Ahnet Basin Research Articles

Sweet Spots Identification: Geological and Engineering Integrated Shale Gas: A Case Study of Shale Gas Ahnet Basin Algeria

Sweet Spots Identification: Geological and Engineering Integrated Shale Gas: A Case Study of Shale Gas Ahnet Basin Algeria

Pore Structure Characterization of Shale Reservoir Using Nitrogen Adsorption-Desorption

This research paper aimed to quantitively characterize the pore structure of shale reservoirs. Six samples of Silurian shale from the Ahnet basin were selected for nitrogen adsorption-desorption analysis. Experimental findings showed that all the samples are mainly composed of mesopores with slit-like shaped pores, as well as the Barrett-Joyner-Halenda pore volume ranging from 0.014 to 0.046 cm3/ 100 g, where the lowest value has recorded in the AHTT-1 sample, whereas the highest one in AHTT-6, while the rest samples (AHTT-2, AHTT-3, AHTT-4, AHTT-5) have a similar average value of 0.03 cm3/ 100 g. Meanwhile, the surface area and pore size distribution were in the range of 3.8 to 11.1 m2 / g and 1.7 to 40 nm, respectively. Nanoparticle size was in the range of 540 to 1589.2 nm, this parameter showed a strong negative correlation with both the surface area and pore volume. Findings can contribute significantly to the pore structure evaluation and characterization of the Silurian shale reservoir in the Ahnet basin. .

In-situ stress analysis of Ahnet Basin, South western Algeria: A 1D geomechanical approach

This work aims to determine the present-day in-situ stress and pore pressure in the Ahnet Basin, Algeria, through a 1D geomechanical approach. We investigated the drilling-induced fracture (DIF) from FMI log data to ascertain the direction of maximum horizontal stress (SHmax) from the Ahnet Field. A mean orientation of N140° ( ± 10°) has been interpreted, which is NW-SE (N140°-N320°), with a local variation of ( ± 20°) compared to fields such as Illizi, Hassi Messaoud, and North Algeria, which can be explained by depth variation and intrinsic rock properties. A 1.05 psi/ft gradient of overburden stress (Sv) has been obtained from density. Pore pressure has been estimated from the sonic log by a normal compaction trend, indicates a hydrostatic regime from the surface to the top of the Silurian unit with an average pore pressure gradient of 0.43 psi/ft and an overpressure regime against the hot shale unit with a gradient of 0.58 psi/ft caused by the high in situ temperature in the study area and possible activity of the mega-shear zone. The poroelastic approach under transverse isotropic vertical conditions (VTI) has been used to calculate the minimum and maximum horizontal stress magnitudes. The outlines indicate a high-stress gradient close to 0.82 psi/ft, for Shmin calibrated with MDT stress points and 1.10 psi/ft for SHmax. The stress magnitudes results, suggest a present-day normal to strike-slip stress regime in the Ahnet Basin. Fault reactivation potential at two Silurian units has been inferred from the frictional theory analysis. The results indicate that increased pore pressure in hot shale formations due to by fluid injection and hydraulic fracturing causing shear slippage of the pre-existing faults, resulting in induced seismicity. Our study has contributed to the understanding of stress state origin in Ahnet Basin, the relationship between in situ temperature and pore pressure, and the fault stability analysis in such unconventional reservoir development.

A Bayesian multivariate model using Hamiltonian Monte Carlo inference to estimate total organic carbon content in shale

The prediction of total organic carbon (TOC) content using geophysical logs is one of the key steps in shale reservoir characterization. Various empirical relations have previously been used for the estimation of TOC content from well-logs; however, uncertainty quantification in the model estimation is often ignored while performing TOC estimation in a deterministic framework. We introduce the problem of TOC estimation in a Bayesian setting with the goal of enhancing the TOC content prediction together with the quantification of the uncertainty in the model prediction. To signify the uncertainty, we draw random samples of model parameters from the posterior distribution by realizing multidimensional stochastic processes within the Hamiltonian Monte Carlo algorithm. The posterior model for the variables that influence TOC estimation is conditioned on the available well-log observations and is further defined by a priori and likelihood distributions. We demonstrate examples of applications of this approach to estimate the TOC content on two real field data sets from the well-known Devonian Duvernay shale of Western Canada and the Silurian shale of the Ahnet Basin. The accuracy in the estimation is arbitrated by comparing the prediction results with those obtained using the two most widely used empirical models. Corroborating the results by the laboratory-measured TOC contents demonstrate that the Bayesian approach offers a more reliable and better confidence in predictions when compared with the empirical models, as it provides additional information on the prediction uncertainty. Finally, the implications of the present approach are derived in terms of depositional environments to characterize the high TOC content zone in the studied organic shale formations.

The lower Silurian black Shales from the Ahnet basin (SW Algerian Saharan platform): a comprehensive mineralogical study and paleoenvironmental implications

The lower Silurian black Shales from the Ahnet basin (SW Algerian Saharan platform): a comprehensive mineralogical study and paleoenvironmental implications

The Lower Cretaceous “Continental Intercalaire” around Tidikelt (Central Sahara, Algeria): Sedimentology, palaeontology and ichnology

The Lower Cretaceous strata are exposed over a large surface in the northern part of the Ahnet basin. They consist of transgressive, horizontal formations overlying the folded Palaeozoic formations by angular unconformity across the Tidikelt region. The present study addresses the respective lithostratigraphical units of the “Continental Intercalaire” succession and their subsequent palaeo-environment in the Tidikelt area using a trace fossil analysis combined with sedimentological and palaeontologic data. The lithological and sedimentological features of the study sections recognised eleven sub-units organised within four lithological units: the Lower Clay Unit of Barremian-Neocomian age consisting of three Sub-units (Akablie Sandstone, Sidi Mahmoud Clays and Pebbly Sands); the Quartzite Sandstones Unit referred to a Late Barremian-Early Aptian age; the Middle Clay-Sandstone Unit consisting of a thick deposit of Early Aptian-Albian age, divided into five Sub-units (Tit Sandstones, In Salah Sand-Clay, In Salah Sandstone, Foum El Bouib Sandstone, and Sandstone-Clay); and the Upper Clay Unit comprising the Oued Taleh Sand and the El-Golea Clays Sub-units. Vertebrate remains all evidence a Late Albian-Early Cenomanian age for the top part of the Continental Intercalary. Ichnologic data analysis documents lower diversity of the invertebrate trace fossils: only ten ichnotaxa are recorded so far and grouped in five ichnoassemblages. Fuvial-lacustrine ichnoassemblage represents a continental ichnofacies of a fluvial-lacustrine environment: Mermia, Scoyenia and Skolithos ichnofacies. Fluvial Ichnoassemblage of the Quartzite Sandstone Unit is represented by insect trace fossils typical of the Coprinisphaera ichnofacies. Fluvio-marginal ichnoassemblage corresponds to a tidal-influenced, fluvial deposit transgressive on Unit B, represented by Skolithos and Psilonichnus ichnofacies. Lower shoreface ichnoassemblage shows the upper part of Unit C, documented by horizontal trace fossils such as Thalassinoides and vertical root traces, all supporting a lower shoreface environment in a semi-arid climate interlined by well-oxygenated episode with nutrient-rich conditions and vegetation dominance. Near to shoreface ichnoassemblage coincides with the upper unit D of coastal deposits with fluvial-tidal influence at the base (Oued Taleh Sands) associated with Scoyenia and Skolithos trace fossils and lagoon deposit of the El-Golea Clays Sub-units.

PHYSICO-CHEMICAL PROPERTIES OF SILURIAN HOT SHALE IN AHNET BASIN, ALGERIA ( CAS STUDY WELL ASS-1)

The prediction of hot shale interval in Silurian formation in a well drilled vertically in Ahnet basin Is by logging Data (Resistivity, Gamma Ray, Sonic) with calculation of total organic carbon (TOC) using ∆ log R Method. The aim of this paper is to present Physico-chemical Properties of Hot Shale using IR spectroscopy and gas chromatography-mass spectrometry analysis, This mixture of measurements evaluation and characterisation show that the hot shale interval located in the lower of Silurian, the molecules adsorbed at the surface of shale sheet are significantly different from petroleum hydrocarbons this result are also supported with gas-liquid chromatography showed that the study extract is a hydroxypropyl

Evaluation of Frasnian Shale reservoir, case studywell DAK-1, Ahnet Basin, southern Algeria

The evaluation of unconventional reservoir in term of future exploration plan where the geochemical data are not unavailable making us different results from logging and Gas Data However this paper aim to define Potential zone throught the estimation of total organic carbon(TOC) using ∆ log R Method and thermal maturity by mean of gas ration technique combined with gamma-ray data of Frasnian shale formation encountered in DAK-1 well drilled in Ahnet Basin from 1552m to 1728m. The results suggest that the frasnian shale have fair to good potential genration with TOC ranging from 2% to 4%, with mature organic matter who producing wet gas,The potential zone positioned in the lower frasnian over a thikness of 10m.

Conodont thermal alteration patterns in Devonian and Carboniferous rocks of the Ahnet and Mouydir basins (southern Algeria)

Conodont colour alteration indices (CAI) have been used to recognize the thermal maturation of the Middle Devonian to Late Carboniferous sedimentary infill of the Ahnet and Mouydir basins in southern Algeria. The study is based on a large set of conodont samples obtained from 69 sections and outcrops. In the Ahnet Basin, the Middle and Upper Devonian rocks at the surface show a uniform thermal maturity level characterized by the CAI values of 2, which suggests maximum burial temperatures about 50 °C. In contrast, the Carboniferous exposures yield a wide range of CAI values from 1 to 3.5. Besides, the general tendency in maturity decreases upward in the sequence, there are anomalies related to heating by small local magmatic bodies. In the Mouydir Basin, the CAI values in the Middle and Upper Devonian outcrops range from 2 to 3.5. The pattern indicates that the majority of Middle Devonian rocks have experienced burial temperatures of about 120 °C, while thermal maturation of the Upper Devonian strata is clearly lower, with maximum heating temperatures about 50 °C at the edges of the basin. The observed thermal maturity has been mostly achieved during Late Carboniferous time, prior to the Variscan inversion, and only locally during the Mesozoic. It is a product of loading by Carboniferous overburden succession of which part is now removed, approx. 200–1300 m in the Ahnet Basin and up to 1200 m in the Mouydir Basin. The analysis of palaeotemperatures suggests an average geothermal gradient of 30 °C/km during Carboniferous time in the Ahnet–Mouydir area. In terms of hydrocarbon exploration the studied Devonian succession is predominantly mature for oil and gas. In contrary, the Carboniferous rocks are immature. Higher maturities are likely to occur in the subsurface.

Infracambrian hydrocarbon source rock potential and petroleum prospectivity of NW Africa

Abstract Proven Infracambrian hydrocarbon plays occur in various parts of the world, including Oman, the former Soviet Union, India, Pakistan and Australia. Organic-rich strata also occur in NW Africa, and gas shows originating from Infracambrian hydrocarbon source rocks are known from well Abolag-1 in the Mauritanian part of the Taoudenni Basin. The distribution of Infracambrian source rocks in North Africa is patchy and deposition commonly occurred in half-graben and pull-apart basins. In these intra-shelf basins, marine, organic-rich shales and limestones were deposited beneath the turbulent wave zone, away from the coarse siliciclastic Pan-African molasse detritus. On the West African Craton (including the Taoudenni Basin) organic-rich horizons were also deposited earlier, in pre- and syn-Pan-African times between 0.5 and 2 Ga (Ga is 10 9 years). The long-lasting sedimentation history in this area contrasts with that of the Pan-African regions, such as Oman, which lies in the Pan-African province of the East African Orogen, where preserved sediments are rarely older than 640 Ma. Infracambrian black phyllites in the Anti-Atlas region of Morocco were deposited on a continental slope of a short-lived ocean lying to the north of the West African Craton. Hydrocarbons generated during Infracambrian times from these deposits, however, have a low preservation potential. Infracambrian organic-rich and/or black-pyritic deposits in North Africa are proven in the Taoudenni Basin, the Anti-Atlas and the Ahnet Basin. Thick carbonate successions exist in the Taoudenni Basin, indicating deposition in areas some distance from contaminating coarse siliciclastic hinterland influx. Infracambrian strata may also occur in the Tindouf Basin. However, their deep burial and consequent early maturation history may be unfavourable for the preservation of Infracambrian-sourced hydrocarbons in this area. Local development of Infracambrian source facies may also occur in the Reggane, Ahnet, Mouydir and Iullemeden basins, as indicated by black shales in wells MKRN-1 and MKRS-1 in the Ahnet Basin. Generally, however, these basins appear to be close to the active Pan-African orogenic belt and, consequently, probably received large quantities of coarse siliciclastic sediment, largely of continental facies, which may have diluted any significant hydrocarbon source potential.

Sedimentary evolution of a Palaeozoic basin and ridge system: the Middle and Upper Devonian of the Ahnet and Mouydir (Algerian Sahara)

The Ahnet and Mouydir regions of southern Algeria are part of one of the world's largest, almost undeformed exposures of Palaeozoic rocks which exemplify a hitherto poorly known early Variscan development of a Devonian basin and ridge system. This area includes a series of intracratonic basins along the northern margin of the West African Craton which consists (from W to E) of the Reggane Basin, Azel Matti Ridge, Ahnet Basin, Foum Belrem Ridge and Mouydir Basin. The depositional and palaeogeographic interpretation is based on 71 sections in this region, which for the first time were biostratigraphically calibrated by means of conodonts, goniatites and brachiopods. The structural evolution during Devonian times was probably controlled by reactivation of ancient N–S- to NW–SE-running faults in the Precambrian basement, which caused differential subsidence and uplift of a previously largely unstructured siliciclastic shelf. A hiatus during Emsian times indicates widespread emergence during this interval. The entire area was flooded during the earliest Eifelian, when the first vestiges of the Azel Matti Ridge become evident by stratigraphic condensation. The palaeogeographic differentiation is most apparent during the Givetian, when a shoal with reduced carbonate sedimentation was established on the Azel Matti Ridge passing towards the west and east into basinal environments of the Reggane and Ahnet basins, respectively. The Foum Belrem Ridge is distinguished by increased subsidence during the early Givetian and by revived uplift during the late Givetian. In the Mouydir Basin further east, up to 1000 m of shales were deposited during the Givetian. The early Frasnian is marked by the ubiquitous sedimentation of black shales and bituminous styliolinites. These lithologies occur repeatedly already during the Middle Devonian and document intermittent anoxic conditions. The basin and ridge topography is levelled by the shallowing-up sequence of up to 1400 m thick upper Frasnian and Famennian shales which grade into a deltaic sequence of uppermost Famennian/Tournaisian sandstones. The up to now only vaguely discriminated lithostratigraphic formations of the Devonian have been biostratigraphically defined in suitable type sections.

Temporal–spatial reconstruction of the early Frasnian (Late Devonian) anoxia in NW Africa: new field data from the Ahnet Basin (Algeria)

Anoxic conditions were widespread in NW Africa during the early Frasnian (Late Devonian) that resulted in deposition of organic-rich shales and limestones with total organic carbon (TOC) values of up to 14%. Organic richness and thickness of these sediments vary laterally, and organic-rich vs. organic-poor facies boundaries are likely to have been diachronous. A precise temporal–spatial reconstruction of this anoxic phase in NW Africa is complicated because the organic matter in outcrops is largely oxidised and biostratigraphic resolution in boreholes is generally low due to the lack of recoverable conodonts. This contribution is based on eight outcrop sections at the margin of the central Algerian Ahnet Basin, where detailed spectral gamma-ray measurements were carried out using a handheld instrument. The pre-weathering organic richness in Frasnian outcrop sections is approximated using the characteristic uranium enrichment in the anoxic facies that, based on well studies, is positively correlated with the total organic carbon content. Conodont biostratigraphic results from these sections suggest that the uranium-enriched interval (the anoxic interval) at the basin margin is most common in the basal Frasnian conodont Biozones 1–2, confirming previous results from the Anti Atlas in Morocco. In three of the eight localities studied the basal Frasnian has not been deposited and the Frasnian here commences with distinctly younger uranium-enriched intervals, including Zones 4–11. Well data from the eastern Algerian Berkine Basin is interpreted to indicate a significantly longer anoxic phase there. Million-year-scale diachroneities of the Frasnian anoxia, therefore, clearly exist across the North Africa shelf. It is assumed that the palaeorelief might have been a major factor in controlling the onset, duration, and intensity of anoxia in the region.

Tectonic development of North African basins

Abstract Mostly the Palaeozoic and Mesozoic basins of North Africa have generally followed, and reworked, earlier basement trends formed by: (1) the NW-SE accretion of continental and oceanic terranes onto a Pan-African nucleus in northeastern Africa, and (2) the collision of this amalgam of accretionary terranes with the West African Craton. During the Upper Precambrian Pan-African Orogeny, the West African Craton formed a rigid block which indented this amalgam of accreted mobile belts to form much of North Africa. Intrusion of this indentor into North Africa caused the expulsion of narrow, triangular-shaped blocks of lithosphere to the north and south in a tectonic style very similar to the Miocene-Pliocene deformation of Tibet. Expulsion reactivated the earlier shear zones to form an anastomosing pattern of steeply dipping shears with left and right lateral sense of displacement. Left lateral shear also affected the northern edge of the West African Craton during this process of indentation. Subsequent rifting of the Pan-African mountain belt resulted in a series of grabens, which were infilled with Upper Precambrian-Cambrian molasse. These are the precursor basins for the Palaeozoic sediments which cover much of North Africa. The effects of rifting continued into the Cambro-Ordovician in the western basins. During the Silurian-Devonian many of the rifts were reworked. A new basin formed in the Atlas and Anti-Atlas, related to the growth of the proto-Tethyan Ocean. Basin inversion characterizes the Palaeozoic structures of the western Atlas and Anti-Atlas, producing thickened crust and a large mountain belt during the Carboniferous. Foreland basins formed on either side of this mountain belt and both the mountains and the adjacent basins were compartmentalized by WNW—ESE-trending transfer zones. Pan-African structures, within the African Plate, were reworked with further indentation of the West African Craton into Pan-African crust. The craton was pushed eastward, generating a left lateral shear couple along its northern margin. NW—SE-trending faults were reworked as dominantly left lateral strike-slip faults and N—S-trending fault blocks were rotated slightly in a clockwise sense. There was probably further lateral expulsion of lithosphere, ahead of the NE—SW-trending front of the indentor, reworking earlier N—S-trending shear zones. The North African Palaeozoic basins were inverted during the Hercynian-Appalachian Orogeny. In the Ahnet Basin the shortening was approximately NNE-SSW, perpendicular to the trend of the structures. This inversion was particularly marked in the Ougarta-Ahnet Basin where it produced a series of open to closed, north-south to NW—SE-trending folds above reactivated basement faults. During the Mesozoic, the Hercynian-Appalachian mountain belt underwent extension to produce deep rift basins infilled with continental sediments and some volcanics. The High Atlas formed as an arm to the Atlantic Basin. Transfer zones have a WNW-ESE trend, indicating that this was the main extension direction, similar to that in western and southwestern Europe. In northeastern Algeria, the orientation of the Mesozoic grabens suggests reworking of the basement fabric formed by Pan-African accretionary tectonics. The structures appear to die out toward the southwest into a broad transfer zone with some NW—SE-trending faults. The northeastern edge of the basin is obscured by later rift basins in the eastern Mediterranean. The Palaeozoic faults of the Amguid Spur, overlying one of the major shear zones of the Hoggar, formed a structural high throughout the Mesozoic with probably several pulses of inversion. An important episode of inversion occurred during the Aptian-Albian with the development of anticlines and associated reverse faults. Crustal extension associated with block faulting occurred in the Sirte Basin of Libya during the Mid- and Late Cretaceous. The block faults trend NNW—SSE to NW—SE, cross-cutting earlier Palaeozoic fold structures at a high angle but possibly parallel to some of the basement shear zones. The faults form the tips of a rift basin which opened between Sicily and Tunisia in the central Mediterranean. The Cretaceous faults have a component of right lateral displacement as well as normal fault movements. The Mesozoic basins of the High and Middle Atlas were inverted during the Late Cretaceous-Early Oligocene. The displacement direction, as seen from the transfer systems, was NW—SE, almost perpendicular to the Middle Atlas, but at a lower angle with the High and Sahara Atlas, which must have had components of oblique or right lateral movement. Minor effects of this inversion are reported from the Saharan basins.

Calcite cement successions in Middle Devonian (Givetian) carbonate mud buildups of the southern Ahnet basin (Algerian Sahara)

In the Ahnet Basin of southern Algeria, carbonate mud buildups were established in a deeper subtidal environment during Givetian times. The primary cavity system (irregular cavities, stromatactis) of these buildups exhibits a calcite cement succession precipitated in marine-phreatic to deep-burial environments. Cementation started in the marine environment. Cloudy, radiaxial-fibrous calcites (RFCs) of originally high Mg-calcite composition coat the walls of the cavities and occupy 50-80% of the pore space. The carbon-and oxygen-isotopic signature of these cements corresponds to that of co-occurring, nonluminescent brachiopod shells and to that of coeval RFCs from the Mader Basin of Morocco. Average values of δ18O=−2.6 (±0.4)%. PDB and δ13C=+2.6(±0.4)%. PDB are assumed to represent the Givetian stable-isotopic seawater signature of the Northwest-African carbonate province. The exceptional high δ18O values, compared to Middle Devonian data of North America, are interpreted to re from the mid-latitudinal (about 40°S), colder-water settings of the Northwest-African carbonate buildups. The87Sr/86Sr ratios of RFCs (0.70802±17) and nonluminescent brachiopod shells (0.70780±3) fit well into established Sr-isotopic curves of Devonian seawater. Cementation in the shallow-burial realm is characterized by a typical non-bright-dull cathodoluminescence sequence of nonluminescent scalenohedral cements (dog-tooth cements) with bright-luminescent outer margins overgrown by slight-ferroan, moderate-luminescent cements. Meteoric influence on this cement sequence can be excluded. Deep-burial cementation following a rather long period of non-cementation is represented by strong-ferroan, dull-luminescent blocky cements. Alternatively to pressure solution, outgassing of CO2 by decreasing pore-fluid pressure caused by Variscan fracturing can be considered as carbonate source for ferroan calcite precipitation in the remaining pore space. Strong δ18O depletion (−10.6 to −14.8%. PDB) of these cements point to the influence of hydrothermal fluids rather than to a pure depth-related temperature control. δ13C values show a trend from a nearly marine signature to more negative values (up to −9.0%. PDB) which suggests the input of methane by hydrocarbon migration during Variscan compression. More radiogenic87Sr/86Sr ratios (0.70826 to 0.71684) with respect to the marine composition are probably caused by the input of radiogenic strontium from K-feldspar dissolution and/or clay recrystallization from adjacent siliciclastic units.

Faulting, fracturing and in situ stress prediction in the Ahnet Basin, Algeria — a finite element approach

Many low-efficiency hydrocarbon reservoirs are productive largely because effective reservoir permeability is controlled by faults and natural fractures. Accurate and low-cost information on basic fault and fracture properties, orientation in particular, is critical in reducing well costs and increasing well recoveries. This paper describes how we used an advanced numerical modelling technique, the finite element method (FEM), to compute site-specific in situ stresses and rock deformation and to predict fracture attributes as a function of material properties, structural position and tectonic stress. Presented are the numerical results of two-dimensional, plane-strain end-member FEM models of a hydrocarbon-bearing fault-propagation-fold structure. Interpretation of the modelling results remains qualitative because of the intrinsic limitations of numerical modelling; however, it still allows comparisons with (the little available) geological and geophysical data. In all models, the weak mechanical strength and flow properties of a thick shale layer (the main seal) leads to a decoupling of the structural deformation of the shallower sediments from the underlying sediments and basement, and results in flexural slip across the shale layer. All models predict rock fracturing to initiate at the surface and to expand with depth under increasing horizontal tectonic compression. The stress regime for the formation of new fractures changes from compressional to shear with depth. If pre-existing fractures exist, only (sub)horizontal fractures are predicted to open, thus defining the principal orientation of effective reservoir permeability. In models that do not include a blind thrust fault in the basement, flexural amplification of the initial fold structure generates additional fracturing in the crest of the anticline controlled by the material properties of the rocks. The folding-induced fracturing expands laterally along the stratigraphic boundaries under enhanced tectonic loading. Models incorporating a blind thrust fault correctly predict the formation of secondary syn- and anti-thetic mesoscale faults in the basement and sediments of the hanging wall. Some of these faults cut reservoir and/or seal layers, and thus may influence effective reservoir permeability and affect seal integrity. The predicted faults divide the sediments across the anticline in several compartments with different stress levels and different rock failure (and proximity to failure). These numerical model outcomes can assist classic interpretation of seismic and well bore data in search of fractured and overpressured hydrocarbon reservoirs.

The World's Most Spectacular Carbonate Mud Mounds (Middle Devonian, Algerian Sahara)

ABSTRACT In the southern Ahnet Basin of Algeria a unique Middle Devonian underwater scenery is revealed, which consists of 191 mud mounds, 15 mud ridges, and 2 mud atolls distributed over nine discrete areas of a shallow-pelagic carbonate shelf. All the buildups are totally exhumed, thus perfectly exhibiting their original morphologies, slope angles, elevations, and the pattern of the onlapping strata as well as their relationship to the surrounding off-buildup sequences. Types of buildups include (1) small mud mounds (1-5 m high, 10-20 m diameter), (2) large mud mounds (up to 40 m high, up to 200 m diameter), (3) lens-shaped mounds(1-30 m thick, 10-15° slopes), (4) mud ridges (up to 100 m high, 150-8500 m long) and (5) semicircular mud atolls (2500-2800 m long, 20-30 m high). The buil ups were constructed during a short time span in the early Givetian (part of the Lower varcus zone), probably not exceeding a few hundred thousand years. Sedimentation rates in coeval off-buildup areas were at least 3-10 times lower. The lithology of the buildups is a massive boundstone with a high proportion of irregular, spar-filled cavities (stromatactis) but only very scattered potential frame builders. The carbonate mud is considered to be an in situ, cyanobacterial precipitate, although evidence of such is poor. Organic components of the buildups are not significantly different from those of the off-buildup lithologies, and include numerous crinoid ossicles, some tabulate and solitary rugose corals, brachiopods, mollusc debris, trilobites, and very rare sponge spicule and bryozoans. The total absence of stromatoporoids, large tabulate and colonial rugose corals, green algae, and micritic envelopes around grains suggests growth of the buildups in the lower part of the photic zone (100-200 m). They are onlapped by a discontinuous sequence, a few meters thick, of skeletal packstones and grainstones, and locally, orthoceratid floatstones that were deposited later in the Givetian. A widespread rise of sea level in the early Frasnian buried both buildups and adjacent areas under shales and siltstones. Postdepositional alteration of the mud buildups includes: (1) formation of stromatactis cavities, probably by dewatering and internal erosion, (2) cementation of stromatactis cavities and interparticle pore spaces by early high-Mg calcite and late, blocky low Mg calcite, (3) neomorphic transformation of Mg-calcitic mud and aragonitic skeletal debris, (4) occasional formation of neptunian dikes, (5) slight erosion of buildup surfaces, (6) compaction and slumping of onlapping capping beds, and (7) local dolomitization. The formation and growth of the mud build-ups is envisaged as a sequence of stages starting with patchy coral thickets evolving into small mud mounds that coalesced into larger mounds. By continuous vertical and lateral growth, closely spaced mounds amalgamated into mud ridges and mud atolls. Regular NW-SE and N-S patterns of some mound rows and ridges show a relationship of buildup formation to rejuvenated late Precambrian tectonic lineaments, which reappear in the Hercynian orogeny. The isotopic signature of matrix and early ements of mound rocks, however, does not support the idea of fluid migration and hydrothermal CO2 supply during buildup growth.

Sequence Stratigraphic Analysis and Development of Lower Devonian Reservoirs in the Central Ahnet Basin - Algeria  :ABSTRACT

Sequence Stratigraphic Analysis and Development of Lower Devonian Reservoirs in the Central Ahnet Basin - Algeria  :ABSTRACT

The hydrocarbon systems of North Africa

North Africa's hydrocarbon reserves are spread over a wide range of reservoirs and play types but derive from only a few key source rock intervals, particularly those in the Silurian, Late Cretacous and Eocene. Source rock maturity, together with the timing of the generation relative to trap forming and destructive events, seem to be the key controlling factors on success or failure at a basin level. In most cases, generation clearly post-dates trap formation, although a more complex relationship is evident in various parts of Algeria, where some traps did not form until gas was being generated. Source rock deposition and maturation can be tied to two main extensional tectonic cycles which ended in the compressional and destructive events of the Hercynian and Alpine orogenies. Remaining exploration potential in mature provinces may be tied largely to subtle and stratigraphic traps. In addition, significant frontier potential can be identified in two main fairways, corresponding to regions where Silurian/Devonian and Cretaceous/Eocene sources respectively are likely to have generated petroleum. Critical factors in these areas are timing of generation and preservation. The gas discoveries in the Ahnet Basin provide analogues for Palaeozoic generation/preservation models in the interior Silurian/Devonian source fairway. Similarly, the key to success for the Mesozoic basins of the Atlas region lies in demonstrating a favourable relationship between trap formation/destruction and generation from Late Cretaceous and Eocene source rocks.

Shell directions as a tool in palaeocurrent analysis

Conical shells (mostly orthoconic nautiloids, locally gastropods and rugose corals) were used to determine current directions in Ludlovian to upper Famennian cephalopod limestones in the eastern Anti-Atlas of Morocco, Ougarta Aulacogen and Ahnet Basin (both Algeria). Data plots established on 50,413 measurements from 217 localities document rather consistent current patterns which show only minor variations through subsequent intervals. A conspicuous feature are currents derived from pelagic platforms and directed towards adjacent basins. Shell accumulations decrease markedly towards platform margins yielding less distinctive information on current directions which, due to lack of shells, cannot be established in the basins proper. Orientation patterns of styliolinids show such a puzzling variation in adjacent samples that their use for current analysis is doubtful. The same is true for the presumed down-stream position of goniatite apertures which shows a highly variable pattern which is rarely consistent with that of concomitant orthoconic nautiloids. The direction of orthocones in cephalopod limestones onlapping lower Givetian mud mounds and ridges in the Ahnet Basin of Algeria shows a radial pattern which is the result of a mere gravitational deposition of shells on the steep slopes of these buildups. Apart from this exception the applicability of conical shells for current analysis is confirmed.

New architectures of deep-water carbonate buildups: Evolution of mud mounds into mud ridges (Middle Devonian, Algerian Sahara)

Spectacular, totally exhumed, carbonate mud buildups of Givetian age can be found in the southern Ahnet basin of the central Sahara. Their geometries range from isolated mud mounds to mud ridges up to 8 km long. The latter are the result of lateral coalescence of closely spaced, individual mounds. Mounds up to 40 m high and ridges up to 85 m high show original slopes of 25° to 65°. Frame-builders in the massive core of the buildups are restricted to scattered small tabulate and solitary rugose corals. The total absence of stromatoporoids, colonial rugose corals, and algae, as well as the lack of debris at the toes of the buildups, shows that they were constructed in deep water below the photic zone and storm wave base. The arrangement of the buildups in north- to northwest-trending clusters and ridges suggests that their distribution was controlled by early Variscan extensional movements inherited from Precambrian patterns.