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Tatyana Kiryanova Glacial Deposits in... the Sahara Desert Natalya Kuznetsova

Geological studies of hydrocarbon reservoirs

This paper discusses an experience of building conceptual model and studies of former Later Ordovician glacial deposits in the territory of present-day North Africa. Studies and modelling of complex reservoirs of this type is based on integrated analysis of core and well log data, drilling wells, and 3D seismics. Keywords: glacial deposits, troughs, intrusions, sedimentation model, seismic facies maps, proportional slices.

Tatiana Kiryanova is the Head of Department of Geological and Engineering intraclasts of black argillite. Solutions in the Ingenix Group. Her research interests include prediction of reservoir Most of the glacial section is properties on the basis of seismic data, and geological risks assessment. composed of periglacial deposits that Natalya Kuznetsova is a Geology Manager in the Ingenix Group. She is interested in developed during the periods of analysis of depositional environments, geological and hydrodynamic modelling, glacial retreat (melting) accompanied various methods of reserves and resources assessment. by formation of large amounts of meltwater. In accordance with core data, the following rock types that belong to various facies and alternate Taking in consideration a number of them are situated within the area of can be identified: 2 large oil and gas fields recently 3D survey (S = 400 km ). One of the ◊ Glaciofluvial outwash (Fig. 2) discovered in the territory of present- wells showed an oil inflow from the quartz sandstones. They are day Sahara, paleo-deposits of glacial Late Ordovician formations with the nongraded, contain textures of genesis attract a growing interest. rate of about 70 m3/day. The mega-ripples and current ripples, accumulated body of data that and faulted to different extents; Study area includes drilling data, well log, core ◊ Glaciolacustrine thin-bedded Late Ordovician was characterised data, 2D and 3D seismic data allows argillite and aleurolite with by formation of huge ice sheet over recommending a methodology stringers of fine grained the most part of Africa, Arabia, and suitable for studies of that kind of sandstones, drop-stones, and north-western South America (Fig. complex reservoirs. deformation textures. Cross- 1). Ice up to three kilometres thick bedded sandstone members that was covering the continent and at the Subsurface structure represent deposits of periglacial moment of maximum glaciation was Core studies in a number of wells deltas are observed (Fig. 3). overlapping shelf of northern revealed the major patterns of the ◊ Thin-rhythmic members of periphery of Gondwana [4]. subsurface structure. Glacial deposits medium-grained turbidite changing According to some reports, ice sheet superpose either the rocks of pre- upward into nongraded occupied an area even larger than the Cambrian basement, or the gradational-, cross-, and area of modern Antarctic glacier. terrigenous Cambrian-Ordovician horizontal-bedding sandstone and During glaciation, glacial valleys rocks. The fresher part of subjacent gravelstone with upward were carved; the valleys were filled section is represented by Late coarsening grain size. Single both by glacial morainic material and Ordovician shallow-marine deposits. Skolithos and numerous periglacial deposits related to glacier Overlying sequence of glacial and Trichichnus bioturbations are melting and subsequent marine water-glacial (periglacial) deposits present. The series represent a transgression. Periodic movement of which infills troughs, belongs to transition from prodelta to Gilbert- ice front resulted in structurally Hirnantian time of top Ashgillian [4] type delta front (Fig. 3). Formation complex sections with variegated or to the Unit IV of the local of Gilbert delta and occurrence of facies composition, which are rather lithostratigraphic column. bioturbation is indicative of difficult to explore. Thin and highly faulted, washed changing glaciolacustrine to fjord The study area is situated within the moraine is associated with glacial settings. territory of People's Democratic deposits; it is identified in the basal All the above-mentioned types of the Republic of Algeria. Operator part of the considered glacial series deposits form complex combinations Company is Russian-Algerian joint (Fig. 2), and is represented by that are characterized by members venture. By now, 14 wells are drilled nongraded conglomerate of pebbles, highly discontinuous along the in the field discovered in 2004; 9 of quartz, quartzite, gruss, and strike, which makes it difficult to

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km, a sharp change in thickness up to pattern. Only two horizons can be Figure 1 Glacier sheet on western Gondwana several tens of meters can be traced fairly confidently and Backstripping. Late Ordovician [6] observed [5]. definitely, they are: Ordovician top Interpretation of wave pattern basing and basement top. As a result, on the seismic data is extremely tracing the rest of the horizons was difficult. Presence of the dipping and performed in “paleo” mode almost sub-vertical boundaries of (levelling up to Ordovician top) on channels (troughs), their overlap, the basis of simultaneous analysis of similar acoustic properties of the amplitude cubes with various media within the same stratigraphic frequency spectra, instantaneous interval - everything mentioned phase cube and relative AI cube. causes problems with reflecting Only a team of seismic interpreter horizons correlation and and geologist working interactively identification (Fig. 4). Therefore, we allowed carrying out geological and used an integrated approach to seismic correlation of the considered achieve the objectives formulated for interval. Boundaries in the wells studies of architecture of Upper were updated on the basis of Ordovician Unit IV. reflectors behaviour, which in turn The most reliable method of analysis were edited with support of well of the target interval is a full-size markers confirmed by

core sedimentological description. sedimentological core description. correlate and identify them even Based on this information, the The conducted petrophysical within the first kilometres [5]. Such boundary surface between the units substantiation using well data have variability can be explained not only III and IV was determined. The not identified favourable by the facies changes but also by the results of well log data clustering prerequisites for quantitative presence of numerous hiatuses and (conducted using the Multi- prediction of the properties of unconformities [1]. Resolution Graph-Based Clustering reservoir under consideration. In the Up the section, the Silurian graptolite mathematical algorithm) were used interval of deposits of glacial shists (oil-source rocks) occur as additional information for tracing genesis, the elastic properties forming a distinct boundary with the geological boundaries. Sonic (DT), (velocities, densities, and acoustic subjacent formations. litho-density (Rhob, Ре), and impedance of reservoir and non- gamma-ray (GR) logs were an input reservoir rocks) completely overlap Glacial deposits: experience of data for clustering. throughout the entire range of their research Non-extension of seismic event axes, values. Consequently, the decision Experience of studying similar lateral variability, and amplitude was made to apply the following deposits in the other North African heterogeneity are typical of wave technologies of qualitative prediction fields suggests the extreme patterns in the target interval: (Fig. of reservoir properties: seismic facies complexity of the sequences of 4). In the study area, a rather dense analysis with the use of automated glacial origin. Even in the nearby fault network and numerous intrusive classification, and multi-attribute wells spaced at intervals of 1 to 2 bodies exist (Fig. 5); this fact analysis. As a result, several tens of significantly complicates the seismic sedimentary slices were analysed

Figure 2 Sedimentation model and typical cross-section of glacial and periglacial deposits of ice sheets

Source: [3] with changes after E.Yu.Baraboshkin Regional Geology August 2014 GEOLOGY AND GEOPHYSICS Accent of the issue

(proportional and parallel to the top Figure 3 of considered interval) over the Sedimentation model of proglacial lacustrine-fjord system numerous seismic cubes, and a number of seismic facies maps. One of them is shown in Figure 6. Efforts of the united multi-discipline team resulted in the conceptual geological model; the following features were identified: incised glacial valleys; water courses of outwash plains; lobes of deltiac system. Basing on the identified facies zonation, seismic data, well log data, core description, and information on regional geology, the interval under consideration was split into three main units. Source: [2] with changes after E.Yu.Baraboshkin Unit IV-1 was formed in the settings of glacial valleys (possibly, mixed in proximity to regional fault. Unit IV-3 was formed in coastal- terms of age) and outwash plains. Unit IV-2 is associated with the marine settings. Cross-bedded Ground moraine deposits are locally deposits of outwash watercourses sandstones, poorly sorted, with grain present in the basal part of the unit. and lacustrine system. The reservoir sizes ranging up to fine gravelstones, Reservoir rocks are represented by is penetrated by one well and represent the reservoir varieties. Due poorly sorted sandstones, represented by low-shale, almost to secondary alterations, the diagenetically affected, characterized pure sandstone with typical reservoir has an extremely low by low porosity (6— 10%) and porosity of 9%. In contrast to the porosity (5 to 7%) and actually inferior reservoir quality. However, fluvial systems with walker lacking pore connectivity. However, separate partings of better porosity riverbeds, primary sorting of the it is possible that zones of better and permeability are identified deposits is very poor, which reservoir quality, which have a within them. There are some reasons considerably deteriorates the genetic association with the deposits to believe that rather high oil reservoir quality. In our case, of Gilbert delta front, could be found flowrate (about 70 cu m per day) was regeneration cement has also in future. This kind of deposits often obtained from reservoirs with open contributed to porosity and roofs a glacial series in the fracture associated with the well permeability decrease. neighbouring areas.

Figure 4 Example of wave pattern in the study area (arbitrary section of instantaneous phase cube)

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Figure 5 structural model of Ordovician Products. International Association of Identification of intrusions as solid bodies paleo-glacial formations. Sedimentologists, Special Publications 39, on the results of spectral (frequency) decomposition Blackwell Publishing Ltd, p.295-319. References 5. Le Heron D. P., J. Craig J., J. L. Etienne J. L 1. Chumakov N.M. Studies of paleo-glacial «Ancient glaciations and hydrocarbon deposits. A practical guide. Geological accumulations in North Africa and the Institute, USSR Academy of Sciences, Middle East», Earth-Science Reviews 93 Moscow, 1990. (2009). 2. Boulton G. S. 1990. Sedimentary and sea 6.Le Heron D., Sutcliff 0., Bourgig K., Craig J., level changes during glacial cycles and their Visentin C, Whittington R. «Sedimentary control on glacimarine fades architecture. architecture of Upper Ordovician tunnel Dowdeswell J.A., Scourse J.D. (Eds.). valleys, Gargaf Arch, Libya: Implications for Glacimarine Environments: Processes and genesis of a hydrocarbon reservoir». GeoArabia, Vol.9, No2, 2004. Sediments. Geological Society, London, Special Publications 53, p.15-52. The authors express their appreciation 3. Einsele G. 2000. Sedimentary basins: Evolution, fades, and sediment budget. and gratitude to Eugeny Baraboshkin, Conclusions Springer-Verlag, Berlin, Heidelberg, New Professor of M.V. Lomonosov Moscow Complex architecture of former York, 792 p., 354 fig., tab. State University, for fruitful 4. Ghienne J.-F., Le Heron D. P., Moreau J., glacial formation of Unit IV cooperation, consultations and for introduces numerous uncertainties Denis M., Deynoux M. 2007. The Late Ordovician glacial sedimentary system of carrying out core description and in delineation of reservoir and the North Gondwana platform. Hambrey M. interpretation of sedimentation reserves assessment. J., Christoffersen P., Glasser N. F, Hubbard B. conditions in the field under The considered reservoir intervals (Eds.). Glacial Sedimentary Processes and consideration. IV-1, IV-2, and IV-3 typically have high vertical and lateral Figure 6 heterogeneity; the reservoir varieties Seismic facies map in the interval of glacial deposits have poor reservoir quality. In this stage of field exploration, it is impossible to identify the reliable criteria for qualitative prediction of reservoir zones with better quality in the interwell space. In this regard, development of accumulations within the reservoir being studied is considered today as economically inadvisable and risky. However, when new well data will be obtained in the course of appraisal and development of underlying shallow-marine terrigenous formations, it will be possible to create more detailed geological and

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