Conference PaperPDF Available

Depositional Environments and Large-Scale Sediment Architecture in the Upper Ordovician of the Murzuq Basin, SW Libya – Melaz Shugran and Mamuniyat Formations

January 2005

January 2005

DOI:10.3997/2214-4609-pdb.11.P08

Conference: 2nd EAGE North African/Mediterranean Petroleum & Geosciences Conference & Exhibition

Authors:

Neil Mcdougall

Independent Consultant

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Content may be subject to copyright.

2nd North African/Mediterranean Petroleum & Geosciences Conference & Exhibition

Algiers, 10-13 April 2005

NC20

NC190

NC186

NC187

NC115

Qarqaf Arch

Ghat

NC200

MIDDLELOWERUMIDDLELOWER(undifeff) MIDDLE UPPER

MESOZOIC

JURASSICTRIASSIC

MIDDLE

CARBONIFEROUSDEVONIANSILURIANORDOVICIANCAMBRIAN

BASEMENT (undiff)

P.C

HASAWNAH

HAWAZ

MELEZ SHUQRAN

MAMUNIYAT

TANEZZUFT

WANIN

TAHARA

AWAYNAT

MRAR

COLLENIA BEDS

ASSEDJEFAR

TAOURATINE

ZARZAITINE

DEMBABA

BDS

P A L A E O Z O I C

HOT SHALF

600

520

409

385

437

439

310

240

ERA

AGE

TERT-

REC

PERIODO

MIDDLELOWERUMIDDLELOWER(undifeff) MIDDLE UPPER

MESOZOIC

JURASSICTRIASSIC

MIDDLE

CARBONIFEROUSDEVONIANSILURIANORDOVICIANCAMBRIAN

BASEMENT (undiff)

P.C

HASAWNAH

HAWAZ

MELEZ SHUQRAN

MAMUNIYAT

TANEZZUFT

WANIN

TAHARA

AWAYNAT

MRAR

COLLENIA BEDS

ASSEDJEFAR

TAOURATINE

ZARZAITINE

DEMBABA

BDS

P A L A E O Z O I C

HOT SHALF

600

520

409

385

437

439

310

240

ERA

AGE

TERT-

REC

PERIODO

Introduction

Respsol-YPF is the operator of Concessions NC115, NC186, NC187, NC190 and NC200 in the

Murzuq Basin of SW Libya. An extensive drilling programme, accompanied by coring and

widespread use of image logs (FMI) together with seismic programmes, both 2-D and 3-D has

provided a significant insight into the development of the key elements of the petroleum system. This

system effectively comprises both Middle and Upper Ordovician reservoirs sealed and sourced by the

shales of the Lower Silurian Tanezzft Formation.

This poster specifically addresses the

development of the uppermost reservoir

unit or Mamuniyat Formation. This

major reservoir horizon was deposited

during the end-Ordovician glaciation

across a highly irregular

palaeotopography in a complex of

rapidly shifting environments

characterised by multiple shifts in sea

level and significant variations in

sediment supply. Subsurface data are

integrated with the results of several

outcrop studies to suggest a genetic

sequence framework and several

geological models improving

understanding of the architecture of this

complex unit.

Stratigraphic

framework

The Upper Ordovician (Ashgillian) is mainly composed of two closely related

formations: Melaz Shugran and Mamuniyat. The former is a highly argillaceous non-

reservoir sequence of greenish, purple and grey varicoloured sandy mudstones with

“floating” clasts, interbedded with fine-grained micaceous sandstone and siltstone.

Subsurface palynology suggest an early Ashgill or Pusgillian age. These sediments

are interpreted as the deposits of glaciomarine environments.

The Mamuniyat Formation is a complex succession of fine to coarse-grained

sandstones and local conglomerates interbedded with often deformed mudstones and

heterolithics which are, in general, quite poorly understood, due to the inherent

Typical outcrop view of the Melaz Shugran Formation

Depositional environments and large-scale sediment

architecture in the Upper Ordovician of the Murzuq Basin, SW

Libya (Melaz Shugran and Mamuniyat Formations)

Z-99

Dr N.D McDougall, *Khaeri Tawengi, Jamie Quin and Judith Vila Pont

Repsol Exploración, 280 Paseo de la Castellana, Madrid, 28046, Spain, *REMSA, Floor 8,

Tower 3, Dhat El Imad, Tripoli, Libya

2nd North African/Mediterranean Petroleum & Geosciences Conference & Exhibition

Algiers, 10-13 April 2005

Tanezzuft Formation

Hawaz Formation

Bir Tlacsin Formation

Upper

Mamuniyat

Middle Mamuniyat

Lower

Mamuniyat

Melaz Shugran Formation

Mamuniyat Formation

LM1

LM2

UM1

UM2

UM3

“Hot Shale” source rock

Tanezzuft Formation

Hawaz Formation

Bir Tlacsin Formation

Upper

Mamuniyat

Middle Mamuniyat

Lower

Mamuniyat

Melaz Shugran Formation

Mamuniyat Formation

LM1

LM2

UM1

UM2

UM3

“Hot Shale” source rock

variability in facies sequences reflecting multiple internal unconformities and rapidly changing

palaeoenvironments. In the subsurface wireline log responses are therefore highly variable, a marked

contrast with Hawaz Formation reservoir, and difficult to interpret beyond individual structures.

However, despite apparent complexity there are recognisable

sequence patterns are based on several outcrop-based studies

and extended to the subsurface.

McDougall and Martin (2000 introduced a subdivision

comprising a Lower, Middle and Upper Mamuniyat each

with distinctive attributes (geometry, internal features,

sedimentary fabric, bounding surfaces and petrophysical

parameters). This subdivision forms the basis for this study

and a brief summary of the key features is provided as

follows:

Lower Mamuniyat

This sand prone unit is subdivided into two packages

separated by a striated pavement. The basal unit (LM1)

comprises fine to medium grained, multistorey and

multilateral channel sands deposited in a tidally-influenced

estuarine setting fed by retreating ice sheets. This subunit

appears mainly to be confined within the axes of major

palaeovalley systems. The uppermost subunit (LM2) is more

widespread filling palaeovalleys and draping palaeohighs. It

is interpreted to comprise sheetflood and ephemeral channel

deposits formed on a distal outwash plain or sandur

Middle Mamuniyat

This distinctive unit truncates the Lower

Mamuniyat along a marked erosion surface of

probable glacial origin. It comprises rippled,

dewatered and slumped heterolithics overlain by

generally fine grained channel sandstones

characterised by large-scale loading. Listric

faulting is also observed in the outcrops of the

SW Qarqaf Arch. Overall deposition occurred

within a rapidly prograding braid-delta system

fed by melting ice sheets

Upper Mamuniyat

This unit forms the best reservoirs in the

subsurface. In the most complete outcrop

sections three packages are defined, UM1 to

UM3, each bounded by erosional surfaces. The

basal unit comprises coarse to very coarse grained, poorly bedded channel sandstones with large-

scale, low nagle trough cross-bedding and localised dewatering, interpreted as a bedrock-confined

anastamosed to braided system developed on an active glacial outwash plain or sandur. Sediment

supply rates were very high and the structures suggestive of catastrophic flood events or Jokulhaups

generated by bursting of ice dams

The middle unit or UM2 is the most aerially extensive of the Upper Mamuniyat packages comprising

a TST and HST in which relatively argillaceous, pebbly, coarse sandstones with abundant deposit

feeding burrows are progressively overlain by cross-bedded sandstones, the whole assemblage

characterised by a marked sedimentary dip of 6°. Initial deposition occurred in shallow marine

sandsheets during transgression of broad, incised valleys followed by the development of tidal sand

ridges. Subsurface examples may even show evidence of megaforesets or Gilbert deltas infilling

paleorelief

Lowermost Middle

Mamuniyat depositional

model

2nd North African/Mediterranean Petroleum & Geosciences Conference & Exhibition

Algiers, 10-13 April 2005

UM3 locally incises into UM2 forming a network of bedrock-confined channels filled by very coarse

grained, pebbly sandstones and occasional conglomerates. Large-scale trough cross-bedding also

characterises the package which is similar in many respects to UM1 as both represent a period of high

sediment supply during catastrophic floods

Bir Tlacsin Formation

The Bir Tlacsin Formation marks the last episode of the Upper Ordovician succession mainly

composed of heterolithic mudstones and may record a separate depositional sequence truncated prior

to deposition of the Tanezzuft Formation of the lowermost Silurian.

Sequence Stratigraphy

Both Isostasy and glacially-related

Eustacy are interpreted to have been

the main controls on the

architectural style of the Ordovician

sediments as a whole. Reactivation

of pre-existing faults in response to

glacial unloading also promotes

glacioisostatic uplifts, inversions

and a network of palaeovalleys and

palaeohighs. 2-D and 3-D

depositional models illustrate this

evolution in response to base level

changes which shows to some

extent a high degree of consistency

across the Basin. Individual steps

relate to major shifts in relative sea

level.

Surface gamma ray and outcrop

to subsurface correlation

Nine sections were measured in

outcrop by means of a portable

gamma ray tool aiming at

constructing a complete outcrop log

section through the Upper

Ordovician succession in the Ghat

area. A composite GR log was then

constructed across the entire

sequence from Middile Ordovivian

Hawaz Formation through to the

Silurian Tanezzuft Formation.

Geoseismic interpretation

In most seismic lines it is generally

possible to recognize the

palaeorelief marking the base of the

Upper Ordovician succession and of

course the base Tanezzuft reflector.

However, within this package,

geoseismic interpretation has been

limited by a restricted number of well ties and the lack of a geological model. Accordingly the key

elements from the outcrop studies, especially cross-sections and sequence boundary character, have

been combined to form conceptual models. Within this framework it is often possible to identify the

multible incision surfaces associated with Top Melaz Shugran and Base Middle or Upper

Mamuniyat. In addition knowledge gained from outcrops can also be applied to the interpretation of

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050 100 150 200 250

API

Hawaz

Melaz Shugran

Lower

Mamuniyat

Middle

Mamuniyat

UM1

UM2

UM3

Comparison between composite outcrop GR and a typical Murzuq

Basin well

2nd North African/Mediterranean Petroleum & Geosciences Conference & Exhibition

Algiers, 10-13 April 2005

different reflector types; notably (a) chaotic suggesting channelling and, in some cases listric faulting;

and (b) parallel to subparallel to largely continuous reflectors

2nd North African/Mediterranean Petroleum & Geosciences Conference & Exhibition

Algiers, 10-13 April 2005

Late Ordovician deglaciation of the Murzuq Basin (SW Libya): a core to seismic-scale characterization of the depositional environments and sedimentary architecture

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Glacial reservoirs have been the focus of numerous studies in various basins around the world, often with the aim of supporting hydrocarbon exploration and development. These reservoirs present a significant challenge due to their inherently complex internal architectures and significant lateral heterogeneities. This is of significant importance, not only for both exploration and reservoir production optimization, but also as an opportunity for stratigraphic trapping and enhanced prospectivity in mature areas. This study is focused on a major glaciogenic reservoir, of Late Ordovician age, in the Murzuq Basin (SW Libya). It aims to integrate subsurface datasets, such as 3D seismic with well-based sedimentological data (core, conventional wireline logs and wellbore resistivity images). The study proposes a comprehensive facies scheme optimized for the sub-surface but consistent with outcrops, and aims to generate depositional models, characterize the complex internal architecture of the reservoir, establish a genetic stratigraphic framework and evaluate facies in terms of reservoir properties and stratigraphic trap potential. Results, integrating subsurface facies analysis and well-calibrated seismic stratigraphy support previous models, derived from outcrop analysis, suggesting that these complex reservoirs cannot be characterised by purely lithostratigraphic criteria. To aid in the assessment of these formations, a facies scheme is proposed, focused on subsurface data but also considering other published, outcrop-based, schemes. Additionally, an interpretation is presented of the internal architecture of the Upper Ordovician based on a succession of several ice advance-retreat cycles, not all of which are present across the study area. Each cycle corresponds to a seismic package interpreted to comprise the deposits of several key sedimentation phases, resulting in a complex mosaic of genetic packages generated by multiple, often areally-restricted, cut and fill events. Predictive stratigraphic models are proposed for this complex package, based on both seismic stratigraphy and well-based sedimentology. The implications for future prospectivity and the significance for stratigraphic trap potential are also discussed.

Ordovician Glacial Paleogeography. Integration of Seismic Spectral Decomposition, Well Sedimentological Data and Glacial Modern Analogues in the Murzuq Basin, Libya

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Jan 2019

Ancient glacial sediments form major hydrocarbon plays in several parts of the world; most notably, North Africa, Latin America, and the Middle East. We have described a methodology for reconstructing broad-scale paleogeographies in just such a depositional system, using an extensive subsurface data set from the uppermost Ordovician glacial sediments of the Murzuq Basin of southwest Libya. Our workflow begins with the analysis of a large, high-quality 3D seismic data set, to understand the frequency content. Subsequently, optimum frequency bands are extracted, after applying spectral decomposition, and then recombined into an R (red) G (green) B (blue) blended cube. This volume is then treated as an image within which paleomorphological features can be distinguished and compared with modern glacial analogs. Mapping at different depths (time slices) of these features is then tied, by integration with core and image-log sedimentology, to specific depositional environments defined within the framework of a facies scheme developed using the well data and published outcrop studies. These depositional environments are extrapolated into areas with little or no well data using the spectral decomposition as a framework, always taking into account the significant difference in vertical resolution between the seismic data set and core-scale descriptions. The result of this methodology is a set of calibrated maps, at three different time depths (two-way time travel), indicating paleogeographic reconstructions of the glacial depositional environments in the study area and the evolution through time (at different depths/time slices 2D + 1) of these glacial settings.

Article Open Access ENHANCED DELINEATION OF GLACIAL CLASTIC RESERVOIRS BY THE APPLICATION OF PETRO-ELASTIC WELL CORRELATION AND SEISMIC INVERSION: CASE STUDY

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Full-text available

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The Murzuq Basin is an intracratonic basin located in the South and Southwest (SW) Libya. Glacial Ordovician sandstones are the primary reservoir. However, identifying the top of the upper Ordovician reservoir and the interpretation of intra-Ordovician sequence directly from seismic using conventional methods are well-known challenges in the exploration and production of Murzuq basin. Even with the advent of 3D seismic and dense well database. Therefore, a detailed seismic and well data analysis of the study area was carried out. Sand-shale elastic baseline cutoffs and Vp and Vs cross-over were incorporated into the petro-elastic wireline interpretation. Also, post-stack seismic inversion was applied to generate elastic attributes (acoustic impedance and modelled VpVs). Vp and Vs cross-over successfully outlined the sand cycles when the Vp is located on the right side of the track, whereas the appearance of Vp on the left side indicates the presence of shale units. Seismic inversion provided a better lateral continuity and vertical top and inter reservoir events separation, which has improved their traceability. The VpVs attribute illuminated the reservoir area by the low percentage ratios of 1.6 and 1.8, which is the expected range for sand bodies with emphasis to their upper, mid and lower contacts. Hence, elastic properties derived from both wells and seismic enhanced the delineation of the top of the Upper Ordovician unconformity unit of Mamuniyat Formation (top reservoir) and enabled clear distinction between a non-reservoir unit of upper Ordovician formation (Bir-Tlaksin) and the primary reservoir.

Application of amplitude spectrum and genetic inversion to determine shaly facies distribution: A case study: Murzuq Basin -Libya

Article

Feb 2018

Late Ordovician glacial deposits of the Mamuniyat Formation are the main oil reservoir in the Murzuq Basin in Libya. Autopicking the strong reflection at the base of the Silurian shales can be used to map the top of the Mamuniyat reservoir in the area where it is in direct contact with the Silurian shales. However, in areas where the Bir Tlacsin Formation, a mud-prone unit, is between the Silurian shales and the Mamuniyat reservoir, the top of the Mamuniyat is difficult to pick because the units juxtaposed across the boundary are too similar to produce a strong reflection. Defining the Bir Tlacsin facies is important because it impacts hydrocarbon accumulation and migration. To predict the distribution of the shaly facies of Bir Tlacsin and enhance mapping of the top Mamuniyat reservoir, we utilized a continuous wavelet transform to identify the distinctive thickness of the Hot Shale and Bir Tlacsin units. We also used genetic inversion to distinguish the bulk density of the Bir Tlacsin facies. A 64 Hz frequency gave good time resolution to the amplitude spectrum and was used to predict the facies distribution of the Bir Tlacsin. In contrast, the 24 Hz frequency showed good frequency resolution of the amplitude spectrum and was used to estimate the temporal thickness of the non-reservoir unit of Bir Tlacsin and Hot Shale. That estimate was then used to modify the autopick horizon for the base of the Silurian reflector to approximate the top of the Mamuniyat reservoir. Because of the large density contrast between the shaly facies of the Bir Tlacsin and the underlying and overlying units, inverted density also provides a way to predict the distribution of the Bir Tlacsin through estimated temporal thickness and to enhance mapping of the top Mamuniyat reservoir through mapping the base of the inverted density of the Bir Tlacsin. A comparison between mapping of the top reservoir using spectral decomposition and inverted density with respect to autopick shows that both methods improved the top of the Mamuniyat reservoir mapping. Prediction of the presence of Bir Tlacsin and improved accuracy of the top of the Mamuniyat reservoir mapping reduce the risk of drilling the shaly facies of Bir Tlacsin, and provides a better estimate of the reservoir reserve.

Using instantaneous frequency and colored inversion attributes to distinguish and determine the sandstones facies of the Late Ordovician Mamuniyat reservoir, R-field in Murzuq Basin, Libya

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Nov 2016

The Mamuniyat petroleum reservoir in southwestern Libya is comprised of clean sandstones and intercalated shale and sand facies that are characterized by spatial porosity variations. Seismic reflection data from the field exhibit relatively low vertical seismic resolution, side lobes of reflection wavelets, reflection interference, and low acoustic impedance contrast between the reservoir and the units underneath the reservoir, which make mapping those facies a difficult task. In the absence of broadband seismic data, optimizing frequency bands of bandlimited data can be used to suppress pseudoreflectors resulting from side-lobe effects and help to separate the clean sandstone facies of the reservoir. We have optimized the data based on our investigation of seismic frequency bands and used instantaneous frequency analysis to reveal the reflection discontinuity that is mainly associated with the reservoir boundary of the sandstone facies of the clean Mamuniyat reservoir. We also performed rock-physics diagnostic modeling and inverted the seismic data using spectral-based colored inversion into relative acoustic impedance. The inverted impedance matches the up-scaled impedance from the well data and the inversion of relative acoustic impedance confirms the conclusion that was drawn from the instantaneous frequency results. The interpretation of facies distributions based on the instantaneous frequency was supported by the inversion results and the rock physics models.

Glaciogenic Reservoir Heterogeneity: an example from the Late Ordovician of the Murzuq Basin, SW Libya

Article

Jul 2006
MAR PETROL GEOL

In North Africa, Late Ordovician glaciogenic reservoirs contain large volume of recoverable hydrocarbon reserves, but their heterogeneity and internal complexity means they are poorly understood. To improve this understanding, this paper presents the case study of the Murzuq Basin, SW Libya, and a synthesis of the stratigraphic architecture of Upper Ordovician glaciogenic reservoirs (the Mamuniyat Formation) within it. Particular attention is paid to regionally extensive stratigraphic boundaries and the geometry of sandstone units of potential reservoir quality. Four disconformity-bound units are recognised, the bounding surfaces of which are flat or of high relief. Unit 1 (the oldest) and Unit 3 are mud-dominated and tend towards non-reservoir. In contrast, Units 2 and 4 (the youngest) are sand-dominated, have poor to excellent reservoir quality but distinctly different sedimentological architectures across the basin. Unit 2 comprises transitions from glaciofluvial/intertidal sandstones to offshore turbidites and formed exclusively during glacial retreat. The occurrence of a significant palaeotopography affected sandbody geometry with glaciofluvial/intertidal deposits concentrating in the lee of these palaeohighs or infilling the bottom of tunnel valleys. Unit 4 is a compound stratigraphic unit recording the effects of glacial retreat and isostatic rebound. In deep marine settings, turbidite fans were deposited during retreat but in shallow water settings, structural reactivation and sediment reworking had a more profound effect on sedimentation. Sedimentological architectures are variable at this level and include prograding coarse clastic wedges (away from a palaeohigh) or the infill of half-graben basins during isostatic rebound.

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Depositional Environments and Large-Scale Sediment Architecture in the Upper Ordovician of the Murzuq Basin, SW Libya – Melaz Shugran and Mamuniyat Formations

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