Jonathan Redfern

Petroleum generation and migration in the Ghadames Basin, north Africa: A two-dimensional basin-modeling study

The Ghadames Basin contains important oil- and gas-producing reservoirs distributed across Algeria, Tunisia, and Libya. Regional two-dimensional (2-D) modeling, using data from more than 30 wells, has been undertaken to assess the timing and distribution of hydrocarbon generation in the basin. Four potential petroleum systems have been identified: (1) a Middle–Upper Devonian (Frasnian) and Triassic (Triassic Argilo Greseux Inferieur [TAG-I]) system in the central-western basin; (2) a Lower Silurian (Tannezuft) and Triassic (TAG-I) system to the far west; (3) a Lower Silurian (Tannezuft) and Upper Silurian (Acacus) system in the eastern and northeastern margins; and (4) a Lower Silurian (Tanezzuft) and Middle–Upper Devonian (Frasnian) system to the east-southeast. The Lower Silurian Tanezzuft source rock underwent two main phases of hydrocarbon generation. The first phase occurred during the Carboniferous, and the second started during the Cretaceous, generating most hydrocarbons in the eastern (Libyan) basin. The Frasnian shales underwent an initial, minor generative phase in the central depression during the Carboniferous. However, the main generation occurred during the Late Jurassic–Cenozoic in the western and central depression. The Frasnian shales are currently only marginally mature in the eastern part of the basin. Modeling indicates that the Alpine (Eocene) exhumation of the eastern (Libyan) basin margin had a significant control on the timing of hydrocarbon generation from the Lower Silurian source rock. The preferred burial-history model calibrates source rock maturity data by incorporating late exhumation and reduced subsidence prior to the Hercynian (Carboniferous) orogeny. As a result, the Tannezuft shales preserve their generative potential into the Mesozoic–Cenozoic, with renewed hydrocarbon generation during subsequent reburial, which can migrate to post-Hercynian (Carboniferous) traps, hence favoring the preservation of hydrocarbon accumulations.

The importance of constraining regional exhumation in basin modelling: a hydrocarbon maturation history of the Ghadames Basin, North Africa

Estimation of eroded overburden at unconformities is essential to accurately constrain burial histories and predict the timing of hydrocarbon maturation. In the Ghadames Basin, three independent techniques, palaeo-isopach construction, sonic velocity and vitrinite reflectance analysis, were employed. The resultant basin model suggests that only the two most significant unconformities, the Hercynian (Late Carboniferous) and Alpine (early Eocene), have a major control on timing of hydrocarbon charge. Modelling indicates only one period of generation from the Lower Silurian shales on the western margin of the basin, with 95% hydrocarbon generation prior to Hercynian exhumation. The central basin and southern margin experienced maximum burial during Eocene time. The Middle–Upper Devonian mudstones are the main source rocks and they did not generate significant volumes of hydrocarbons over the basin centre until Cretaceous time; they are currently within the peak oil generation zone. In Libya, on the eastern/northeastern flank of the basin, results indicate Cenozoic maximum burial, followed by up to 2000 m of Alpine exhumation. The magnitude of this exhumation has not been recognized previously, although it is a critical component of the basin model as it has a major impact on potential hydrocarbon charge in this area. Maturation models predict that the Lower Silurian source underwent two generative phases: (1) pre-Hercynian (Carboniferous) generation; and, significantly, (2) post-Hercynian (Late Jurassic–Cenozoic) generation. The identification of late hydrocarbon generation offers potential for oil and gas to migrate into post-Hercynian traps. Over the western, northern and eastern flanks, the Devonian source rocks remain immature/ marginally mature at present day.

Glaciogenic Reservoirs and Hydrocarbon Systems

Glaciogenic reservoirs and hydrocarbon systems occur intermittently throughout the stratigraphic record, with particular prominence in Neoproterozoic, Late Ordovician, Permo-Carboniferous and Late Cenozoic strata. Recent interest in glaciogenic successions has been fuelled by hydrocarbon discoveries in ancient glaciogenic reservoirs in North Africa, the Middle East, Australia and South America. Glaciogenic deposits of Pleistocene age are noteworthy for their content of groundwater onshore and potentially prospective and/or hazardous gas accumulations offshore. The abundant imprints of Pleistocene glaciations in both hemispheres can be used to reconstruct complex histories of repeated ice cover and retreat, and glacier-bed interactions, thus informing our view on the dynamics of older ice caps and predictions of future glaciations. This volume aims to provide a better understanding of glaciogenic processes, their stratigraphic record and reservoir characteristics of glaciogenic deposits. The book comprises 3 overview papers and 16 original case studies of Neoproterozoic to Pleistocene successions on 6 continents and will be of interest to sedimentologists, glaciologists, geophysicists, hydrologists and petroleum geologists alike.

Integration of digital outcrop models (DOMs) and high resolution sedimentology – workflow and implications for geological modelling: Oukaimeden Sandstone Formation, High Atlas (Morocco)

ABSTRACT Outcrop analogue studies provide key information for reservoir modelling which is difficult to obtain from traditional subsurface datasets (i.e. seismic data, well data). Terrestrial laser scanners or LiDAR (light detection and ranging), combined with digital photography, provide a new technique to create high resolution 3D digital outcrop models (DOMs). These DOMs generate exhaustive information which is used to build more realistic three-dimensional facies-based geocellular models and populate the different model zones. This paper documents the use of an extensive dataset, which combines high resolution traditional field data and DOMs. We provide an accurate description of the workflow followed in the geocellular modelling of a fluvial-dominated continental formation. Geocellular facies association and connectivity models are well constrained by outcrop observations and a number of different techniques are used in the quality control of the final model. The study also qualitatively discusses the uncertainties identified during the workflow and proposes methods to reduce them. The workflow and results shown in this paper can be applied in similar analogue systems in order to help improve model building for subsurface reservoirs.

Glaciogenic reservoirs and hydrocarbon systems: an introduction

Abstract Glaciogenic reservoirs host important hydrocarbon and groundwater resources across the globe. Their complexity and importance for exploration and palaeoclimate reconstruction have made glaciogenic successions popular subjects for study. In this paper we provide an overview of the palaeoclimatic and tectonic setting for Earth glaciation and a chronological account of glaciogenic deposits since c. 750 Ma, with particular emphasis on their reservoir potential and associated hydrocarbon systems. Hydrocarbon accumulations within glaciogenic reservoirs occur principally in Palaeozoic (Late Ordovician and Permo-Carboniferous) sandstones in South America, Australia, North Africa and the Middle East, with relatively minor occurrences of shallow gas hosted in Pleistocene deposits in the North Sea and Canada. Groundwater reserves occur within glaciogenic sandstones across the northern European lowland and in North America. The main glaciogenic environments range from subglacial to glacier front to proglacial and deglacial. Rapidly changing environments, hydrodynamic regimes and glacier-front and subglacial deformation often result in very complex glaciogenic sequences with significant challenges for reconstruction of their origin and resource importance, which this volume seeks to address.

LiDAR-based digital outcrops for sedimentological analysis: workflows and techniques

Abstract Recent developments in workflows and techniques for the integration and analysis of terrestrial LiDAR (Light Detection And Ranging) and conventional outcrop datasets are demonstrated through three case studies. The first study shows the power of three-dimensional (3D) data visualization, in association with an innovative surface-modelling technique, for establishing large-scale 3D stratigraphical frameworks. The second presents an approach to derive reliable geometrical data on sediment-body geometries, whereas the third presents a new technique to quantify the proportions, distributions and variability of sedimentary facies directly from outcrop. In combination, these techniques provide essential conditioning data for geocellular and stochastic facies modelling. Built upon robust, reproducible and quantitative data, the resultant models combine realistic 3D geological architectures with sufficient quantities of reliable numerical data required for stable statistical analysis and establishing uncertainty. Together this new information provides detailed understanding and quantification of the 3D complexity of the sedimentary systems in question, thus offering insights of value for predicting the subsurface anatomy of analogous petroleum systems. As such, use of LiDAR, when combined with conventional field geology, offers a powerful tool for quantitative outcrop analysis, tightly constraining 3D structural and stratigraphical interpretations, and effectively increasing the statistical significance of outcrop analogues for reservoir characterization.

An integrated study of Permo-Triassic basins along the North Atlantic passive margin: implication for future exploration

Permo-Triassic rift basins offer important hydrocarbon targets along the Atlantic margins. Their fill is dominated by continental red beds, comprising braided fluvial, alluvial fan, aeolian, floodplain and lacustrine facies. These relatively lightly explored basins span both the Atlantic and Tethyan domains and developed above a complex basement with inherited structural fabrics. Sparse data in offshore regions constrain understanding of depositional geometries and sedimentary architecture, further impeded by their deep burial beneath younger strata, combined with the effects of later deformation during continental breakup. This paper provides results from a multidisciplinary analysis of basins along the Atlantic margin. Regional seismic and well data, combined with geochemical provenance analysis from the European North Atlantic margins, are integrated with detailed outcrop studies in Morocco and Nova Scotia. The research provides new insights into regional basin tectonostratigraphic evolution, sediment fill, and reservoir distribution, architecture and quality at a range of scales. Regional seismic profiles, supported by key well data, indicate the presence of post-orogenic collapse basins, focused narrow rifts and low-magnitude multiple extensional depocentres. Significantly, Permo-Triassic basin geometries are different and more varied than the overlying Jurassic and younger basins. Provenance analysis using Pb isotopic composition of detrital K-feldspar yields new and robust controls on the sediment dispersal patterns of Triassic sandstones in the NE Atlantic margin. The evolving sedimentary architecture is characterized by detailed sedimentological studies of key outcrops of age equivalent Permian-Triassic rifts in Morocco and Nova Scotia. The interplay of tectonics and climate is observed to influence sedimentation, which has significant implications for reservoir distribution in analogue basins. New digital outcrop techniques are providing improved reservoir models, and identification of key marker horizons and sequence boundaries offers a potential subsurface correlation tool. Future work will address source and seal distribution within the potentially petroliferous basins.

Geochemical evaluation of East Sirte Basin (Libya) petroleum systems and oil provenance.

Abstract With cumulative reserves exceeding 23 gigabarrels oil recoverable (GBOR), the East Sirte Basin is a prolific oil province hosting supergiants such as the Amal, Augila-Nafoora and Sarir fields. Production from Precambrian-Oligocene reservoirs yields low sulphur and often highly waxy oils. The Late Mesozoic-Cenozoic Agedabia and older Hameimat, Maragh and Sarir troughs provide the main structural features of the habitat and control hydrocarbon prospectivity. Paleogene subsidence has facilitated the generative process with Mesozoic basin-fill sediments hosting source rocks for productive petroleum system(s). Traditionally the marine Upper Cretaceous Sirte Shale Formation source was thought to provide the dominant charge. Application of geochemical inversion procedures to oil data, however, indicates a greater diversity in oil provenance. Delineation of eight end-member generic oil families indicates a number of complex contributory petroleum systems, mixed-system hybrid oils also being evident. Non-marine (lacustrine) source inputs are also in evidence, enhanced waxiness differentiating petroleums of such provenance. Systematic screening of the stratigraphic section has additionally identified source potential in Nubian (Triassic and Lower Cretaceous), Rachmat-Tagrifet (Upper Cretaceous), Harash (Paleocene) and Eocene formations. Assignment of oil provenance has been achieved via multivariate oil data analysis and application of a carbon isotope-based source kerogen-oil correlation procedure. End-member petroleum systems have been definitively identified involving the Sirte Shale Formation, Rachmat-Tagrifet Formations and Nubian (Triassic) as the contributory sources. The remaining major systems rely upon Pre-Upper Cretaceous lacustrine sediments specific to the Hameimat and Sarir troughs. Whereas numerous archetypal Sirte Shale Formation oils were recognized (e.g. Messla, Hamid, Sarir-L etc.), reserves for many of the giant fields, including Amal, Augila-Nafoora and Sarir-C, rely on hybrid system charging. These results confirm that the prospectivity of the Sirte Basin is not exclusively dependent upon the Sirte Shale Formation, with other petroleum systems in operation, often involving hybrid-sourcing.

Constraining burial history and petroleum charge in exhumed basins: New insights from the Illizi Basin, Algeria

Intracratonic sag basins commonly have relatively simple tectonic histories; however, later tectonic activity involving exhumation can make reconstructing the burial history a challenging task. This is important because the relative timing of hydrocarbon generation and trap formation can be a key factor in risk assessment. If trap formation postdates peak hydrocarbon generation, exploration plays are typically downgraded. Mechanisms for charge in such exhumed basins are critical factors for understanding exploration risk. This study uses data collected from an Ordovician gas-condensate field in the Illizi Basin of Algeria to document the charging of a trap formed, or modified, during exhumation of the basin following maximum burial. Integrated analysis of sonic compaction data, thermal history indicators, and stratigraphic well data was used to constrain the burial and thermal history of the region. Hydrocarbon generation in the lower Silurian source rock is interpreted to have occurred during the Carboniferous (prior to Hercynian exhumation) and during the Late Cretaceous–early Eocene maximum burial (prior to Eocene exhumation). Structural reconstructions indicate that the field was initially located on the southern flank of a long-lived, intrabasinal, Paleozoic paleohigh. The large, low-relief structural closure that defines the present-day accumulation formed as a result of northward tilting of the Illizi Basin during Eocene uplift of the Hoggar massif. The study demonstrates that the timing of trap formation at the Ordovician field postdates the main local hydrocarbon generation events within the basin, suggesting that alternative hydrocarbon charge mechanisms are required. This study indicates considerable potential to charge updip traps on the flanks of exhumed petroliferous basins via redistribution of the preexisting hydrocarbons within the basin.

Advances in the study of naturally fractured hydrocarbon reservoirs: a broad integrated interdisciplinary applied topic

Abstract Naturally fractured reservoirs, within which porosity, permeability pathways and/or impermeable barriers formed by the fracture network interact with those of the host rock matrix to influence fluid flow and storage, can occur in sedimentary, igneous and metamorphic rocks. These reservoirs constitute a substantial percentage of remaining hydrocarbon resources; they create exploration targets in otherwise impermeable rocks, including under-explored crystalline basement, and they can be used as geological stores for anthropogenic carbon dioxide. Their complex fluid flow behaviour during production has traditionally proved difficult to predict, causing a large degree of uncertainty in reservoir development. The applied study of naturally fractured reservoirs seeks to constrain this uncertainty and maximize production by developing new understanding, and is necessarily a broad, integrated, interdisciplinary topic. Some of the methods, challenges and advances in characterizing the interplay of rock matrix and fracture networks relevant to fluid flow and hydrocarbon recovery are reviewed and discussed via the contributions in this volume.