposed upon the infra- structural configuration. Infra- source rocks are ainly distributed in the Maqteir and Taoudeni depressions. The Taoudeni depression is located in Mali. It covers a large area and was buried to a great depth. The Maqteir depression is located in Mauritania and it covers an area uch saller than the Taoudeni depression. Paleozoic source rocks are ainly confined to the Paleozoic depressions Reservoir Rock Integration of cores, cuttings, wireline logs and oil/gas testing data fro the Ouasa-1 and the Abolag-1 wells indicates that six sets of beds are present in the Taoudeni Basin (Table 1): Lower infra-, Middle infra- paleo-karsted and fractured liestone, Middle/ Upper infra-,, and basal (Figs. 2, 3, 4 and 5). 1) Lower infra- beds: They ainly occur in the upper part of the lower infra-. These beds were coposed of s with an average thickness of ore than 6. Due to great burial depth, old age and strong diagenesis, petrophysical properties are generally poor. The CNL porosity is alost and the pereability is very low. It is difficult for the to be effective beds Upper Middle Residual eroded Fig. 2 Profile showing source--cap rock asseblage for Ouasa-1 well RT Reser Source voir Seal Carboniferous - Jurassic Lower Carboniferous 1 6 Table 1 Reservoir rocks in the two wells in western Taoudeni Basin Well Abolag-1 Well Ouasa-1 Reservoir bed Thickness Reservoir bed Thickness Basal 4 Sandstone Basal 53.2 Sandstone Sandstone Middle and upper upper infra- 151 Upper upper infra Tight Basal upper infra- 76 Tight Middle upper infra Tight Middle infra- Karsted and fractured liestone 19 Carbonate rock Basal upper infra Tight Lower infra- >6 Tight quartz Middle infra-

3 26 Pet.Sci.(28)5:24-3 CNL AC 2 1 DEN 15 5 CAL 2 CALL 2 Source ReseSeal rvoir Cenozoic 1 RT Source Reservoir Seal Mesozoic Upper Fig. 3 Profile showing source--cap rock asseblage for Abolag-1 well RT Source Resevoir Seal Infra Fig. 5 Profile showing source--cap rock asseblage for Abolag-1 well Upper Infra Fig. 4 Profile showing source--cap rock asseblage for Ouasa-1 well 2) Middle infra- karsted and fractured liestones: The total thickness for this interval can reach 19 eters. During its deposition, there was no input of detrital clasts. Stroatolite was developed in a typical shallow epicontinental sea and tidal flat environent. There were active oil and gas shows during drilling. Reservoir beds were coposed of stroatolite. The porosity type varies greatly. Intra-particle and dissolved pores are ost developed. There is little priary porosity whereas secondary porosity is well developed, so could be filled copletely in the later diagenesis There is a weathered surface on the top of the interval where karsting and dissolution took place, which led to the developent of dissolution porosities. As a result, the stroatolite could becoe fine oil and gas beds. Due to copression and copaction, stroatolite could be easily fractured to becoe fractured beds. 3) Basal upper infra- s: In the Ouasa-1 and Abolag-1 wells, the interval constitutes an 8 thick sequence in the basal Upper Infra. The porosity is very low. The beds are doinated by tight s. 4) Middle-upper upper infra- s: Sandstone beds with a thickness of up to 1 are distributed in the iddle and upper parts of the Upper Infra in the Ouasa-1 and Abolag-1 wells. Their average porosity is 8-15%. 5) s: The is doinated by s (Chen et al, 27). Its thickness is between 4-5. The sandy sequence is a ain interval in the Taoudeni Basin, with oil and gas shows. The porosity is approxiately 7-15% and can reach as high as 18 to 22% in soe intervals. The pereability is.2-1 d. 6) Basal s: The overlies the older sequences unconforably. Over the unconfority, tillite and were developed. The basal in the Ouasa-1 well is a set of beds and is a suit of brittle fine to ediu grained. The average porosity is 1%. 1.4 Cap rocks Cap rocks are developed in various stratigraphic intervals in the Ouasa-1 and Abolag-1 wells. They are doinated by

4 Pet.Sci.(28)5: udstones and shale. During the Middle infra-, depositional environent was neritic and udstone was deposited. The tight calcareous udstone thickness is up to 2. Sealing capacity of this set of udstone is very good. During the Late infra-, sea level changed frequently. There were nuerous cycles of sand-udstone sequences. Mudstone thickness varies for uddy intervals in different cycles. Individual udstone layers are 1-2 thick and soe of these can reach 5-6. The udstones are gray and brown in color and norally interbedded with s. They have experienced intensive diagenesis and have good seal ability. During the, two large scale sand-udstone cycles were developed. The udstone is gray and fairly thick. udstones are approxiately 19.thick in the Abolag-1 well and 36 in the Ouasa-1 well. They have good sealing capacity. During the, two periods of deposition hiatus occurred. But the as a whole is a transgressive sequence. Thick udstone was developed in the Taoudeni depression. The udstone can act as a good seal for the s. 1.5 Types of source--cap rock asseblage In the basin, three types of source--cap rock asseblages, in situ source and, lower source & upper, and upper source & lower are developed. They are illustrated in Fig Petroleu syste Based on the distribution of effective source rocks in the Taoudeni Basin, cobined with the structure characteristics of the basin and the distribution characteristic of the beds (Bai and Zheng, 27), two petroleu systes are recognized in the basin. One is the infra- petroleu syste and the other is the petroleu syste. They are illustrated in Fig Infra- gas syste In this syste, source rocks are confined to the Middle and upper infra-. They are widespread in the Taoudeni Basin and best developed in the Maqteir and Taoudeni depressions. Except the rap regions, the source rocks are ature. The axiu Ro value is over 3.6% at the base of the iddle infra- and 2.6% at its top. The infra- source rocks began to generate oil in the late infra- and entered the peak of oil and gas generation in the -. The source rocks becae over ature by the end of the Carboniferous. The post-hercynian uplifting halted hydrocarbon generation in the infra-. The oil and gas generated by the Middle and Upper Infra- udstones could accuulate in the sae stratigraphic interval, the overlying strata or the underlying strata by vertical igration through faults or fractures. Oil and gas shows fro this syste were found throughout the infra-. A tested gas flow was recorded fro Abolag-1 well. The distribution of the syste is shown in Fig. 8. The beds of the gas syste are ainly coposed of tight s of the upperost part of the lower infra-, the iddle infra- carbonate rocks and the upper infra- s. The beds have a cuulative thickness of 2 in the upper infra- and the carbonate beds have a cuulative thickness of 19 in the iddle infra-. The beds are doinated by shore zone quartzose s. The carbonate s were coposed of neritic and inter-tidal stroatolitic liestones. Nuerous intervals occur in this syste but their petrophysical properties vary greatly. Porosities fall in the range of -28%. claystones constitute the regional seal for the syste. They are widely distributed and have a good sealing capacity. Source Reservoir Cap Asseblage Carboniferous Lower source & upper Upper source & lower U. Infra- Lower source & upper M. Infra- L. Infra- In situ source and Upper source & lower Fig.6 Types of source--cap rock asseblage

5 28 Pet.Sci.(28)5:24-3 Altitude, ABOLAG-1 5 OUASA-1 Mauritania Mali ATOUILA-1 YARBA Upper Infra- Late Early Carboniferous Syste Petroleu Early Syste -4 Lower Infra- Infra k Gas shows Oil shows Upper Infra- Petroleu Stroatolite source rock Middle Infra- Fig. 7 Cross-section showing source rocks and oil/gas shows 2.2 petroleu syste This syste includes the, and. The was deposited in an extensive area, larger than the current Taoudeni Basin. The claystone probably was present originally within the entire basin. But subsequent uplifting and erosion led to the preservation of the source rocks only in the Taoudeni depression, which evolved into the present source kitchen center. The source rock is coposed of black or dark gray udstones. Copared with the infra- gas syste, the distribution of the petroleu syste is very liited (Fig. 9). The source rock is ainly located in the Taoudeni depression. Outside the depression, source rocks are very thin and their aturity is low so that they contributed little to the petroleu syste in the basin. The s of the syste are the s and the basal s. The diagenesis of these two sets of beds is weak so that their petrophysical properties are good. They are generally oderate to good beds. shows have been found in these s. The beds have a thickness of 5. Porosities generally fall in the range of 7%-1% and soe can reach as high as 18%-22%. Pereability is.2-1 d. The basal s are ediu grained. Their average porosity is 1% and their thickness is approxiately 5. The udstone and the udstone in the upper part of the Middle constitute the cap rock of the petroleu syste. The cap rocks are thick and have a good sealing ability. The udstone is the cap rock of Reguibat Massif(>1Ma) Mauritania Infra- source rock hydrocarbon generating center 1.2 Ouasa-1 Infra- source rock hydrocarbon generating center 2. Atouila-1 2k Mali N Mauritania Fold Belt (Metaorphised Palaeozoics) 1.2 Abolag Yarba-1 Ta13 Infra- petroleu syste Ta21 1. Fig. 8 Distribution of infra- gas syste in Taoudeni Basin The top of iddle infra- Ro,%

6 Pet.Sci.(28)5: Reguibat Massif(>1Ma) Mauritania sandy siltstone outcrops Ouasa-1 Effective source rocks Atouila-1 Mali N Mauritania Fold Belt (Metaorphised Palaeozoics) Abolag-1 Yarba-1 sandy siltstone outcrops Boundary of syste 2k Fig. 9 Distribution of the petroleu syste in Taoudeni Basin the beds and the udstone the cap rock of the basal beds. The tiing of hydrocarbon igration and accuulation is in the Carboniferous. Like the infra- gas syste, uplifting and denudation at the end of the Carboniferous led to the destruction of ost accuulations. As source rocks lost the ability of hydrocarbon generation, the accuulations are regarded as relic ones. Therefore, the petroleu syste is also considered as a relict one and favorable sealing conditions are required for preservation of earlier fored petroleu accuulations. 3 Integrated analyses of the ain controls on the types of hydrocarbon accuulations Based on the above discussions, the features of hydrocarbon accuulation can be suarized as follows: 3.1 Structural uplift and tiing of petroleu generation controlled the tiing of petroleu charging and preservation of hydrocarbon accuulations The Taoudeni Basin suffered post-hercynian erosion at the end of the Carboniferous period. Then the basin experienced liited deposition during the Cretaceous period. The source rocks in the basin were not buried again to a uch deeper depth to be able to generate hydrocarbons for the second tie. Therefore, hydrocarbon generation took place fairly early. The late Carboniferous is the latest tie for hydrocarbon generation. The subsequent gas supply was unable to copensate the hydrocarbon loss resulting fro the post-carboniferous diffusion. For the infra- petroleu syste, source rocks are highly ature to over ature. They were in the gas window by the Carboniferous tie so that the generated gas could not be preserved in large quantities since that tie (Fig. 1). 3.2 Maturity, evolution history, and distribution of effective source rocks controlled hydrocarbon richness The aount of gas generation by infra- source rocks is very liited after the Carboniferous. The distribution of the Infra- source rocks controlled the distribution of natural gas. The long period of uplifting led to the destruction of the accuulated oil and thus only a sall proportion of the originally generated gas could be preserved. source rocks have a lower aturity than Infra- source rocks. They generated a larger proportion of liquid hydrocarbons. Although hydrocarbon generation was halted early, oil was ore easily preserved than gas. Thus, the petroleu syste controlled the generation and distribution of oil. 3.3 The ain controls and geological processes controlled the type of petroleu accuulations The generation tie of hydrocarbon and processes of petroleu presentation controlled the concentration of natural gas (Huang et al, 1996a and 1996b; Li et al, 27; Dai, 1985; Dai et al, 27; 23; Zhao et al, 25). Petroleu accuulations in the Infra- petroleu syste are doinated by relic gas accuulations. Gases in the stratigraphic and lithologic traps tend to be ore easily preserved. These gas traps are ainly distributed within the depression or on the slopes of the depressions, but their sizes are sall. Petroleu accuulations in the petroleu syste, which covers a sall area, are doinated by relic oil and gas accuulations. In suary, lacking a thick Mesozoic sedientary cover eans that the hydrocarbon prospect would probably be

7 3 Pet.Sci.(28)5:24-3 Basin Stratu generation history Taoudeni Basin M-infra- Stratu age Legend L-infra M-infra U-infra C O S D C P T J K E Oil generation Gas generation Dispersed tie Fig. 1 -generation history and loss of oil and gas in Taoudeni Basin oderate in the Taoudeni Basin. 4 Conclusions Two petroleu systes: infra- and systes, are identified in the Taoudeni Basin. The petroleu accuulations in the infra- petroleu syste are doinated by relic gas accuulations and those in the petroleu syste by relic oil and gas accuulations. The petroleu syste has a better exploration potential than the infra- petroleu syste. For the infra- petroleu syste, the favorable exploration area is around the source kitchen area. The favorable exploration area for the petroleu syste lies in the eastern part of the basin. References Bai G P and Zheng L. Paleozoic coposite petroleu syste of north Africa: hydrocarbon distribution and ain Controlling factors. Petroleu Science (1): Che n S P, Wang Y and Jin Z J. Controls of tectonics on both sedientary sequences and petroleu systes in Tari Basin, Northwest China. Petroleu Science (2): 1-9 Dai J X. Coal-fored gas in west Siberian basin and its controlling accuulation rules. Natural Gas Geosciences (1): 4-11 (in Chinese) Dai J X, Wei Y Z and Zhao J Z. Iportant role of the foration of gas accuulations in the late stage in the foration of large gas fields. Chinese Geology (1): 1-19 (in Chinese) Dai J X, Zou C N, Tao S Z, et al. Foration conditions and ain controlling factors of large gas fields in China. Natural Gas Geosciences (4): (in Chinese) Hua ng Z L, Gao Y B and Hao S S. Quantitative evaluation and application of concentration seal of caprock. Acta Sedientologica Sinica. 1996a. 14(2): (in Chinese) Hua ng Z L, Hao S S and Yang J Q. The dynaic equilibriu study on natural gas igration and accuulation in Pingluoba Field of West Sichuan. Natural Gas Geosciences. 1996b. 16(2): 9-11 (in Chinese) Li J, Huang Z L and Wang H. Control of the tie effect of hydrocarbon generation on distribution of oil and gas: Taking Lishu fault depression in southeastern uplifted area, the southern Songliao Basin, as an exaple. Natural Gas Geosciences (9): (in Chinese) Mac gregor D S. Giant fields, petroleu systes and exploration aturity of Algeria. in Macgregor D S, Moody R T J and Clark-Lowes D D (eds.), Petroleu geology of North Africa: Geological Society, London Special Publication 132: Mak hous M, Galushkin Y and Lopatin N. Burial history and kinetic odeling for hydrocarbon generation, part II, Applying the GALO odel to Saharan basins. Aerican Association of Petroleu Geologists Bulletin (1): Mann P, Gahagan L and Gordon M B. Tectonic setting of the world s giant oil and gas fields. In Halbouty M T (ed.), Giant oil and gas fields of the decade AAPG Meoir 78. Tulsa: AAPG. 23: Ton g X G and Guan Z Y. Atlas of Petroleu Exploration and Exploitation of the World (Africa Monograph). Beijing: Petroleu Industry Press. 22 (in Chinese) USGS. U.S. Geological Survey World Petroleu Assessent 2-Description and Results. USGS Digital Data Series. 2. DDS-6 Zha o W Z, Wang Z Y, Wang Z C, et al. Concept, Connotation and evaluation index of high effective gas source kitchen and its action to the foration of high effective gas pool. Acta Sedientologica Sinica (4): (in Chinese) (Edited by Yang Lei)