Depositional model and allostratigraphic architecture of Late Ordovician syn-glacial strata from the Tiguentourine Field (Illizi Basin, Algeria) By Stephane Roussé 2, Stig E. Sandvik 1, Bruno Murat 2, Alasdair Hutchinson 1, Kamel Saadi 1, Erwan Le Guerroué 2 1 BP/STATOIL/SONATRACH, In Amenas Joint Venture, BP 513, Hassi Messaoud 30500, Wilaya de Ourgla, Algerie 2 BEICIP-FRANLAB, E&P Consultancy, 232 Av. Napoléon Bonaparte, 92500 Rueil Malmaison, France. Corresponding autor:stephane.rousse@beicip.com Setting The Tiguentourine field (Illizi Basin, Algeria; Figure 1) holds gas condensate reserves within pre-glacial Cambrian sediments (fluvial to shallow marine), and syn-glacial late Ordovician sediments (C040 glaciogenic and C050 post-glacial shallow marine). At the scale of North-Gondwana platform, the Late Ordovician glacial episode led to an extensive icesheet growth. Ice-sheets development implied glacially-cut topographies (e.g. dug into pre-glacial substrate) ranging in scale from 100 km wide mega-valleys, to 10 km wide (palaeo-valleys and/or tunnel valleys). The lack of biostratigraphic constrains and the low resolution of the available seismic data set also strongly limits the layering of the syn-glacial reservoir. However the stratigraphy records a number of glacial erosion surfaces, sub-seismic in scale, strongly complicating the sedimentology of the syn-glacial deposits that bears the bulk of the gas reserves. In the light of a detailed sedimentological analysis performed on 22 cored wells, a depositional model of the syn-glacial succession was build. Allostratigraphic concepts have been applied to resolve part of the stratigraphic complexity and offer guide-lines for further geological modeling.
Subglacial topography in the Tiguentourine Field The isochore of the syn-glacial strata (Figure 2) underlined by well penetrations and seismic survey exhibits two elongate sub-parallel depositional thicks, trending approximately SE to NW and separated by a high (ridge). The troughs are inferred to be glacially-cut palaeo-valleys and filled by an overthickened CO40 unit reaching 250 m of thickness whereas the ridge exhibits a reduced syn-glacial series of less than a 100 m. However, it is to stress that isochore maps amalgamate multiple, stacked glacial cycles. Sedimentology of CO40 glacial deposits A glacial depositional sequence may be defined as a group of strata, bounded at top and base by two glacial erosion surfaces (GES) (Ghienne et al., 2008). As a consequence, a glacial depositional sequence (GS) is the record of a single glacial phase (e.g. a simple ice advance and subsequent retreat) where deposition mainly occurs during ice-sheet/ice-stream retreat. Based on cored intervals observations, an idealized suite of glaciogenic facies can be defined between two successive GES (Figure 3). Conceptually, this complete facies suite evolves from subglacial environments to pro-glacial and/or to peri-glacial settings.
The recognition of glacially-driven deformation processes below GES (Figure 3A) such as shear structures, folding, fracturing (hydraulically induced) as well as associated clastic material injections within fractured network, is critical for GES interpretation and subsequent GES recognition in cored wells. Above an identified GES, the typical glaciogenic, vertical facies suite may be described as follow: At base, glacial-tractional sandy deposits (40-FA0, Figure 4) may be found incased in pre-glacial strata, older glacial sequences, and/or in coeval sub-to pro-glacial diamictites (inferred therefore as tillite and/or proglacial diamicton). The glacio-tractional deposits are typically made of cross-bedded to massive gravel sandstones (Figure 3D) and are interpreted to be deposited within narrow elongated meltwater channels (anastomozed fluvio-glacial channels network). At the immediate subaqueous outlet of the subglacial drainage system, sustained high-density flow deposits (megaripples and turbidites) may be associated. These deposits testify for an ice-contact fan occurrence, constructed by jet efflux streams, resting as fan-shaped sand body. In absence of the above-described facies-association, and/or above it, a blanket of subglacial «till» deposits (40-FA1, Figures 3 & 4), which corresponds to breccia-conglomerate and/or clast-rich diamictites, may be found. Intense deformation due to glacial drag is generally associated within and below the tillite layer, attesting of sub-glacial stressed conditions.
Above and/or grading upward from the tillite, sub-glacial to pro-glacial subaqueous diamictons (40- FA2, Figures 3E & 4) are deposited. These facies types often consist of mud-prone thick successions and their deposition may be related to a wide range of depositional processes and associated deformation structures. Deformation is mainly represented by fractures, folding, and clastic material injections within hydraulically induced fracture network (Figure 3F&G). All these arguments attest of pressurized depositional environment and argue for a moraine deposition in immediate front of icebody (marine-terminating ice front or grounded line deposits). Immediately above the sub- to proximal pro-glacial succession, thin argillaceous heterolithic ice-distalshelf (mudstones and rainout diamictites, 40-FA4; Figure 3J) and/or distal outwash (40-FA3c; Figure 3K) sediment intervals may occur (Figure 4). This thin unit (few meters thick) is followed and/or interfingered and/or replaced by mass flow/debris flow deposits and coeval dewatered low to high density turbiditic sandstones (Figures 3H & 4). These facies represent infills of glacially cut topographies during early phase of ice-retreat (contemporaneous of ice-sheet instability) under flooddriven and gravitational sedimentary processes. On top, a general coarsening upward succession is present (Figure 4), implying high to low density turbidites at base, passing to high sustained turbiditic flows (40-FA3c; Figure 3I & L) and finally sharply overlain by clean mega-rippled sandstones (40-FA3ab, Figure 3M to O). This facies suite corresponds to the development of the inner part of a pro-glacial subaqueous to subaerial outwash plain system. It is characterized by an aggrading/prograding to purely aggrading stacking pattern. The megaripples complex is composed (as observed in Algerian outcrop analogs) of amalgamated low sinuosity anastomozed channels passing to sheets organized in extensive sandy lobes and sheets eventually ending distally/laterally to turbiditic sandstone sheets. Each above-described glacial depositional sequence (vertical suite of facies-types, Figure 4), may reach 30-100m in thickness, and corresponds to a basal fining-upward trend (e.g. retrograding suite of facies from sub-glacial facies to pro-glacial facies) followed by an overall coarsening upward succession (prograding suite of facies evolving from distal-outwash/ice-distal shelf, peri-glacial flooddominated turbidites, to pro-glacial sandy outwash fans (megaripples and sustained turbidites)). Depositional model for CO40 glaciogenic succession A conceptual model illustrating the stratigraphic development and the resulting architecture of a glacial depositional sequence may be proposed (Figure 5), highlighting 3 main phases of evolution. Glacial advance and glacial maximum phase: This phase is generally poorly recorded on the glaciated shelf, and corresponds mainly to the glacial erosion surface (GES) formation. Glacio-tectomites deposits such as subglacial tillites and deformed diamictites (40-FA1/40-FA2) are deposited at ice-substrate contact. Locally glaciotractional facies may be formed in sub-glacial channels (40-FA0) and/or at the mouth of subglacial drainage systems, forming discrete, so-called ice-contact fans. Ice-front recession: The initial ice-sheet retreat is often associated to a relatively thick glacio-marine succession (diamictite-dominated (40-FA2) outwash system at grounded line) deposited above a thin till, and/or blanketing the GES. Rapid retreat of marine terminating ice-front is associated to masswasting and flooding of the deglaciated area. High rate of ice front recession, coeval to collapse of the ice-sheet, prevents sandy outwash fan development. Depending on the local topography, part of the material deposited during this phase may also be sourced by ice-bodies, located on the marginal valley edges, depositing lateral morainal banks that can record large amount of diamicton and other poorly sorted debrites (40-FA2).
Ice-front stabilization: A subsequent decreasing in the rate of ice-sheet recession is inferred, coeval with the ice-front stabilization. This change in ice-dynamic enables the development and growth of voluminous proglacial sandy outwash systems (40-FA3) such as submarine fans or fan-deltas. Due to an increasing sediment supply and a concomitant decreasing rate of relative sea level rise (isostatic rebound uplift), the outwash fan system progrades, causing a rapid subsequent filling of the accommodation space. The outwash fan system development marks transitional conditions from pure marine terminating ice-front, to progressively more land-terminating ice-front. Eventually, in the case of a complete ice-sheet melting (e.g. no immediate ice re-advance), interglacial conditions prevail. They result in a high sea level, associated to transgressive erosion of former glaciogenic deposits and/or on previously subaerial exposed areas. Finally the deglaciated shelf progressively starves. As glacial depositional sequences are recording ice advances and retreats (driving accommodation space creation or destruction), they provide excellent potential for regional allostratigraphic correlations and could be used therefore as basic blocks for further 3D geomodel construction.
Early post-glacial transgressive interval (CO50 unit) CO40 glacigenic succession is capped by transgressive shallow marine shelf deposits belonging to the CO50 unit (Figure 6). Offshore marine dropstones-bearing mudstones (CO50.1) are draping glacigenic topographies and deposits. This mud-prone unit is rapidly followed by the deposition of progradational, bioturbated heterolithics sandstones and/or sharply overlain and/or replaced by clean coarse-grained cross-bedded sandstone (CO50.2). CO50 marine shelf to shoreline unit represents the transgressive remnants of the post-glacial Early Silurian flooding (comparative to above-described inter-glacial stage). CO50 facies succession is thought to be the record of the eustatic rise and isostatic rebound interplay towards the end of glaciation. The transgression peak was reached above, marked the deposition of the Silurian Hot Shales. Silurian shales (S10) Early Silurian high gamma Hot Shales, which sealed and sourced the gas horizons contained in the Cambrian and Upper Ordovician reservoirs, correspond to an abrupt deposition of anoxic, graptolite rich shales following early post-glacial CO50 shallow marine strata. The deposition of fines, blanketing the residual shallow marine topography suggests a rapid decrease of energy within the basin, certainly as the result of a global sea level rise triggered by the complete African polar ice cap melting. Syn-glacial and post-glacial stratigraphic architecture From facies analysis on cores and wireline electrofacies identification, syn-glacial (CO40) and postglacial (CO50) strata may be further subdivided respectively into Lower & Upper CO40 and CO50.1 & CO50.2. These divisions, which are thought to represent the 2 main glacial cycles recorded during the Late Ordovician Glaciation and the subsequent early post-glacial episode, have been defined, correlated, and mapped (Figure 6). CO40 glaciogenic strata are deposited within a complex palaeovalley-shaped topography mainly cut during the first and major glacial episode (Lower CO40). The Glacial Erosion Surface at the base of the Lower CO40 unit (GES-1) created multi parallel palaeovalley incisions (5 to 15 km wide, 60 to 120 m deep) deeply dug into pre-glacial units (Figure 6). Size and shape of such broad-scale incision may be attributed to tunnel valleys, and/or glacial troughs/ridges (mega-scale glacial lineations). During the subsequent early phase of retreat and/or during late glacial maximum phase, localized ice-contact fans associated to subglacial channels are developed at the base of the depressions. During glacial retreat and subsequent ice-front stabilization (e.g. Southwards of the studied area), outwash dominated sand-prone facies are partly filling remnants of topography. The Glacial Erosion Surface associated to Upper CO40 (GES-2) consists of a relative low relief erosion surface, that creates wide and shallow glacial ridges and troughs. Localized sub-glacial channel shapes (e.g. tunnel channel?) are evidenced. During the late phase of advance and/or early retreat thick sub-glacial to pro-glacial diamictite-prone units are developed testifying of stagnant icefront in the area. These mud-prone units form diamictite ridges inferred as subaqueous grounded-line morainal banks (Figure 6). Subsequent retreat of ice-front leads to the growth of a voluminous and correlative sandy outwash delta. The correlative thick (up to 60m thick) and widespread outwash delta sand body is regarded as a progradational (coarsening and cleaning upward) trending unit, deposited at the top of the first high frequency ice-advance cycle of Upper CO40. A last and minor inferred glacial re-advance may be recognized (xges, Figure 6) dissecting the above-mentioned proglacial outwash delta sand body. Subsequent flash retreat of ice-front left underfilled glacial topography, itself corrected and partly filled during early post-glacial times (CO50). CO50 unit records transgressive early post-glacial conditions. Influences of underfilled glacial topography and coeval isostatic rebound processes are the major control over its distribution (e.g. in terms of facies-type and thickness). Mud-prone deposition (CO50.1 unit) particularly overthickens in underfilled depressions (glacial troughs), whereas both isostatic process and local topographic influences (remnant palaeo-highs; glacial ridges), enhance erosion and reworking, that lead to shallow marine sand-prone shelf development (CO50.2 unit) in the more elevated areas (Figure 6).
Conclusions Applying allostratigraphic approach on Late Ordovician syn-glacial strata (CO40) of the the Tiguentourine Field (Illizi basin, Algeria) allowed the recognition and lateral correlations of glacial depositional sequences (GS) bounded by glacial erosion surfaces (GES). These sequences represent minor ice-sheet advance/retreat cycles, where deposition occurred exclusively during ice-sheet retreat and subsequent ice-front stabilization. Depositional environments are ranging from sub-glacial to proglacial and/or peri-glacial conditions. Stratigraphic correlations of the glacial depositional sequences allowed deciphering the sedimentary architecture (e.g. internal layering and sizes of sedimentary bodies) of syn- and post-glacial strata. Within the syn-glacial CO40 succession, two main glacial cycles are recognized and correlated across the Tiguentourine Field. Each cycle involves a complex stratigraphic architecture that reflects the complexity of glaciogenic depositional environments and superimposed minor higher-frequency ice advance/retreat. As a consequence, each cycle may be tentatively further subdivided into several iceadvance/retreat episodes (as obviously inferred for the Upper CO40 cycle). Immediately following the last glaciation, the CO50 post-glacial unit was deposited throughout the Tiguentourine field area. CO50 package includes transgressive shallow marine sediments deposited on a drowned glacially-cut topography ensuing isostatic rebound, suggesting definitive retreat of icesheet of the area, prior to the Silurian flooding. Established correlations indicate the development of heterogeneous reservoirs within glacial sequence ranging in two end-members: (1) discrete and highly variable sandy sub-glacial facies, (2) extensive and voluminous pro- to peri-glacial sandy outwash fan facies. The very locally developed transgressive sandstones, deposited during the post-glacial isostatic rebound, may be considered as an additional reservoir play.