Sedimentology of a “nonactualistic” Middle Ordovician tidal-influenced reservoir in the Murzuq Basin (Libya)

The subsurface of the highly productive Murzuq Basin in southwest Libya remains poorly understood. As a consequence, a need exists for detailed sedimentological studies of both the oil-prone Mamuniyat Formation and Hawaz Formation reservoirs in this area. Of particular interest in this case is the Middle Ordovician Hawaz Formation, interpreted as an excellent example of a “nonactualistic,” tidally influenced clastic reservoir that appears to extend hundreds of kilometers across much of the North African or Saharan craton. The Hawaz Formation comprises 15 characteristic lithofacies grouped into 7 correlatable facies associations distributed in broad and laterally extensive facies belts deposited in a shallow marine, intertidal to subtidal environment. Three main depositional sequences and their respective systems tracts have also been identified. On this basis, a genetic-based stratigraphic zonation scheme has been proposed as a tool to improve subsurface management of this reservoir unit. A nonactualistic sedimentary model is proposed in this work with new ideas presented for marginal to shallow marine depositional environments during the Middle Ordovician in the northern margin of Gondwana.

). However, since the mid-1990s, the Murzuq Basin has 4 developed into a major oil and gas producing province. The Hawaz Formation 5 constitutes one of the most important reservoirs in a number of producing fields 6 in the central and northern part of the basin. The generally high reservoir quality 7 (average 5-15% porosity and 0.1-150md permeability) and lateral continuity, 8 characteristic of the Hawaz are key factors in the development and production 9 of these accumulations. However, despite the well-documented potential of the 10 Hawaz Formation, its subsurface character remains poorly understood. 11 To date, only a few sedimentological studies of this formation have been carried 12 out and all are exclusively based on surface geology (Vos, 1981  actually possible to compare these paleoenvironments with any 'actualistic' 6 sedimentary model? 7 The limitations of the approach become apparent when the uniformitarian 8 principle is extended to depositional environments in the most ancient 9 geological record. In particular, the assumption that modern environments can 10 provide analogues for all geological successions must be questioned (Nichols, 11 2017). It is broadly accepted that earth dynamics have changed considerably 12 throughout geological history and accordingly, factors controlling sedimentation 13 have changed also, such as a lack of flora stabilizing river banks, greenhouse 14 vs icehouse periods defining coastal geomorphology, tidal ranges controlling 15 facies belts or characteristic ichnofacies during a particular period of geological 16 time. The analysis of some of these factors suggests that the facies succession 17 of the Hawaz Formation reflects rather different depositional processes from 18 those observed in modern environments. From this point forward we will use the 19 term 'non-actualistic' to describe those processes affecting the geological 20 signature of the Hawaz Formation which are difficult to compare with any 21 modern depositional environment analogue. 22 Consequently, the main aim of this article is to present a sedimentological Ordovician. In addition the overall analysis aims to build a genetically-3 based zonation through sequence stratigraphy which will improve reservoir 4 management and provide tools for maximizing hydrocarbon recovery efficiency. 5 Finally, it is intended that these sedimentological and stratigraphic models 6 should be a well-documented subsurface analogue for clastic reservoirs in 7 similar settings.  10 The structure and stratigraphy of the Murzuq Basin 11 The Paleozoic succession of the Murzuq Basin is an erosional remnant of a 12 much more extensive regional succession extending along the northern margin 13 of the Gondwana supercontinent (Davidson et al., 2000;Shalbak, 2015). Its 14 present extent reflects several periods of uplift and unroofing during the late 15 Paleozoic, Mesozoic and Cenozoic, which together are responsible for its 16 modern architecture. As a consequence, the present-day basin geometry bears 17 little relation to the broader and larger pre-existing sedimentary basin. The 18 current basin is composed of a central Cretaceous depression bounded to the 19 northwest by the Atshan arch, the Gargaf high to the north, and the Tibesti and 20 Tihemboka highs on the southeast and southwest, respectively (Figure 1). 21 These structural highs were formed by multiphase tectonic uplifts from the  17 can be subdivided into four main units: 1) Cambrian-Ordovician, 2) Silurian, 3) 18 Devonian-Carboniferous, and 4) Mesozoic (Figure 2). 19 The lower Paleozoic succession comprises the terrigenous Cambrian- 20 Ordovician Gargaf Group consisting of at least five formations -from bottom to  Exxon discoverd gas at Atshan region and Gulf tested oil at low rates from 18 Ordovician sandstones. However, in 1958, industry attention shifted east with 19 the discovery of a major oil accumulation in the Sirte Rift province and there 20 was little further exploration of the Murzuq Basin for the next 20 years. During 21 the late 1980s to 1990s, Rompetrol and later Repsol drilled up to 57 exploratory 22 wells in the basin, all of which targeted Ordovician prospects. This exploratory 23 activity resulted in many significant oil discoveries highlighting the rapidly 24 growing potential of the basin.

GEOLOGICAL SETTING
The most recent hydrocarbons-in-place estimation for the Murzuq Basin is 1 about 6 billion barrels (bbl) of oil and about 35 trillion cubic feet (TCF) of gas, 2 which represent about 6.5% of the Libya's resources and 30% of the Libya's 3 current oil production (Shalbak, 2015). 4 The main petroleum system in the Murzuq Basin comprises a basal Silurian 5 (Tanezzuft) hot-shale source rock, Ordovician sandstone reservoirs and a thick  The Ordovician sandstone reservoirs, associated with the primary petroleum  The Hawaz Formation In the subsurface of the northern Murzuq Basin, the Hawaz Formation is 1 represented by a detrital succession of slightly more than 200 m (650 ft) thick, 2 composed of fine-grained quartz arenites and subarkosic arenites, with 3 subordinate sublithic arenites, similar to the equivalent succession exposed on 4 the Gargaf High (Ramos et al., 2006). terms, nearshore to shoreface facies are dominated by dense 'pipe rock' fabric 10 formed by Skolithos and Siphonichnus. In contrast, storm-dominated heterolithic 11 facies are characterized by horizontal deposit feeding Cruziana bioturbation.   12 The present study was based on data from 36 wells located across the north 13 central sector of the Murzuq Basin ( Figure 3). This data included core 14 descriptions, high-resolution image logs (FMI), gamma-ray (GR), sonic (DT), 15 neutron porosity (NPHI) and density (RHOZ) wireline logs. The methodology 16 followed consisted of: 17 1) Well data synthesis and standardization from the 36 wells by means of 18 building well composite charts with the wireline logs available for each well.  These log profiles were then used to identify facies associations in wells 5 lacking core or FMI data.    Small to medium scale cross-bedded sandstones (Sx2) 17 Fine to medium-grained, well-sorted and cross-bedded sandstones   Parallel-laminated sandstones (Sl) 8 Fine-grained sandstones with parallel lamination (<5º) (Figure 4). Bioturbation 9 was not recognized ( Figure 4). Organized in sets 10 to 100 cm (4 to 39 in) thick.

10
It is interpreted to record sand deposition from nearshore currents under a 11 moderate to high-energy, upper flow regime. A similar lithofacies has been 12 described by Ramos et al. (2006) in outcrops as parallel-laminated sandstones 13 with occasional parting lineation and very scarce bioturbation.
14 Cross-laminated sandstones (Sxl) 15 Fine-grained sandstones with low-angle cross-lamination ( Figure 4). Climbing-16 ripple lamination and mud drapes are also occasionally present. In general, it is 17 a nonbioturbated lithofacies, although sparse Skolithos were occasionally 18 observed. Set thicknesses range from 10 to 140 cm (4 to 55 in). This lithofacies 19 is interpreted as the deposits of storm events in a nearshore environment.   direction, locally bimodal towards south-southeast. 13 Massive sandstones (Sv) 14 Fine-grained, clean, generally well sorted sandstones with poorly defined planar  were carried off in suspension. The cross-bedding records the migration of dune 11 and bar bedforms whereas the vertical to oblique burrows suggest a shallow, 12 high energy marine environment. 13 Burrowed cross-laminated sandstones (Sxlb) 14 Fine-grained, variably argillaceous and micaceous sandstones with low-angle 15 cross-lamination and local mud laminae and mudstone intraclasts. This 16 lithofacies is moderately bioturbated with an ichnofabric dominated by Skolithos 17 and Siphonichnus, indeterminate burrows and meniscate backfilled burrows 18 ( Figure 4). The minimum thickness observed of this lithofacies is 70 cm (28 in). 19 The moderately intense bioturbation, dominated by mainly vertical, suspension-20 feeding burrows suggests a shallow, high-energy subtidal environment. 21 However, the mud laminae also reflect low-energy conditions. Thus, depending

16
Burrowed sandstones with Siphonichnus (Sb) 17 Fine-grained well-sorted sandstones locally with mud laminae. This lithofacies is 18 highly bioturbated, with an ichnofauna dominated by Siphonichnus burrows, 19 locally up to 100 cm (39 in) in length, giving rise to a distinctive 'pipe rock'  Burrowed sandy heterolithics (HSb) 9 Thinly interbedded very fine-grained, micaceous, argillaceous sandstone and 10 micaceous, argillaceous siltstone (>50% sand content). Locally, the argillaceous  Burrowed muddy heterolithics (HMb) 18 Argillaceous siltstone interbedded with minor fine-grained sandstone layers and 19 sandstone laminae (>50% clay content). It is characterized by a variable degree  Facies associations 6 The proposed scheme based on the previously described lithofacies establishes          When core data was not available for several sections in the studied wells, 20 image log data was key to characterize the seven facies associations previously 21 mentioned (Figure 7).  However, the Earth has changed significantly through geological history. 6 Indeed, even from the early Paleozoic until present day, some processes and 'present' to be the 'key of the past' probably choosing the most recent 'present' 10 is not the best idea. 11 After careful study of the Hawaz Formation and the sedimentary processes 12 involved in its deposition, several significant concepts have been developed 13 which require further discussion in this respect (Table 2) (Table 2).

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The other remarkable aspect worthy of note is the effect of vegetation on the 6 generation of clay minerals (Table 2). Many Precambrian to Ordovician    to those produced during greenhouse periods, as much of the Cambrian- 6 Ordovician actually was. The relative sea level, during much of the 7 Ordovician (at least until the onset of the Hirnantian glaciation), was 8 probably tens of meters higher than at present time, which in the case study 9 would represent a very extensive area of land flooded, across a very low 10 relief cratonic margin (Table 2). Thus, confined estuary systems produced 11 by incised valleys during sea-level drop are not expected in this setting. This 12 discussion can be applied to the depositional model of the Hawaz 13 Formation. As such, classical estuarine environments are inherently unlikely.
14 Indeed, conventional lowstand systems tracts would be, in any case, 15 extremely difficult to identify, as major erosive features related to sea-level 16 drop would not be produced in this low gradient, cratonic transitional setting.

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3) It is also relevant to our study that tidal range has not been constant through 18 the whole of Earth's history. Tides are largely controlled by differential 19 gravitational forces exerted between the Earth and the Moon, but the  1 Some if not many or even all of the organisms responsible for these 2 ichnofabrics are already extinct (Table 2). Thus, the occurrence of these 3 ichnofacies in such a very low gradient, cratonic platform is highly unlikely in 4 the present day.

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After these comments, it is also worthwhile considering that the geomorphology   However, the presence of clay drapes in most of the lithofacies described does 16 strongly support an important tidal effect throughout the depositional system. 17

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The purpose of this section is to recognize and correlate stratigraphic surfaces 21 representing changes in depositional trends and to interpret the resulting The top of the Ash Shabiyat Formation is marked by a sharp or slightly more 3 gradational shift from the blocky, low GR response, characteristic of this 4 formation, to a notably more spiky or serrate GR response typical of much of 5 the lower Hawaz. This shift is interpreted not only as a maximum regressive 6 surface but also as a sequence boundary. As such it is a compound surface 7 and might be considered in terms of marine erosion as a ravinement which 8 marks the base of the depositional sequence 1 (DS1) (Figure 9). 9 The overlying HWZ1 is broadly transgressive in character, comprising stacked 10 fining-upwards parasequences (including a regionally distinctive and extensive 11 abandoned subtidal complex) capped by a regional flooding surface (Figure 9), 12 and finally a cleaning-upwards, progradational parasequence or parasequence 13 set. 14 The boundary between HWZ1 and HWZ2 is marked in all the wells by an abrupt 15 change in lithology to more argillaceous facies recording a marked deepening in 16 the basin. This is an excellent and consistent correlatable surface but is not fully 17 genetic as the maximum flooding surface of the DS1, only rarely coincides with 18 the lithological change and is instead typically picked a short distance above the 19 shift at the highest GR peak in the well (Figure 9). separates a dirty sandy package from a cleaner sandy package within a 1 coarsening-upwards parasequence or parasequence set as suggested by the 2 GR response and facies analysis. However, this surface is not easily 3 recognizable in all wells and has not been used as a regional correlative surface 4 due to its probable diachronous nature.

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The HST of DS1 is truncated by an erosive surface interpreted as a shoreline 6 ravinement unconformity (Figure 9) generated by the action of wave and tidal   Zone HWZ4 comprises stacked fining-upwards parasequences, mainly formed 5 by tidal sand to mixed flat deposits cut by tidal creeks (Figure 9). Similar  'actualistic' or 'near-actualistic' systems, but in many cases it might be hard to 1 apply to very ancient coastal to shallow marine depositional systems, notably 2 those of the Precambrian to lower Paleozoic due to major differences in Earth 3 surface dynamics. Nevertheless, while some of these ancient depositional            Week, Oran, Algeria.

Land flora
Vegetation in continental to transitional environments helps to stabilize river banks limiting channel shifting, changing river style from braided to meandering in low gradient systems Chemical weathering and related clay generation.
The lack of vegetation in subaerial conditions led to the development of high energy fluvial systems (mainly braided style) characterised by rapid channel shifting of rivers even in very low gradient systems Lack of clay generation by induced chemical weathering due to the absence of vegetation in subaerial environments. Clay-size particles alternatively sourced from volcanic ash, hydrothermalism, diagenesis, etc.

Greenhouse / Icehouse
Incision of valleys during sea level fall in recent icehouse periods and subsequent development of estuarine environments with marine transgressions. Fluvial sediments are common in proximal parts of the systems and related hyperpycnal deposits in more distal settings during lowstand stages.
Epeiric seas in large cratonic basins during greenhouse periods developing areally extensive paralic environments. Very difficult to identify lowstand deposits due to very limited incision in proximal environments. Very low gradients imply major paleoshoreline shifts with only limited relative sea level rises