Lower Silurian `hot shales' in North Africa and Arabia: regional distribution and depositional model

[1] Three cross sections of the Moroccan High Atlas illustrate the structural geometry and relationship between tectonic shortening and topography in this Cenozoic intracontinental mountain range. The structure is dominated by thick-skinned thrusting and folding, essentially by inversion of Mesozoic extensional faults and by buckling of both pre-Mesozoic basement and its sedimentary cover. Detached, thin-skinned thrusting is limited and apparently related to basement underthrusting, which did not always create structural relief. Despite the high topography, tectonic shortening is moderate, with faults and folds being spaced and separated by broad tabular areas. Section restoration indicates that shortening decreases along strike from east to west in the High Atlas, while topographic elevation generally increases. This inverse correlation suggests that crustal thickening does not fully explain the observed topography and suggests a mantle contribution to uplift. This is supported by geophysical indications of a thin lithosphere and by alkaline volcanism in the vicinity. Mantle-related uplift, which occurs in a broad region, may also explain the scarce foreland basin record adjacent to the High Atlas. The relief of the Atlas Mountains is interpreted as a combination of isostatic and dynamic topography.

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Tectonic shortening and topography in the central High Atlas (Morocco)

Abstract The Palaeozoic petroleum systems of North Africa contain five large giant (> 1 billion barrels of oil equivalent) and 24 giant (> 250 million barrels of oil equivalent) oil and gas fields with total recoverable reserves discovered to date of more than 46 billion barrels of oil equivalent. This article presents a classification of these petroleum systems based upon their productivity and maturity. Productivity of each system has been estimated from the associated hydrocarbon reserves and maturity from an analysis of their geological history ranging from initial genesis to maturity, destruction and final extinction. Key factors controlling both productivity and maturity include hydrocarbon charge, style of drainage and entrapment, and intensity of post-entrapment tectonic, thermal and hydrodynamic destructive processes. The regionally extensive Lower Silurian Tanezzuft Formation is the origin of 80–90% of Palaeozoic sourced hydrocarbons, with a further 10% from the Upper Devonian Frasnian shales, charging a number of intra-Palaeozoic and basal Triassic reservoirs. Triassic fluvial sands are the most important of these, hosting just over half of the total reserves, while Cambro-Ordovician and Lower Devonian F6 sandstone reservoirs are the second and third most significant, respectively. Three categories of Palaeozoic petroleum systems have been identified: (1) Mesozoic to early Tertiary charged systems with Triassic-Liassic shale and evaporite seals in the Mesozoic sag or ‘Triassic’ Basin of the northern Sahara Platform. These include > 78% of the total discovered reserves, with > 56% in the supergiants, Hassi R’Mel and Hassi Messaoud fields (2) Mesozoic to early Tertiary charged systems with intra-Palaeozoic shale seals in basins of south and east of the Triassic Basin. These include > 18% of the total discovered reserves, mostly in the prolific Illizi Basin. (3) Now largely extinct Palaeozoic charged systems with intra-Palaeozoic seals in basins of southwest Algeria and Morocco with 3% of discovered reserves. The productivity of these systems varies considerably. Hassi R’Mel and Hassi Messaoud are classified as super-productive, located on the crests of broad Palaeozoic arches which encouraged extremely efficient lateral migration focusing, and a very high impedance entrapment style. Other petroleum systems within the Triassic Basin are of high productivity with somewhat less effective migration focusing and impedance characteristics. Because of a regional evaporite seal and minimal late stage modification, these systems are all preserved in a mature phase of evolution. Basins south and east of the Triassic Basin are in various stages of destruction with variable productivities reflecting both less robust seals and post-entrapment modification by Austrian and mid-Tertiary uplift, tilting, remigration, spillage and freshwater flushing. The Illizi Basin is the least affected by these late stage destructive processes with some 15% of total discovered reserves still remaining. The Palaeozoic charged systems of southwest Algeria and Morocco were largely destroyed by Hercynian, Austrian and mid-Tertiary deformation. Only the high relief Hercynian anticlines of the Ahnet-Gourara Basin retained their trapping integrity and still reservoir a very significant amount of gas. Apart from scattered hydrocarbon shows and a few small residual accumulations the other basins in this region now all appear to be extinct.

/pdf/palaeozoic-petroleum-systems-of-north-africa-5390a5wly4.pdf

Palaeozoic petroleum systems of North Africa

An edited version of this paper was published by the American Geophysical Union (AGU). Copyright 1998, AGU.
See also:
http://www.agu.org/pubs/crossref/1999/1998TC900015.shtml;
http://atlas.geo.cornell.edu/morocco/publications/beauchamp1999.htm

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1998TC900015

Inversion tectonics and the evolution of the High Atlas Mountains, Morocco, based on a geological-geophysical transect

Discovery of giant polygonal terrains on Mars has prompted a 30-year debate over how they formed. The prevailing hypothesis is that small-scale Martian polygons formed by thermal contraction, as in terrestrial permafrost environments. Large-scale (>1 km) Martian polygons in the northern plains are visible in THEMIS, MOLA, Viking, and Mariner data, but how they formed remains enigmatic. We suggest that terrestrial deep-water marine polygons are morphological and perhaps genetic analogs to largescale Martian polygonal features. The terrestrial, deep-water polygons are imaged in three-dimensional seismic-reflection data acquired by the oil and gas industry in offshore Norway and the Gulf of Mexico. How deep-water polygonal fault systems form is a debated topic beyond the scope of this work. However, similarities between terrestrial deep-water polygonal fault systems and large-scale Martian polygonal terrains suggest that the latter could have formed during deep-water marine deposition. Deep-water polygonal faults form within fine-grained sediment at shallow burial depths. Increases in slope angles can trigger downslope disaggregation of deep-water polygons and mass wasting (forming debris flows). Physical models indicate that multidirectional extension can cause polygonal features to break up on a slope over a mobile substrate. Some knobby terrains in the Vastitas Borealis Formation seem to originate from disaggregation of large-scale Martian polygonal terrains. These analogies suggest a possible deep-water subaqueous origin for large-scale Martian polygonal terrains and support the idea of a late Hesperian–early Amazonian ocean on the northern plains of Mars.

Deep-water polygonal fault systems as terrestrial analogs for large-scale Martian polygonal terrains

The lower continental slope of Morocco's west coast consists of Triassic-age salt manifested in the form of diapirs, tongues, sheets, canopies, and toe thrusts. Active salt diapirism and regional tectonics greatly influence the morphology of the modern sea floor, forming a severely rugose expression with ongoing minibasin development and episodic submarine failure. Detailed mapping of a 1064-km2 (411-mi2) seismic survey acquired in the Safi Haute Mer area revealed that Jurassic to Holocene salt mobilization continually affected distribution of sediment, causing a range of depositional flow styles, from slumps to sheet slides and mass-transport complexes (MTCs). Large sediment waves (20 km [12 mi] long, 1.5-km [0.9-mi] wavelength) were also documented at the end of the Aptian. An east-west–trending structural anticline downdip of the salt activated during initiation of the Atlas uplift in the latest Cretaceous to earliest Tertiary and shaped much of the lower slope into the Tertiary with a persistent canyon system and slope channels. The largest of the debris flows is a Cretaceous-age MTC, a 500-m (1640-ft)-thick flow that spans an area of up to 20,000 km2 (7722 mi2). Composing the MTC are (1) chaotic, mounded seismic facies; (2) internal syndepositional thrusts; and (3) transported megablocks (3.3 km2 [1.3 mi2]) with preserved internal stratigraphy. The MTC originated from an upslope collapse of a narrow shelf during the earliest phases of the Alpine orogeny.

Seismic geomorphology of offshore Morocco's east margin, Safi Haute Mer area

The Tadla basin, Morocco, is the easternmost major structural subdivision of the Meseta south of the Central Massif. After the basin remained relatively stable through the Paleozoic, movements during the Variscan orogeny divided the basin into two subbasins, a western subbasin where compressional forces formed large anticlines and an eastern subbasin where intense folding and faulting are associated with uplift and erosion. The sedimentary basin fill includes numerous organic shales, which provide the source beds. Also deposited were numerous shallow-water to continental clastics and to a lesser extent carbonates, providing several possible reservoir rocks for hydrocarbons. The results from computer modeling show that the chief formations of hydrocarbon generation are Carboniferous and Silurian. Most of these formations may have generated hydrocarbons before Variscan movements, which probably destroyed preexisting oil and gas traps. The only source rocks in the generation stage from Mesozoic to the present are the upper Visean shales. The western part of the Tadla basin was the most favorable area for the generation and maturation of hydrocarbons of the upper Visean shales.

Haddou Jabour

Papers

Seismic geomorphology of offshore Morocco's east margin, Safi Haute Mer area

Basin Modeling of Tadla Basin, Morocco, for Hydrocarbon Potential