Berkine Basin Research Articles

A regional chemostratigraphically-defined correlation framework for the late Triassic TAG-I Formation in Blocks 402 and 405a, Algeria

The Triassic Argilo-Gréseux Inférieur Formation (TAG-I) is one of the principal hydrocarbon reservoirs in the Berkine Basin of Algeria. Sedimentological studies have shown that it exhibits marked spatial and temporal facies variations on both a local field scale and a regional basinal scale. This variability, combined with a lack of diagnostic flora and fauna, makes regional correlation within the unit difficult. In turn, the lack of a consistent regional stratigraphic framework hampers the comparison of the various correlation schemes devised by operators in the basin. Contrasting the TAG-I in Blocks 402 and 405a exemplifies the problems encountered when attempting regionally to define a correlation framework for the interval. Between these two blocks, a distance of approximately 200 km, there are marked changes in the style of deposition from sand-dominated, proximal fluvial systems in the SW (Block 405a, MLN, MLC, KMD and MLNW fields) to a more distal, more clay-prone system in the NE (Block 402, ROD/BRSE/BSFN, SFNE and BSF fields). A chemostratigraphic study of the TAG-I in these two blocks has allowed a four-fold correlation framework to be defined, where each chemostratigraphic package has distinctive geochemical features. Chemostratigraphic Package 10, the oldest unit, lies above the Hercynian Unconformity, but beneath a geochemically identifiable hiatal surface. Chemostratigraphic Package 20 lies above the hiatal surface but is separated from the overlying packages by a mineralogical change identifiable in both claystone and sandstone geochemistry. Chemostratigraphic Packages 30 and 40 are chemically somewhat similar, but are separated by a regional event interpreted as a period of dolocrete and lacustrine development. By combining the geochemical differentiation of the units and recognition of their stratal boundaries, it is possible to define a correlation for the TAG-I between Blocks 402 and 405a. The proposed correlation between the two blocks suggests that the northern parts of Block 405a may have been occupied by a spur or subsidiary channel from the main SW–NE-trending fluvial system, resulting in one of the chemically defined packages being demonstrably absent in the MLNW, MLN, KMD and MLC fields when compared with the other areas of the study.

Successful Miscible-Gas Injection in Rhourde El Krouf Field, Algeria

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 94138, "Successful Peripheral Miscible-Gas Injection in RKF Field, Berkine Basin, Algeria - Field Case History," by H. Cubillos, SPE, D. Stright, SPE, and J.M. Navarro, SPE, CEPSA, prepared for the 2005 SPE Europec/EAGE Annual Conference, Madrid, Spain, 13–16 June. The full-length paper discusses the discovery, development, and operation of a miscible-gas-cycling project in a highly under-saturated, volatile-oil Trias Argilo-Gréseux Inférieur (TAGI) reservoir in the center of the Berkine basin, Algeria. Miscible gas injection was initially considered to be high risk and marginally economic; however, with a flexible development plan and careful reservoir management, a successful project was implemented. Introduction The Rhourde El Krouf field (RKF) in Block 406A of the Berkine basin, 480 miles southeast of Hassi Messaoud, has been producing under partial pressure maintenance by miscible-gas injection from the TAGI since 1996. Deeper volatile-oil and retrograde-gas-condensate reservoirs provide makeup gas for the project. After 9 years of peripheral, downdip, high-pressure gas injection, RKF has recovered more than 50 million bbl of oil with 90 Bcf of gas injection. Primary recovery was predicted to be less than 40 million bbl of oil. The field still is producing at the 20,000 BOPD designed plateau rate. Background The RKF was discovered in 1992 by the Rhourde El Krouf 1 exploration well drilled to a total depth of 16,670 ft. Sustained production from this well was 5,000 BOPD from two siliciclastic reservoirs, the TAGI and the Carboniferous, from 9,843 to 11,155 ft. The RKF was brought on stream in 1996 in a record time for a remote area with no infrastructure. In 1997, the field was placed on full production with seven producing wells and three gas-injection wells. As dynamic production data were acquired, facilities were expanded and wells added. Reservoir Description The TAGI reservoir consists of stacked fluvial deposits, including porous sandstone units interbedded with claystones, resting unconformably over Carboniferous rocks. The TAGI was deposited over a relatively stable continental surface closely related with the regional Hercynian unconformity. A combination of well logs and core descriptions was used to subdivide the 492-ft TAGI section into three well-constrained stratigraphic sequences (Upper, Middle, and Lower) that have excellent lateral continuity. Pressure and tracer data indicate good lateral and vertical communication. Significant anisotropy with a southwest/northeast azimuth has been observed that is aligned with the main fault patterns. Vertically, high-permeability layers exist together with lower-permeability sublayers within individual flow units that are, on average, 33 ft thick. Porosity varies from 12 to 18%, with a 16%average. Permeability varies from 10 to 1,000 md, with an average of 200 md. Producing-sand bodies are mainly composed of quartz (94%) and clays (6%), which are mainly chlorite, illite, and kaolinite.

FLOODPLAIN DEPOSITS AND SOIL CLASSIFICATION: THE PREDICTION OF CHANNEL SAND DISTRIBUTION WITHIN THE TRIASSIC ARGILOGRESEUX INFERIEUR, BERKINE BASIN, ALGERIA

Middle to Late Triassic fluvio‐lacustrine sandstones referred to as the Trias Argilo‐Grèseux Inférieur (TAG‐I) are an important hydrocarbon reservoir in the Algerian Berkine Basin. Three depositional sequences have been identified within the TAG‐I and a fourth sequence overlies it, forming part of the “Trias Argilo‐Carbonaté”. Subtle changes in the style of sedimentation through these sequences have been identified and are attributed to periodic rises in base‐level together with changes in subsidence rate and climate, leading to basinward, progradational shifts of the fluvial systems. The palaeoclimate changed from semi‐arid to sub‐humid with seasonal wetting and drying, as indicated by the evolved style of the deposition, the abundance of vegetation and the nature of associated palaeosols. Four main types of palaeosol profile have been identified and consist of green/grey, red, and red/brown to purple palaeosols together with green/grey to red pedogenically modified mudstones, depending on the level of maturity.The presence of sphaerosiderites in green palaeosols is a clear indication of the occurrence of wetland conditions. Palaeosol development and maturity are useful aids to stratigraphic correlation and as an indication of the proximity of reservoir channel sandstones. With improved prediction of the sandbodies’location, better models of reservoir distribution can be made leading to an enhanced field development plan

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.

Optimizing 3-D seismic technologies to accelerate field development in the Berkine Basin, Algeria

Abstract A radical approach to 3-D acquisition in country with large sand dunes was developed to improve seismic data quality. A methodology was devised which allowed accurate removal of near-surface statics and transmission effects giving enhanced depth prognosis. An adaptive noise-attenuation technique, based upon an image-processing procedure, was developed to improve the signal-to-noise ratio of 3-D data that were contaminated with ‘acquisition footprint’. Finally, parallel survey geometries were devised that reduced the noise contamination of the next generation of 3-D surveys in the Berkine Basin. Most of the 3-D surveys acquired in the Berkine Basin are now designed with parallel templates rather than cross-spreads. This survey technique has been shown to provide better spatially sampled data, despite the interference caused by the huge sand dunes, which, in turn, respond more positively to standard pre-stack noise-attenuation processing. Attribute and coherency analysis, as well as stratigraphic interpretation, is now possible on these datasets with a much greater degree of confidence.

Frasnian organic-rich shales in North Africa: regional distribution and depositional model

Abstract During the Frasnian, organic-rich shales were deposited across much of North African, most notably in parts of Morocco, Algeria, southern Tunisia, western Libya and the Western Desert of Egypt. They are estimated to be the origin of about 10% of all Palaeozoic-sourced hydrocarbons in North Africa. The depositional, palaeoecological and geochemical characteristics of this black shale unit can be best studied in the eastern Algerian Berkine (i.e. western Ghadames) Basin where the thickest and organically richest ‘hot shales’ occur. In wireline logs, the Frasnian hot shales are marked by high gamma-ray values, often in excess of 300–400 API, which, according to gamma-ray spectrometry, almost exclusively originate from an elevated uranium content. Comparison with total organic carbon (TOC) data shows that the gamma-ray curve can be used as a proxy for the TOC content of the Frasnian shales, with 150 API correlating approximately with TOCs of about 3% in eastern Algeria. The hot shale unit usually consists of high-frequency, high-amplitude, metre-scale gammaray cycles; however, especially in the thicker hot shale units, the lower frequency envelope curve of the high-frequency gamma-ray cycles has a gradual, bell-shaped form. The gradual increase and subsequent decrease in organic richness over time may be interpreted as evidence for a gradual rise and subsequent fall of the oxygen minimum zone (OMZ), with invasion of oxygen-depleted waters onto the North African shelf. The rise of the OMZ may have been triggered by the Early Frasnian transgression, which has been described in detail from Morocco, where it is now well-dated by conodonts and is associated with characteristic black shales and carbonates. Additional high-resolution biostratigraphic data are still needed in order to better correlate the Frasnian hot shales of Algeria, Tunisia and western Libya with other Late Devonian dysaerobic/anaerobic facies in Morocco, western Egypt, Europe, South and North America.

Diagenesis, provenance and reservoir quality of Triassic TAGI sandstones from Ourhoud field, Berkine (Ghadames) Basin, Algeria

The Triassic TAGI (Trias Argilo-Gréseux Inférieur) fluvial sandstones are the main oil reservoirs in the Berkine Basin, Algeria. Nonetheless, their provenance and diagenesis, and their impact on reservoir quality, are virtually unknown. Samples from the Ourhoud field, representing the Lower, Middle and Upper TAGI subunits, were studied using a combination of petrographic, mineralogical and geochemical techniques. The Lower TAGI sandstones have an average framework composition of Q98.3F0.6R1.1 and 95% of the quartz grains are monocrystalline. By contrast, the Middle–Upper TAGI sandstones have an average framework composition of Q88.3F9.8R1.9 and 79.7% of the quartz grains are monocrystalline. The Lower TAGI quartz arenites derived from Paleozoic siliciclastic rocks, whereas the Middle–Upper TAGI subarkoses originated mainly from metamorphic terrains. This change in provenance is a potential criterion for correlation within the TAGI. Also, this change has contributed to the significantly different diagenetic paths followed by the Lower TAGI quartz arenites and the Middle–Upper TAGI subarkoses.Grain-coating illitic clays are abundant in the Lower TAGI, where they exert a critical control on reservoir quality. These clays are interpreted as pedogenic and/or infiltrated in origin and to have had, in part, smectitic precursors. Shallow burial Fe-dolomite cementation was favored in the downthrown block of the field-bounding fault, where it contributed to the poor reservoir quality. Magnesite–siderite cements are multiphase. The earliest generation is composed of Fe-rich magnesite that precipitated during shallow burial from hypersaline fluids with high Mg/Ca ratios, probably refluxed residual brines associated with the Liassic evaporites. Later magnesite–siderite generations precipitated during deeper burial from waters with progressively higher Fe/Mg ratios. Authigenic vermicular kaolin largely consists of dickite that replaced previously formed kaolinite. Dickitization was followed by late-stage illitization related to the dissolution of detrital and authigenic K-feldspar. Quartz, the most abundant cement, was mainly sourced by the pressure- or clay-induced dissolution of detrital quartz and is a critical factor controlling the reservoir quality. Overall, quartz cement is more abundant in the Lower TAGI than in the Middle–Upper TAGI, and this increase correlates with a decrease in average porosity. Within the Lower TAGI, quartz cement abundance is stratigraphically very variable, which is in part related to facies controlled variations in grain-coating clay, resulting in major vertical variations in reservoir quality. Anhydrite and barite cements postdate quartz overgrowth. The sulfate necessary for their formation was likely sourced by deep subsurface dissolution of Late Triassic–Liassic evaporites.

Sequence stratigraphy and sedimentology of the late Triassic TAG-I (Blocks 401/402, Berkine Basin, Algeria)

The Berkine Basin is an intra- or pericratonic basin that developed during the Middle to Late Triassic on the margin of the Saharan platform. The basin lies to the east of the north–south trending Hassi Messaoud Ridge which separates it from the Oued Mya Basin to the west. These Algerian basins lie to the south and east of the network of rift basins that developed in the Iberian peninsular and along the margins of the North Atlantic seaboard. The principal hydrocarbon reservoir is the Upper Triassic Argilo-Gréseux Inférieur (TAG-I) which, in Blocks 401a and 402a, is Carnian to Norian in age. Elsewhere in North Africa, especially to the south and east, there are older Triassic formations. For example, in Zarzaitine in the extreme south east of the Triassic Algerian outcrop, Anisian vertebrates are documented. The TAG-I sits unconformably on Palaeozoic basement rocks and with the basal Lower Carbonate comprises a laterally and vertically variable sequence which has been sub-divided into four depositional sequences: Sequence 1, an unconformity-bounded, ephemeral fluvial interval that fills palaeorelief on the Hercynian unconformity; Sequence 2, an initially upward-fining, and subsequently upward-coarsening package of perennial fluvial sandstones and floodbasin shales with thin crevasse splay elements and interfluve palaeosols; Sequence 3, an erosively based, fluvio-lacustrine section characterised by fluvial sandstones with associated crevasse sandstones and floodbasin/lacustrine shales. This sequence is the main hydrocarbon reservoir section and is divided into two main packages 3A and 3B; the base of 3B is distinguished by basin-wide fluvial incision and the widespread channel sand deposition; Sequence 4 (Lower Carbonate) is a coastal plain and shallow marine system comprising shales, sabkha-type evaporites and bay-fill sandstones. The four depositional sequences reflect differences in depositional style resulting from base level shifts, tectonics and climate throughout TAG-I times. The overall increase in relative sea level was interrupted by periods of incision, which may relate to periods of rifting and erosion of the rift shoulders of the Berkine Basin. The initial valley fill (Sequence 1) was deposited under relatively arid or semi-arid conditions. During Sequences 2 and 3A, perennial fluvial systems with anastomosed channels and floodplain lakes became dominant and the climate increasingly humid. At this time a major longitudinal drainage, divide developed due to intrabasinal rifting. The basin was ultimately flooded by the transgressive systems tract of Sequence 4.

Adapting to noisy 3-D data: Enhancing Algerian giant field development through strategic planning of 3-D seismic in Berkine Basin

In 1995, after several successful exploration wells, Anadarko Algeria Corporation and partners (Sonatrach, Lasmo Oil Algeria, and Maersk Olie Algeriet) sought to accelerate development in Berkine Basin (Figure 1) where they had rights to five million acres.

Petroleum generation and accumulation in the Berkine basin, eastern Algeria

The Berkine (Ghadames) basin is regarded as an important hydrocarbon play in eastern Algeria. An evaluation of organic source facies, the application of petroleum geochemistry, and an extensive basin modeling study were carried out to improve the understanding of the petroleum charge of the basin. Geochemical data indicate that the Llandoverian-Wenlockian and the Frasnian hot shales are the most important oil-prone source rocks for the known hydrocarbon accumulations in the basin. Maturity data reflect the Late Cretaceous-Tertiary burial and are consistent with a 60-63 mW/m2 heat flow in the vicinity of the anticlines and 50 mW/m2 heat flow farther away from the folded area. A two-dimensional basin modeling study revealed the importance of poorly constrained parameters, such as the amount and rate of Hercynian erosion, on hydrocarbon generation and trapping efficiencies. The model shows that a Cretaceous-Tertiary petroleum generation and emplacement for both source rocks can better explain the different gas and oil fields than can an earlier, Hercynian event. The faults related to the Triassic-Jurassic rifting are most certainly permeable and can be considered as major migration pathways for petroleum. In contrast, the overpressure that developed along the El Biod-Hassi Messaoud axis in the Ordovician reservoirs was probably caused by the low permeability of the faults bounding the anticlines that therefore prohibited any lateral or vertical fluid movements. The model also indicates two petroleum systems that have Triassic reservoirs: (1) a Frasnian-sourced system that is confined to the central part of the basin and (2) a Llandoverian-Wenlockian-sourced system in the western part of the basin. Migration into Paleozoic reservoirs was followed by dismigration into overlying Triassic traps after the Cretaceous compressive movements.

ABSTRACT: 2D Modeling of Fluid Flow and Destruction Versus Conservation of Hydrocarbon in the Berkine Basin, Algeria

ABSTRACT: 2D Modeling of Fluid Flow and Destruction Versus Conservation of Hydrocarbon in the Berkine Basin, Algeria

Abstract: Characterization of North African Fluvial Reservoirs, Berkine Basin, Algeria: Applications of New Technology to the Reservoir Description Process

Abstract: Characterization of North African Fluvial Reservoirs, Berkine Basin, Algeria: Applications of New Technology to the Reservoir Description Process

Abstract: Climatically controlled stratigraphic architecture of the Trias Argilo Greseux Inferior (TAGI) of the Berkine Basin, North Africa

Abstract: Climatically controlled stratigraphic architecture of the Trias Argilo Greseux Inferior (TAGI) of the Berkine Basin, North Africa

Abstract: Sedimentology and Reservoir Characterisation of the Hassi Berkine South (HBNS) Field Block 404, Berkine Basin, Algeria

Abstract: Sedimentology and Reservoir Characterisation of the Hassi Berkine South (HBNS) Field Block 404, Berkine Basin, Algeria

Abstract: The Structural Evolution and Reservoir Architecture of the HBNS Field from  3D Seismic, Berkine Basin, Algeria

Abstract: The Structural Evolution and Reservoir Architecture of the HBNS Field from  3D Seismic, Berkine Basin, Algeria

Abstract: Advances in Stochastic Modeling Techniques Applied to Reservoir Characterization of the Fluvial TAGI Sandstones, Berkine Basin, Algeria 

Abstract: Advances in Stochastic Modeling Techniques Applied to Reservoir Characterization of the Fluvial TAGI Sandstones, Berkine Basin, Algeria 

Abstract: Semi-Arid Lacustrine Cycles, a Controlling Mechanism for Triassic Reservoir Geometries and Characteristics, Berkine Basin, Algeria

Abstract: Semi-Arid Lacustrine Cycles, a Controlling Mechanism for Triassic Reservoir Geometries and Characteristics, Berkine Basin, Algeria

Missing response data: To impute or not a impute?: J.M. Boswick, K.L. Lee, R.M. Califf, E.J. Topol, and the TAMI study Group Duke University, Durham, North Carolina(67)

The “red beds” of the Triassic succession outcropping at Tejra-Medenine (southern Tunisia, Saharan Platform) have yielded rich fossil assemblages of both freshwater and brackish-marine invertebrates and vertebrates. The new discovered fauna indicates an Anisian-Lower Ladinian age for the Tejra section. Its lowermost part is considered as equivalent of Ouled Chebbi Formation, while the medium and upper parts are considered as equivalent of the Kirchaou Formation. Both sedimentological characteristics and fossil assemblages indicate the increasing marine influences within the middle part of the section and the migration of brackish and freshwater fauna into the lacustrine/playa environment at the top. The marine fauna-rich interval of the Tejra section correlates well with the well-known Myophoria-rich carbonate stratigraphic marker confirming the Middle Triassic (Ladinian) major transgression well recorded eastward in the Tunisian Jeffara basin and in Libya. The use of this Ladinian stratigraphic event in addition to the Carnian carbonate transgressive events of the Jeffara escarpment outcrops was of great help for regional lithostratigraphic correlations between the Triassic outcropping series and those currently buried in Ghadames and Berkine basins. The age of the sandstones of “Trias Argilo-Gréseux Inférieur” (TAGI) which forms the main oil and gas reservoir in the Saharan domain is attributed to the Anisian-Carnian and considered as coeval of Ouled Chebbi and Kirchaou Formations of the Dahar escarpment. An updated synthetic stratigraphic chart is proposed for the Triassic of Saharan Platform domain on the basis of the compilation of the new obtained results and the subsurface data taken from published literature.

The Lower Paleozoic succession in the Tassili outcrops, Algeria: sedimentology and sequence stratigraphy

The EAGE Conference held in Algiers in April 2005 was the opportunity for 23 attendees to participate in a post-conference field-trip organized in the Tassili N’Ajjer outcrops in southern Algeria. The stratigraphic succession from the infra-Cambrian to the Middle Devonian is exposed in these exceptional outcrops, desert conditions favouring continuous exposure. The field trip started in the city of Djanet in the Infracambrian granites and shales and ended five days later in Illizi in the Lower Devonian Emsian sandstones (Figure 1). During the trip, these outcrops were described in terms of sedimentology, fracturation, and sequence stratigraphy. The outcrops of this area are of particular interest for petroleum geologists as the same sedimentary succession produces hydrocarbons in the nearby Illizi and Berkine basins and in the Hassi- Messaoud area. The Cambrian, Ordovician, and Devonian sandstones observed in the Tassili outcrops have an equivalent in the reservoir units in the subsurface. The Silurian hot shales are also the prolific source rock of the petroleum system in these basins. The succession presented in the outcrops can then be directly compared with the subsurface data of the Illizi and Berkine basins. The area was first surveyed by Killian (1922) who described the sedimentary succession of the region. Then, the lithostratigraphy and the Siluro-Devonian succession in the outcrops were established by Dubois et al. (1967). The reference study of the Lower Paleozoic outcrops in Algeria was published by Beuf et al. (1971). These authors proposed a lithostratigraphic nomenclature for the Lower Paleozoic and a sedimentological description of the formations. They also identified and interpreted the main unconformities in the Lower Paleozoic with a modern approach, discussing the relative role of tectonics and climate during sedimentation. A detailed mapping of the area was also performed by Sonatrach and Beicip under the supervision of the Ministere de l’Industrie of Algeria. However, since these pioneer studies, little has been published about the outcrop geology although hydrocarbon exploration and development has been very active in the nearby Illizi and Berkine basins. The objective of this paper is to propose a reference section for the Lower Paleozoic in the western Tassili with an updated sedimentological interpretation. An interpretation in terms of sequence stratigraphy will also be proposed. We then summarize the lithostratigraphy in use in the outcrops and propose a correspondence with the sub-surface.