DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Geology and Petroleum Resource Assessment of Onshore Northwestern Africa By James A. Peterson* Open-File Report 86-183 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Missoula, Montana 1986 Contents Page Abstract ......................................................... *.. 1 Introduction............................................................. 1 Regional geology......................................................... 2 Summary of tectonic and sedimentary history.......................... 2 Late Precambrian (Infracambrian)................................ 3 Early and Middle Paleozoic...................................... 3 Carboniferous................................................... 3 Permian......................................................... 3 Triassic........................................................ 4 Jurassic.*...................................................... 4 Early Cretaceous................................................ 4 Late Cretaceous................................................. 4 Tertiary........................................................ 4 Petroleum geology and resource assessment.........*....*................. 5 Atlas orogenic belt............................................. 6 Tindouf, Colomb-Bechar, and Reganne Basins...................... 7 Summary.................................................................. 8 Atlas folded Geosynclinal belt.................................. 8 Tindouf, Colomb-Bechar, and Reggane Basins...................... 8 Taoudeni Basin.................................................. 8 Acknowledgments.......................................................... 8 Selected references...................................................... 8 Illustrations Page Figure 1. Index map, Northwest Africa, showing main basins and uplifts and assessment area................................... 14 2. Index map, Atlas folded geosynclinal belt and adjacent area, showing main basins and uplifts and main oil and gas fields... 15 3. North-south generalized stratigraphic-structural cross-section A-Af , Tunisian Atlas and adjacent area........................ 17 4. North-south generalized stratigraphic-structural cross-section B-B 1 , Tellian Atlas and adjacent area......................... 18 5. North-south generalized stratigraphic-structural cross-section C-C 1 , Essaouri basin to south end of Gao trough........(in pocket) 6. West-east generalized stratigraphic-structural cross-section D-D 1 , Senegal basin to Taoudeni basin..................(in pocket) 7. Approximate thickness, sedimentary cover, Northwest Africa........................................................ 19 Illustrations (continued) 8. Approximate thickness, Paleozoic rocks, Northwest Africa........................................................ 20 9. Approximate thickness, Mesozoic rocks, Northwest Africa....... 21 10. Approximate thickness, Tertiary rocks, Northwest Africa....... 22 11. Northwest Africa, Atlas folded geosynclinal belt Undiscovered conventionally recoverable oil............................... 23 12. Northwest Africa, Atlas folded geosynclinal belt Undiscovered conventionally recoverable gas............................... 23 13. Northwest Africa, Tindouf, Colomb-Bechar, and Reggane basins Undiscovered conventionally recoverable oil.................. 24 14. Northwest Africa, Tindouf, Colomb-Bechar, and Reggane basins Undiscovered conventionally recoverable gas.................. 24 15. Northwest Africa, Taoudeni basin Undiscovered conventionally recoverable oil.............................................. 25 16. Northwest Africa, Taoudeni basin Undiscovered conventionally recoverable gas.............................................. 25 Tables Table 1. Assessment of undiscovered conventionally recoverable petroleum resources of onshore Northwestern Africa............. 12 2. Supplementary and comparative data supporting the resource assessment for onshore Northwestern Africa..................... 13 11 GEOLOGY AND PETROLEUM RESOURCE ASSESSMENT OF ONSHORE NORTHWESTERN AFRICA by James A. Peterson ABSTRACT The main onshore basins of northwestern Africa are: (1) basins in the Atlas folded geosynclinal belt adjacent to the Mediterranean Sea, (2) the Tindouf, Colomb-Bechar, and Reggane basins of western Algeria and southern Morocco, and (3) the Taoudeni basin of Mauritania and Mali. Much of the Atlas folded geosynclinal (erogenic) belt contains a very thick but highly disturbed section of marine Paleozoic, Mesozoic, and lower Tertiary carbonate and clastic rocks that includes a substantial thickness of probably highly mature or overmature petroleum source rocks. The small, elongate, infolded Tertiary intermontane basins appear to have some hydrocarbon potential, but limited drilling there has had little success thus far. Accumulations of oil and gas are likely to be small and difficult to find. The Tindouf, Colomb-Bechar, and Reggane basins contain a thick section of marine Paleozoic rocks that has some gas potential and minor oil potential. Carbonate mounds and reefs of Devonian and Carboniferous age in these basins may not have been adequately explored. The Taoudeni is a very large basin and has a high exploration risk. Most of its sedimentary section is early Paleozoic and late Precambrian in age. Problems include questionable source-rock quality, inadequate burial depths in much of the basin, and lack of extensive regional seals. Because of the location of the basin in a remote part of Africa, discoveries would have to be large to be commercial. U.S. Geological Survey (USGS) estimates of original recoverable petroleum resources (ultimate) of the northwestern Africa onshore provinces as of December, 1982 at probability levels of 95 percent, 5 percent, and statistical mean are for oil: approximately 0.15, 6, and 1.8 BB (billion barrels); and for gas: approximately 0.5, 30, and 12 Tcf (trillion cubic feet). INTRODUCTION This investigation of the petroleum resource potential of the onshore northwestern Africa region includes geological, geophysical, production or other petroleum engineering data available through 1982. The resource assessment was conducted by the Resource Appraisal Group (RAG) of the U.S. Geological Survey, Branch of Oil and Gas Resources, following the standard procedures developed since 1974 for domestic petroleum resource analysis (Dolton and others, 1981). The technique requires study of a given area, with particular attention to the geologic factors controlling the occurrence, quality, and quantity of the petroleum resource. Standardization of critical elements of the investigations is achieved by the preparation of data forms for each basin or province, which call for specific volumetric, areal, and rock-quality measurements, as well as the determination of basin analogs for comparison purposes. In addition, finding-rate histories and projections are constructed, when possible. From these data and analyses, various analytical techniques are used to calculate a set of resource numbers. The assessment process itself is subjective; the results of the geological investigation and of the resource calculations are presented to a team of USGS assessment specialists, who make their personal estimates conditional upon recoverable resources being present. Initial assessments are made for each of the assessed provinces as follows: (a) a low-resource estimate corresponding to a 95 percent probability of more than that amount; this estimate is the 95th fractile (Fg^). (b) a high-resource estimate corresponding to a 5 percent probability of more than that amount; this estimate is the 5th fractile (Fc). (c) a modal (most likely) estimate of the quantity of resource associated with the greatest likelihood of occurrence. The individual estimates are then posted and averaged, and the results debated from the perspective of the personal experiences of the individual assessors; a second and third iteration of the procedure may follow depending on consensus. If no commercial oil has been heretofore discovered in the province, then a marginal probability is subjectively assessed reflecting the probability that any commercial oil will ever be discovered. The results of the final estimates are averaged, and those numbers are computer processed using probabilistic methodology (Crovelli, 1981) to show graphically the resource values associated with a full range of probabilities and to determine the 95th fractile, the 5th fractile, and the mean, as well as other statistical parameters. REGIONAL GEOLOGY Onshore northwestern Africa includes the West African Craton and the adjacent Alpine Zone (Atlas orogenic belt) to the north (figs. 1, 2). This region has been separated into three assessment provinces: (1) the Atlas orogenic belt (Alpine Zone), (2) the Tindouf, Colomb-Bechar, and Reganne basins, and (3) the Taoudeni basin (fig. 1). The Alpine Zone includes the complex of uplifts and basins adjacent to the Mediterranean Sea, separated from the southern region by the South Atlas fault. This complex structural region is generallly separated into several structural provinces, including the Rif, Chelif, Kabylia, and Tellian Alpine thrust belts bordering the Mediterranean, and the Moroccan Meseta, Middle Atlas, High Atlas, High Plateaus, Saharan Atlas, and Tunisian Atlas foreland provinces of the middle and southern Atlas (fig. 2). Summary of Tectonic