Impact of depositional facies on the distribution of diagenetic alterations in the Devonian shoreface sandstone reservoirs, Southern Ghadamis Basin, Libya

Sedimentary Geology 329 (2015) 62–80

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Impact of depositional facies on the distribution of diagenetic alterations in the Devonian shoreface sandstone reservoirs, Southern Ghadamis Basin, Libya Muftah Ahmid Khalifa a,⁎, Sadoon Morad b a b

Department of Earth Sciences, Uppsala University, 752 36 Uppsala, Sweden Department of Petroleum Geosciences, The Petroleum Institute, PO Box 2533, Abu Dhabi, United Arab Emirates

a r t i c l e

i n f o

Article history: Received 23 July 2015 Received in revised form 8 September 2015 Accepted 8 September 2015 Available online 18 September 2015 Editor: Dr. J. Knight Keywords: Shoreface sandstones Diagenesis Reservoir quality Ghadamis Basin Libya

a b s t r a c t The middle Devonian, shoreface quartz arenites (present-day burial depths 2631–2588 m) are important oil and gas reservoirs in the Ghadamis Basin, western Libya. This integrated petrographic and geochemical study aims to unravel the impact of depositional facies on distribution of diagenetic alterations and, consequently, related reservoir quality and heterogeneity of the sandstones. Eogenetic alterations include the formation of kaolinite, pseudomatrix, and pyrite. The mesogenetic alterations include cementation by quartz overgrowths, Fedolomite/ankerite, and illite, transformation of kaolinite to dickite, illitization of smectite, intergranular quartz dissolution, and stylolitization, and albitization of feldspar. The higher energy of deposition of the coarsergrained upper shoreface sandstones combined with less extensive chemical compaction and smaller amounts of quartz overgrowths account for their better primary reservoir quality compared to the finer-grained, middle-lower shoreface sandstones. The formation of kaolin in the upper and middle shoreface sandstones is attributed to a greater flux of meteoric water. More abundant quartz overgrowths in the middle and lower shoreface is attributed to a greater extent of stylolitization, which was promoted by more abundant illitic clays. This study demonstrated that linking the distribution of diagenetic alterations to depositional facies of shoreface sandstones leads to a better understanding of the impact of these alterations on the spatial and temporal variation in quality and heterogeneity of the reservoirs. © 2015 Published by Elsevier B.V.

1. Introduction Shoreface sandstones are important hydrocarbon reservoirs in basins around the world (Ambrose et al., 1998). Improving hydrocarbon recovery requires thorough understanding of the depositional facies, sequence stratigraphy, and diagenesis of such sandstone reservoirs (Emery and Myers, 1996; Posamentier and Allen, 1999; Morad et al., 2000, 2010; Lima and De Ros, 2002; Al-Ramadan et al., 2005). Depositional facies, which control the primary porosity and permeability, pore water chemistry, and sand-body geometry, exert substantial impact on the distribution of eogenetic alterations (Hartmann and Beaumount, 1999; Morad et al., 2000, 2010; Salem et al., 2000; Stonecipher, 2000; Luo et al., 2009; Brenner et al., 2010; Odigi, 2011). Mesogenetic alterations are controlled mainly by burial-thermal history, formation-water chemistry, and distribution patterns of eogenetic alterations (Burley et al., 1985; Morad et al., 2000, 2010, 2012; Khalifa and Gasparrini, 2014). There are relatively few studies integrating diagenesis into depositional facies, which should provide a better understanding of the spatial and temporal distribution of reservoir quality ⁎ Corresponding author. E-mail addresses: [email protected] (M.A. Khalifa), [email protected] (S. Morad).

http://dx.doi.org/10.1016/j.sedgeo.2015.09.003 0037-0738/© 2015 Published by Elsevier B.V.

in shoreface sandstones (Al-Ramadan et al., 2005, 2012a, 2012b). The goals of this study are to constrain the impact of depositional facies on reservoir quality and diagenetic evolution of the Devonian shoreface sandstones of the southern Ghadamis Basin, western Libya (Fig. 1). The study demonstrates the importance of facies-controlled diagenesis, particularly the distribution of clay minerals and quartz overgrowths, on reservoir quality distribution of quartzose shoreface sandstones. 2. Geological setting The Ghadamis Basin is a large intracratonic sag basin developed on the passive northern margin of Gondwana (Sutcliffe et al., 2000; Hallett, 2002). The basin covers an area of 350,000 km2 with the depocenter located west of Tihemboka Uplift in Algeria and Tunisia (Fig. 1). The basin is bounded by the Amguid El-Biod Uplift to the west, Hoggar Massif and Gargaf Arch to the south, Nafusah Uplift to the north, and to the east the basin wedges out beneath the western part of the Sirt Basin (Baudet, 1988; El-Arnauti and Shelmani, 1988; Echikh, 1998) (Fig. 1A, B). The basin is characterized by complex stratigraphic thickness variations as a result of several major tectonic (uplift, erosion, folding, and faulting) events, including the Caledonian,

M.A. Khalifa, S. Morad / Sedimentary Geology 329 (2015) 62–80

8

A

Dahar

Tripoli

Arch

H. Messaoud Field

32

Nafusah Uplift

H

GERIA

L I B Y A un ra G

Basin

n

Al A Amguid El Biod Arch

30

be

Ghadamis

Ghadamis gh

ra

Al

Tin Fouye

Hi

am

H

Jable Al Sawda

Gargaf Uplift

M

as

n tsha Al A dle d Sa Sabha 26

Tihem

Lower Devonian

gg

ar

boka

Illizi Basin

Ho

Murzuk Basin

Odov

sif

B

Medierranean Sea

16

12

TUNISIA

63

ician

N

80 Km

MEDITERRANEAN SEA Libya

Tripoli

TUNISIA

Nafus

a Upli

32 N

ft

Al GERIA

ft

n

be

ra

Ghadamis

pli

nU

da

nG

Hu

30 N

ad

GHADAMIS BASIN

W

air on Kb i El- ress p De

Study area Gargaf Arch

28 N

nS tsha

Al-A

200 Km

Illiz

10 E

Al Harouj Al Aswad

Sabha

le

add

Illiz Basin

Zallah Trough

Jabal Al-Sawda

12 E

14 E

16 E

Fig. 1. Geological map showing: (A) location and boundaries of the Ghadamis Basin western Libya. The gray areas represent the volcanic rocks of the Al-Haroj Al-Aswda in central Libya, and the Hoggar Massif in SE Algeria. (B) The eastern part of the Ghadamis Basin indicating the study area (modified after Echikh, 1998; Elruemi, 2003). The red area represents the Ghadamis Basin in NW Libya, whereas the green arrow indicates the eastern part of the Ghadames Basin.

Hercynian, and Austrian phases (Echikh, 1998). Tectonic evolution of the basin occurred in three major phases: (i) subsidence through reactivation of Pan-African fault systems of a subsiding Paleozoic Basin, (ii) uplift and erosion of much of the Paleozoic section during the Hercynian Orogeny, and (iii) a north-west tilting and superimposition of the Mesozoic extensional basin (Echikh, 1998).

3. Devonian stratigraphy The stratigraphic column of the Ghadamis Basin shows that Devonian sandstones of the Tadrart, Ouan Kasa, Aouinet Ouenine, and Tahara formations unconformably overlie the Tanezzuft Shale (early Silurian) and Acacus sandstones (late Silurian) (Massa and Moreau-Benoit, 1976; Belhaj, 1996; Underdown, et al., 2007) (Fig. 2). The alternating

M.A. Khalifa, S. Morad / Sedimentary Geology 329 (2015) 62–80

CRE TACE OUS

TERTIARY

AL HAMRA NAFUSA

UP

compression

C SSI

L

L

L

L

L

L

L

L

L

L

SSIC

BIR EL GHANAM AL AZIZIA RAS HAMIA

HERCYNIAN

UP

Stephanian Westphalian

TIGUENTOURINE

MID

Namurian

ASSED JAFFAR

LO

Visean Tournaisian

MID

Marine

M'RAR TAHARA

IV III II

Famennian Frasnian Givetian Eifelian

F4 and F3

I

INTRA-GIVETIAN

Aouinet Ounine

UP

uplift

DEMBABA

Emsian-Pragian EMGAYET SH. OUAN KASA SH. LO Gedinnian

TADRART SS.

ORDOVICIAN SILURIAN

Marine

AUSTRIAN

TAKBAL

TRIA

DEVONIAN

GROUP

ABREGHS GYPSIUM

IFER OUS BON CAR

DEPOSITIONAL TECTONIC EVENTS ENVIRONMENTS

GROUP

KIKLA

LO

Strunian

P A L E O Z O I C

LITHOLOGY

FORMATION / UNITS

STAGE

PERIOD

JUR A

M E S O Z O I C

CENOZOIC

64

UP

LO

Continental, deltaic and shallow marine

CALEDONIAN

compression

ACACUS TANEZZUFT SHALE

UP

MEMOUNIAT

MID

MELEZ CHOGRANE

Marine

Continental to marginal marine Regressive marine Marine Fuvial to marine

HAOUAZ

LO

CAMBRIAN

ACHEBYAT HASSAOUNA

PAN AFRICAN

PRE- CAMB.

uplift

Sandstones

Dolomite

Metamorohic

Limestone and sandstone

Mudstone

Unconformity

Limestone

Evaporites/Anhydrite

Mudstone and limestone

L L

Evaporites/Salt

UP.: Upper MID.: Middle LO.: Lower PRE-CAMB.: Pre-Cambrian

Fig. 2. Stratigraphic column of the Ghadamis Basin showing the Devonian sandstones/Aouinet Ouenine Formation (F3 sandstone) (modified after Massa and Moreau-Benoit, 1976; Hammuda, 1980; Montgomary, 1994; Belhaj, 1996; Echikh, 1998; Klett, 2000; Elruemi, 2003; Underdown et al, 2007).

M.A. Khalifa, S. Morad / Sedimentary Geology 329 (2015) 62–80

Devonian sandstones, siltstones, and shales were deposited in continental, deltaic, and shallow-marine environments (El-Rweimi, 1991; Hallett, 2002). The studied Aouinet Ouenine Formation is Middle-Upper Devonian (Eifelian to Famennian) in age (Massa and Moreau-Benoit, 1976; Vos, 1981; Stirling and Guinn, 2012). Maximum thickness of the formation varies from 683 m in the eastern margin of the basin (Bellini and Massa, 1980; El-Rweimi, 1991) to about 115 m to the south (Stirling

Well: A30-169

Depth

Facies

Grain size and Sedimentary structure

Meter

2832 (-2103.7)

and Guinn, 2012). The formation was deposited during a widespread transgressive event, which extended south to the Gargaf Arch (Bracaccia et al., 1991; Dardour et al., 2004; Stirling and Guinn, 2012). The formation is subdivided into four members (I, II, III, and IV) (Fig. 2). Member I overlies the eroded Tanezzuft shale and is characterized by fine-grained ferruginous sandstones (Bracaccia et al., 1991; Arduini et al., 2003). Member II consists of sandy mudstones and siltstones, which pass to thick, massive, fine- to medium-grained

F3 sand

Lithology

carbonate cemented sandstones

Silt clay

F

C

65

G

P

KB: 2220

Porosity (%) 0

5

10 15 0

Horizontal permeability (mD)

200

400 600

Upper shoreface

2833 (-2104.7)

M

Middle shoreface

2844 (-2115.5)

Ferroan dolomite cemented sandstones Medium to lower coarse sandstones Medium to fine sandstones

M

Fine sandstones Very fine sandstones

2858 (-2129.3)

Lower shoreface

Shale/claystone M

Wavy ripples Planar beding Trough cross stratification . .

2871 (-2142.4)

. .

. . . .

. . .

. . . . .

. . . .

.

2876 (-2147.4) 2882 (-2153.9) 2884 (-2155.5)

Transitional/ offshore

Bioturbated

.

.

2874 (-2145.7)

Massive

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

End of measuring porosity and permeability

Stylolites Shell fragments

Offshore End of the core

Irregular contact surface

Fig. 3. Graphic log of the cored F 3 sandstone unit of the Aouinet Ouenine Formation in southern Ghadamis Basin showing depositional environments, lithology, structure, grain size, and distribution of porosity and permeability.

M.A. Khalifa, S. Morad / Sedimentary Geology 329 (2015) 62–80

40

0

80

120

y Facies and depositional environments

og

l ho

Gamma Ray (API)

t Li 140

Formation

Age

Transgressive shale

2833 (-2104.7) m

14 m

Middle shoreface

17 m

Lower shoreface

9.5 m

Transitional/ Offshore

2857.6 (-2129.3) m

Aouinet Ounine Formation

at 2844 (-2115.5) m

Middle Devonian (Eifelian-Givetian)

11m

Upper shoreface

Aounite Ounine members I and II (F3 sand)

Extensively carbonate cemented sandstones

2832 (-2103.7)

OWC

Me mb er

66

2861 (-2145) m

2883.9 (-2155.5) m

End of the core Offshore

End of the log

Carbonate cemented sandstones Sandstones Shales Cored section

Fig. 4. Combination of gamma ray (GR) curve pattern of the F3 sandstones and lithology suggesting that the studied sandstones were deposited in shoreface environments.

Table 1 Summary description of the F3 sandstone depositional facies of the Aouinet Ouenine Formation indicates the relationship between depositional facies and reservoir quality. Facies/thickness Rock type

Grain size

Sorting

Structure

Depositional environments

Reservoir quality

A (~11 m)

Sandstones

Middle shoreface

Moderate

C (~13 m)

Sandstones

Massive, cross stratification, faint wave ripples structure, low-angle trough cross stratification, and extensively oil stained Low-angle cross stratification, horizontal bedding, horizontal to sub-horizontal planar bedding, swaley stratification and irregular contact surfaces, locally bioturbated with horizontal burrows, and shell fragments commonly occur Slightly to intensively bioturbated, low-angle cross stratification, and hummocky cross stratification/lamination Intensively bioturbated

Good

Sandstones

Moderately to well Well sorted

Upper shoreface

B (~14 m)

Medium to coarse Fine to lower medium

Lower shoreface

Poor

D (~4 m) E (~10 m)

Fine to very Well fine sorted Claystone/siltstone Very fine/silt/clay Shale/claystone

Black shale with thin horizontal laminae of siltstone, lenticular, and hummocky cross laminae with commonly scattered shell fragments

Transition/offshore Offshore

M.A. Khalifa, S. Morad / Sedimentary Geology 329 (2015) 62–80

A(I)

B(I)

C(I)

A(II)

67

Scale: 0.1 m

B(II)

B(III)

C(II)

D

B(IV)

E

Fig. 5. Core images of sandstones from the various shoreface facies. (A) Core samples of upper shoreface sandstones showing intensive oil stain and vague wave ripples. The sample depth is 2839 (−2110.7), 0.4 m thick, and at 3 m above the oil–water contact (OWC). (B) Core samples of the middle shoreface sandstone showing (I) tight bed, massive with scoured surfaces, occurs at oil–water contact (OWC), depth of 2843.8 (−2115.5) m. The sample thickness is b0.2 m. (II) Low-angle cross stratification and characterized by an irregular contact surface. The sample depth is 2844.6 (−2116.3), 0.2 m thick, and at 0.8 m below OWC. (III) Dense layer with swaley cross stratification. The sample depth is 2847.4 (−2119), 0.3 m thick, and at 3.6 m below OWC. (IV) Planar horizontal bedding. The sample depth is 2857.8 (−2129.4), 0.2 m thick, and at 10.3 m below OWC. (C) Core sample of the lower shoreface sandstones showing: (I) fine grained and bioturbated, the sample depth is 2864.5 (−2136.2), 0.3 m thick, and at about 20.8 m below OWC and (II) very fine grained and extensively bioturbated, with scoured surfaces The sample depth is 2868.6 (−2140.3) and thickness of 0.2 m, at 21.2 m below OWC. (D) Core sample of the lower shoreface sandstone-transitional offshore (claystone/siltstone). The sample depth is 2871.2 (−2142.9), 0.3 m thick, and at 27.4 m below OWC. (E) Core samples of the offshore depositional facies consist of claystone with thin laminae of siltstone (white) and local occurrence of shell fragments. The sample depth is 2882.7 (−2154) m, 0.3 m thick layer at 38.8 m below OWC.

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M.A. Khalifa, S. Morad / Sedimentary Geology 329 (2015) 62–80

av. 0.6 mD). The facies texture, structure, and gamma ray patterns suggest lower shoreface deposits. Lithofacies D comprises shale (about 4 m thick) with horizontal laminae of siltstones and very fine-grained sandstones, which are characterized by hummocky cross stratification, horizontal to sub-horizontal lamination, lenticular bedding, and rare horizontal burrows (Fig. 5D). The facies is interpreted as lower shoreface/transitional offshore depositional environments. Lithofaces E shales/claystones (9.5 m thick; Fig. 5E), which are black, fissile, with scattered shell fragments particularly at base, are characterized by very high gamma ray (N140 API) values, which suggest deposition in mud rich, offshore depositional environments.

sandstones. Member III consists of shales and siltstones associated with ferruginous oolitic deposits (Collomb, 1962; Bellini and Massa, 1980). Member IV consists of mudstones and siltstones, which were suggested to be deposited during regression event in an open shelf environment; this member has a sharp contact with the overlying Upper Devonian (Strunian) Tahara Sandstone Formation (Bellini and Massa, 1980; Arduini et al., 2003). Members I and II have been subdivided into F4 (Eifetian) and F3 (Eifetian to Givetian). The F4 unit unconformable overlies the Lower Devonian sandstones (Ouan Kasa Formation) (Massa and Moreau-Benoit, 1976; El-Rweimi, 1991; Hallett, 2002) (Fig. 2). The F3 sandstones (40 m thick), which are the target of this study, display variation in thickness from 0 to 50 m in the Al-Wafa field area in the south part of the basin, and pinch out toward the south and southeast of the Al-Wafa and Alrar fields (El-Rweimi, 1991). The top of the F3 sandstone ranges from 2833 to 2786 m thickness recorded in wells A30 and A37-NC169, respectively, which are located 5 km apart. The unit is bounded at base by an erosional unconformity (sequence boundary) and at top by an erosional transgressive surface (Rahmani, 1993; Elruemi, 2003). The gamma ray curve patterns of the F3 sandstones display a funnel shape, which indicates coarsening upward. The sedimentary structures, texture, and the low clay contents suggest shoreface depositional environments (Figs. 3 and 4). The unit is characterized by good reservoir quality in the south, moderate to the east, and poor to the west (Arduini et al., 2003). The F3 sandstones are subdivided into five lithofacies (Table 1): Lithofacies A comprises 11 m thick, medium- to coarse-grained, moderately to well-sorted sandstones. The sandstones are oil stained and are massive or show faint low-angle trough cross stratification, wave ripples, and trace marks of vertical burrows (Fig. 5A). These sandstones have good reservoir quality (porosity 6–14%; av. 12%; and permeability 4–632 mD; av. 259 mD), no detrital clay content, which suggest upper shoreface deposits (Chaouchi et al., 1998). Lithofacies B comprises about 14 m thick, fine to medium-grained, well-sorted sandstones, which show trough and swaley cross stratification, and subhorizontal planar bedding, with vertical and horizontal burrows, and trace of marine shell fragments in the lower part (Fig. 5B). The facies is characterized by moderately to poor reservoir quality (porosity 4–11%; av. 9%, and permeability 0–80 mD; av. 26 mD). The facies structure, texture, and low gamma ray curve patterns suggest middle shoreface depositional environments. Lithofacies C comprises about 13 m thick, very fine to fine-grained, well-sorted sandstones, which show low-angle cross stratification, planar and horizontal bedding, lenticular siltstone and is intensively burrowed, which suggest high energy deposition conditions (Nichols, 2009) (Fig. 5C). The sandstones have poor to nil reservoir quality (porosity 6–13%; av. 10%, and permeability 0–7 mD;

4. Samples and methodology Fifty-eight representative samples were collected from cored sandstone reservoirs of the F3 unit of the Aouniet Ounine Formation from two wells (well A30-169 and A37-NC169; Fig. 1). The selected samples cover the various depositional facies both from the water and oil zones. The upper shoreface sandstones (about 11 m thick) are oil saturated, whereas the middle and lower shoreface sandstones occur below the oil–water contact at 2844 m (− 2115.5 m subsea depth) and hence are water saturated. The length of the whole core is about 51 m, the core length below OWC is about 40 m. Only the upper 16 m of the middle and lower shoreface sandstones are included in this study, whereas the lower 24 m of the lower shoreface and the mudstones of the offshore were not sampled. Thin sections were prepared for all samples subsequent to vacuum impregnation with blue epoxy. The modal composition and porosity were obtained by counting 300 points in each thin section. Sixteen representative samples were coated with gold and examined with a JEOL JSM-T330 scanning electron microscope (SEM). A JEOL electron microprobe equipped with a back-scattered electron detector (BSE) was used to analyze the chemical composition of carbonate cements. The operation conditions were an accelerating voltage of 20 kV, a measured beam current of 8n A and a beam diameter of 1 mm. The standards and count times used were wollastonite (Ca, 10 s), (Mg, 10 s), (Mn, 10 s), hematite (Fe, 10 s), and strontianite (Sr, 20 s). Analytical precision was better than 0.01% for all elements. Six Fe-dolomite/ankerite cemented samples were analyzed for C and O isotope analyses. The samples were crushed and reacted in vacuum with 100% phosphoric acid at 50 °C for 24 h (Al-Aasm et al., 1990). The CO2 collected was analyzed using a delta plus mass spectrometer. The phosphoric acid fractionation factor used 1.01060 for dolomite (Rosenbaum and Sheppard, 1986). Data are reported in per mil (‰) relative to V-PDB standard (Craig, 1957).

Table 2 The framework grain composition and diagenetic cements distribution within depositional facies in the shoreface deposits of the F3 sandstone, Aouinet Ouenine Formation (Middle Devonian) in the southern of the Ghadamis Basin. The values represent volume in %, and SD stands for standard deviation. Rock composition

Upper shoreface

Middle shoreface

Lower shoreface

Min

Max

Mean

SD(σ)

SD 2(σ)

Min

Max

Mean

SD(σ)

SD 2(σ)

Min

Max

Mean

SD(σ)

SD 2(σ)

Detrital component Monocrystalline quartz Feldspar Rock fragments Mica Heavy minerals

60 0 0 0 0

78 2 0 0 0.6

70 0.2 0 0 0.3

5 0.6 0 0 0.2

10 1 0 0 0.4

62 0 0 0 0

73 0.6 0 0 0.4

67 0.2 0 0 0.2

3 0.2 0 0 0.2

7 0.4 0 0 0.4

62 0 0 0.3 0

69 0.3 0 1 0.3

65 0.2 0 0.5 0.1

3 0.2 0 0.4 0.2

6 0.4 0 0.8 0.4

Diagenetic component Quartz overgrowths Fe-dolomite/ankerite Calcite Kaolin Illite Pyrite Intergranular porosity Moldic porosity

8 0.3 0 0 0 0 5 0

16 11 0 3 0 0 17 0.3

11 3 0 0.9 0 0 12 0.4

3 4 0 1 0 0 3 1

5 7 0 2 0 0 7 2

16 0 0 0 0 0 1 0

20 1 0 0.3 3 0 10 0.6

18 0.4 0 0 1 0 6 0.2

2 0.4 0 0.1 1 0 3 0.2

3 0.8 0 0.2 3 0 6 0.4

14.7 0 0 0 1 0 8 0

16.7 15 0 0 23 1 10.1 0

16 5 0 0 10 0 9 0

0.9 5.7 0 0 8 0 1 0

2 11 0 0 16 0 0 0

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Porosity and permeability were measured on core plugs 3.8 cm in diameter obtained from the sandstone sample. The porosity measurements were performed using a helium porosimeter, whereas permeability measurements were achieved using a helium permeameter by applying a confining pressure of 100 to 400 psi. Prior to measurements, the core plugs were examined carefully for microfractures, cleaned in oil extractor, and dried in a vacuum oven at 60 °C for 24 h.

69

monocrystalline. Rock fragments, chert, plagioclase, mica, zircon, epidote, and anatase are present in trace amounts in some of the sandstones. Apart from the common presence of deformed mud intraclasts along laminae in the lower and middle shoreface sandstones, there are no detectable differences in the framework composition between the various shoreface depositional facies.

6. Diagenetic minerals 5. Framework composition of the sandstones 6.1. Quartz cements The Devonian F3 sandstones (Table 2) are quartz arenites according to Folk's (1968) sandstone classification scheme, with an average composition of Q99 F1 L 0. The quartz grains are nearly exclusively

Quartz, which is the main diagenetic minerals in the sandstones, occurs as syntaxial overgrowths covering the detrital quartz grains partly

Fig. 6. Petrographic features of quartz overgrowths: (A) Optical photomicrograph (XPL) showing syntaxial quartz overgrowths (arrows) around detrital quartz grains. Note intergranular porosity (P). Fine to medium-grained middle shoreface sandstone. (B) SEM image showing poorly coalesced quartz overgrowths (arrows) around detrital quartz grains, development was probably retarded by oil emplacement, which as a result preserve the intergranular porosity (P) via inhibiting formation of syntaxial quartz overgrowths. Medium to coarse-grained upper shoreface sandstone. (C) SEM image showing quartz grains extensively coated by poorly coalesced quartz overgrowths, which preserved the intergranular porosity (P). Note that the tiny crystals coalesced together to form coarser crystals (arrows). Medium to coarse-grained upper shoreface sandstone. (D) SEM image showing quartz overgrowth (Q Og) engulfing kaolin (K). Medium to coarse-grained upper shoreface sandstone. (E) SEM image showing quartz overgrowths (Q Og) engulf lath-like illite (arrows), which bridging the pore-throat (T), and hence are later in age. Fine-grained lower shoreface sandstone. (F) SEM image showing framboidal pyrite crystals (Py) engulfed by quartz overgrowths (Q Og). Fine to medium-grained, well-sorted lower shoreface sandstone.

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Fig. 7. (A) Optical photomicrograph (XPL) showing quartz overgrowth (arrows) covered by Fe-dolomite and ankerite (Dol), which partly fills the intergranular porosity. Fine to mediumgrained, moderately to well-sorted upper shoreface sandstone. (B) Optical photomicrograph (XPL) showing abundant syntaxial quartz overgrowths (arrows), which completely occluded the intergranular porosity. Medium-grained middle shoreface sandstones.

to completely (Fig. 6A, B). Quartz overgrowths occur as small crystals, which display variable extents of coalescence toward complete coverage of the quartz grains (Fig. 6C). Quartz overgrowths engulf kaolin (Fig. 6D), illite (Fig. 6E), and pyrite (Fig. 6F) and are engulfed by Fe-dolomite/ ankerite (Fig. 7A). Quartz overgrowths are more abundant (av. 17%) in the middle and lower shoreface (Fig. 7B) than in the upper shoreface sandstones (av. 11%) (Fig. 6B). Poorly coalesced quartz overgrowths are most abundant in the upper shoreface sandstones, whereas wellcoalesced quartz overgrowths are common in the lower part of the middle and lower shoreface sandstones.

6.2. Carbonate cements Fe-dolomite and ankerite cement (trace to 15 vol%; av. 3%) in the shoreface sandstones occur as patches or discrete saddle-like crystals (Fig. 8A), which are scattered throughout the shoreface sandstones and along the stylolites (up to 300 μm a cross) (Fig. 8B, C). Electron microprobe (EMP) analyses revealed the following molar compositional ranges of Fe-dolomite/ankerite: CaCO3 (51.1–63%), MgCO3 (14.1–28.6%), MnCO3 (0.8–3.1%), FeCO3 (11.6–32.6%), and SrCO3 (0.02–0.08%). Zoned ankerite with Fe plus Mn contents ranging between 20% and 38% occurs as

Fig. 8. Petrographic features of dolomite. (A) Optical photomicrograph (XPL) showing euhedral saddle dolomite (arrow) covering quartz overgrowths (Q Og), which suggest mesogenetic origin of Fe-dolomite and ankerite. Fine-grained lower shoreface sandstone. (B) Optical photomicrograph (PPL) showing Fe-dolomite and ankerite crystals (black arrows) along stylolite (red arrow). Fine-grained, well-sorted lower shoreface sandstones. (C) Optical photomicrograph (XPL) showing close view of the image (B), indicating occurrence of Fe-dolomite crystals (Dol) along the stylolite (arrow). (D) BSE image showing euhedral Fe-dolomite (Dol) with ankerite overgrowths (Ank), partly filling the intergranular pores, and engulfs quartz overgrowths (arrows), which suggest mesogenetic origin. Fine to medium-grained, well-sorted lower shoreface sandstone.

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overgrowths on, and partly replaces, the Fe-dolomite (Fig. 8D). Fecarbonate cements have δ18 OV-PDB values between − 17.6‰ and − 13.2‰, and δ13CV-PDB values between − 6.6‰ and − 3.7‰. 6.3. Clay minerals Clay minerals in the middle Devonian sandstones include kaolin and illite. Kaolin (up to 3 vol.%) occurs only in the upper and middle shoreface sandstones and is absent in the lower shoreface sandstones. Kaolin occurs as scattered patches (up to 400 μm across), which are constituted of vermicular and booklet-like aggregates (b10 μm across) (Fig. 9A). Two kaolin crystal morphologies recognized which are thin (b0.5 μm thick) pseudohexagonal crystals, and blocky shapes (up to 5 μm thick) with smooth surfaces (Fig. 9B). Illite (1–25 vol.%; av.10%) is encountered in the lower shoreface sandstones while it is less abundant (b3 vol.%; av. 1%) in the middle shoreface and absent in the upper shoreface sandstones. Illite has replaced mud intraclasts, which have been squeezed to form pseudomatrix between the quartz grains (Fig. 10A, B). Tiny discrete quartz crystals are embedded within the mud intraclasts and pseudomatrix (Fig. 10A). Pseudomatrix occurs along 200–400 μm thick laminae in the lower shoreface sandstones and as scattered patches (Fig. 10C, D). Illite occurs as flaky crystals (Fig. 10E), flaky crystals with filamentous and hair-like terminations (Fig. 10F), and as honeycomb-like aggregates (Fig. 10C). 6.4. Albite and pyrite The limited amounts of partly dissolved feldspar (b2 %vol.), which dominate the coarser-grained upper and middle shoreface sandstones (Fig. 11A, B), contain tiny (b 2 μm) parallel-aligned crystals and thin overgrowths (Fig. 11B, C). The albite crystals are engulfed by illite, which partly fills intragranular pores in the feldspar (Fig. 11D). The albite overgrowths are engulfed by, and hence pre-date, quartz overgrowths. Pyrite (b 1 vol.%) occurs as tiny framboids (b5 μm across) embedded in pseudomatrix (Fig. 11E). 6.5. Compaction The most direct evidence of mechanical compaction in the studied sandstones is the formation of pseudomatrix by squeezing of mud intraclasts between the quartz grains. The sandstones display evidence of chemical compaction such as concave–convex and sutured contacts between quartz–quartz grains (Fig. 12A) and straight contact between quartz–mica grains. Chemical compaction is also manifested by low amplitude stylolites and dissolution seams (100 μm thick, 1 to 4 cm long) along which there are thin, discontinuous films of illitic clay (Fig. 12B). Bitumen (Fig. 12C) and saddle Fe-dolomite occur along the stylolites (Fig. 8B, C). Chemical compaction is more extensive in the very fine to

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fine-grained lower shoreface sandstones than in the upper and middle shoreface sandstones. 6.6. Porosity and permeability of the sandstones Porosity in the studied sandstones is dominated by primary intergranular pores (up to 17%) compared to intragranular and moldic porosity (b2%). The latter pores are encountered in partly to extensively dissolved feldspar grains (Fig. 11A, B), which may contain albite crystals. The upper shoreface sandstones and in some cases, the upper part of the middle shoreface sandstones have larger amounts of intergranular pores (Fig. 12D) compared to the lower part of the middle shoreface and lower shoreface sandstones (Fig. 7B). The core plug porosity is 2– 15% (av. 11%), horizontal permeability is b1–632 mD (av. 136 mD), and vertical permeability from b 0.1 to 642 mD (av. 123 mD). The upper shoreface sandstones have higher porosity (6–14%; av. 12%), horizontal permeability (b4–632 mD; av. 259 mD), and vertical permeability (0 to 579 mD; av. 239 mD) than the other shoreface sandstones. The middle shoreface sandstones display moderate porosity (4–11%; av. 9%), relatively low but strongly variable horizontal permeability (0–80 mD; av. 26 mD) and vertical permeability (b 0.1–64 mD; av. 22 mD), whereas the lower shoreface sandstones have moderate porosity (6–13%; av. 10%), and very low to nil horizontal permeability (0–7 mD; av. 0.6 mD) and vertical permeability (0–3 mD; av. 0.4 mD). The cross plot of horizontal versus vertical permeability (Fig. 13A) displays a strong positive correlation, whereas the cross plot of porosity versus permeability displays a very weak positive correlation (Fig. 13B). 7. Discussion Diagenetic alterations and their impact on reservoir quality of the Devonian shoreface sandstones are related to depositional facies. Depositional setting of paralic sediments influences (A) changes in pore water chemistry from marine to meteoric owing to fall in relative sea level; (B) changes in sediment texture (size and sorting), and hence primary porosity and permeability of the sand, which in turn impact rates of fluid flux and related diagenetic alterations; and (C) the incorporation of intrabasinal grains (e.g., mud intraclasts) in the sandstones (Morad et al., 2000, 2012). These parameters have direct impact on the distribution of eogenetic alterations, which in turn affect the mesogenetic alterations (sensu Morad et al., 2000). The latter alterations are further controlled by the geochemical evolution of formational waters and burial-thermal history (Gaupp et al., 1993; Morad et al., 2000, 2010, 2012; Reed et al., 2005; Kim et al., 2007). The paragenetic sequence of the diagenetic alterations, which is obtained on textural relationships among the various diagenetic components, is illustrated in Fig. 14.

Fig. 9. Petrographic features of clay minerals. (A) SEM image showing patchy form of extensively altered/kaolinitized feldspar grain (K) between quartz grains (Q). Coarse-grained, upper shoreface sandstone. (B) SEM image showing tiny booklet-like hexagonal kaolin. The thin crystals (b1 μm thick) are kaolinite (K), and the blocky crystals (N3 μm thick) are dickite (Dk). Medium to coarse-grained, middle and upper shoreface sandstone.

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Fig. 10. (A) Optical photomicrograph (XPL) showing squeezed mud intraclast (MI) between quartz grains. Note the presence of tiny quartz grains within the intraclast (arrow) may represent quartz silt grain in these intraclasts. Fine-grained, lower shoreface sandstones. (B) Optical photomicrograph (XPL) showing squeezed mud intraclasts resulting in formation of pseudomatrix (arrows), which is stained black with bitumen, inhibiting precipitation of quartz overgrowths. Fine-grained, lower shoreface sandstone. (C) SEM image showing illitized grain between quartz grains. Note the honeycomb-like illite (arrows). Fine-grained, lower shoreface sandstones. (D) SEM image showing probable illitized fecal pellets (smectite illitization) of semi-spherical shaped in the centre (arrow) with flay illite. Fine-grained, lower shoreface sandstones. (E) SEM image showing smectite replaced by illite. Fine-grained, lower shoreface sandstones. (F) SEM image showing flaky illite with filamentous terminations that extends into pores to bridge pore throats (arrow). Fine-grained, lower shoreface sandstones.

7.1. Eogenetic alterations The quartzose composition of the Devonian shoreface sandstones has resulted in limited water–rock interaction and, consequently, limited chemical eogenetic alterations. The quartzose composition is attributed to the dominantly quartzitic source rocks of the Al-Gargaf Arc (e.g., Boote et al., 1998) and, to a lesser extent, diagenetic alteration of the feldspars. Apart from grain rearrangement by mechanical compaction, the eogenetic processes include mainly kaolinitization of the limited amounts of feldspars and deformation of mud intraclasts. Kaolinitization of feldspar grains is evidenced by the occurrence of scattered patches of kaolin that have the same size as the framework grains and by remnants of etched feldspar (e.g., Rossi et al., 2002; Marfil et al., 2003; Abouessa and Morad, 2009; De Ros and Scherer,

2012). Kaolinitization and dissolution of feldspars indicate flushing of the sediments by meteoric waters, which commonly occurs during a fall in relative sea level (e.g., Morad et al., 2000, 2012; Ketzer et al., 2002; Worden and Morad, 2003). The greater abundance of kaolin in the medium to coarse-grained upper shoreface sandstones suggests that kaolinitization was probably favored by high permeability, which allows more efficient flushing by meteoric waters (Morad et al., 2000, 2010; Lanson et al., 2002; Ketzer et al., 2003; Al-Ramadan et al., 2012b). The eogenetic origin of kaolinite is evidenced by its engulfment by quartz overgrowths and its occurrence as thin pseudohexagonal crystals (e.g., Macaulay et al., 1993; El-ghali et al., 2006). The intraclasts are expected to be originally smectitic in composition (Moraes and De Ros, 1990; De Ros et al., 1994). The formation of only trace amounts of framboidal pyrite is attributed to the lack of detrital

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Fig. 11. (A) Optical photomicrograph (PPL) showing intergranular (blue) and moldic porosity (MP) (arrows) resulting from partial dissolution of feldspar grain (Fd). Medium to coarsegrained, middle, and upper shoreface sandstones. (B) SEM image showing extensively dissolved feldspar grain and development of moldic porosity (MP) outlined by thin overgrowths (Fd Og). Medium to coarse-grained, middle to upper shoreface sandstones. (C) SEM image showing albitized feldspar grain. Note occurrence of albite crystals (arrows) and formation of moldic porosity (MP). Medium to coarse-grained, upper shoreface sandstones. (D) SEM image showing albite crystals (white arrows), partly filling intragranular pores in dissolved feldspar grain and engulfed by flak-like illite. Note occurrence of filamentous and hair-like terminations (thick yellow arrows). Fine-grained, lower shoreface sandstones. (E) SEM image showing framboids of pyrite (Py), which is engulfed by flaky illite. Fine-grained, lower shoreface sandstones.

Fe-minerals (e.g., hematite, biotite) and low organic matter content of the sandstones (Raiswell, 1982; Morad et al., 2000). 7.2. Mesogenetic alterations The mesogenetic alterations, which occurred at burial depths ≥ 2 km and temperature ≥ 70 °C (Morad et al., 2000), include cementation by quartz overgrowths and Fe-dolomite/ankerite, dickitization of kaolinite, illitization of smectite, intergranular quartz dissolution and stylolitization, and albitization of feldspars. 7.2.1. Origin and distribution of quartz overgrowths The precipitation of quartz overgrowths occurs typically at temperatures greater than 100 °C (McBride, 1989), which is also the case in sedimentary basins in Libya (Khalifa and Morad, 2012). However, it is uncertain whether such temperatures were achieved during deep burial

at present-day geothermal gradients, times of higher heat flux in the basins (i.e., higher geothermal gradient than today), or by flux of basinal fluids. Events of high heat flow were encountered in the Ghadamis Basin due to extension and thinning of the lithosphere associated with basin rifting, during the Triassic and Upper Cretaceous-Cenozoic (Underdown and Redfern, 2007, 2008) (Fig. 15). If no event of substantial uplift was encountered, such events of high heat flux in the basin are required to explain the presence of substantial quartz cement in the present-day burial depths of the sandstones (2833 to 2786 m). Silica needed for quartz cements was presumably sourced internally from concomitant intergranular quartz dissolution and stylolitization of the fine-grained, lower shoreface quartz sandstones (Fig. 12B). The smaller amounts of quartz overgrowths (av. 11%) in the oil saturated upper shoreface sandstones compared to the water saturated middle and lower shoreface sandstones (av. 18% and 16%, respectively) are attributed to (i) greater extent of stylolitization in the

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Fig. 12. (A) Optical photomicrograph (XPL) showing concave–convex contact boundary (arrows) between quartz grains (Q) due to compaction. Medium-grained, upper shoreface sandstone. (B) Optical photomicrograph (XPL) showing stylolite (arrow) marked with iIlitic clay. Fine to medium-grained, middle shoreface sandstones. (C) Optical photomicrograph (PPL) showing stylolite (arrow) marked with bitumen, fine to medium-grained, upper shoreface sandstones. (D) Optical photomicrograph (PPL) showing high intergranular porosity (P) and lack of quartz overgrowth, with evidence of chemical compaction at the contacts of the quartz grains (arrows). Medium to coarse-grained, upper shoreface sandstones.

latter sandstones owing to the presence of illite laminae, which were formed by squeezing and mesogenetic illitization of the presumed smectite ductile mud intraclasts between the rigid quartz grains by mechanical compaction, and (ii) retardation of quartz cementation owing to oil emplacement (e.g., Walderhaug, 1994, 1996; Bjørkum et al., 1998). Walderhaug (1994), Bjørkum et al. (1998) suggested that quartz cementation continues at slower rates after oil emplacement, via silica diffusion in irreducible water saturation. Retardation of quartz cementation may explain the presence of abundant poorly coalesced quartz overgrowths in upper shoreface sandstones. 7.2.2. Origin of stylolites and related Fe-dolomite/ankerite cement The common presence of illitic clays along sutured intergranular quartz contacts and stylolites, particularly in the lower shoreface sandstones, suggests that these clays promoted quartz dissolution (e.g., Oelkers et al., 2000). The greater amounts of quartz overgrowths in these sandstones compared to in the upper shoreface sandstones suggest that silica was redistributed locally by diffusion over short distances (Oelkers et al., 2000). The presence of bitumen along the stylolites suggest: (i) the possible role of stylolites as conduits for oil migration (Baron and Parnell, 2007) or (ii) chemical compaction proceeded after early, partial oil saturation. Moreover, the presence of saddle dolomite along the stylolites is attributed to the migration of basinal brines along stylolites (Fontana et al., 2014). Stylolites might be opened during lateral tectonic compression events and growth of anticlines (Koepnick, 1988; Marfil et al., 2005) and hence act as conduits for fluid flow (Wong and Oldershaw, 1981; Braithwaite, 1989; Marfil et al., 2005; Ben-Itzhak et al., 2014). The

Ghadamis Basin underwent tectonic compression/folding in the Cretaceous–Tertiary (Boudjema, 1987; Echikh, 1998; Underdown et al., 2007). Using the δ18 OV-PDB values of Fe-dolomite/ankerite (− 17.6‰ and − 13.2‰), the dolomite–water oxygen fractionation equation of Land (1983) and assuming δ18Ov-SMOW of Devonian sea water was − 1‰ (Joachimski et al., 2004; Van Geldern et al., 2006) suggest precipitation temperatures of about 90–150 °C. These temperatures are in agreement with post-quartz precipitation of the carbonate cement. The δ13CV-PDB values (−6.6‰ and −3.7‰) of the carbonate cement fall within the range for dissolved carbon derived from thermal maturation of organic matter (Irwin et al., 1977). The presence of dissolution seams in porous sandstones suggests that (i) compaction by overburden pressure is not a key factor in dissolution of quartz, but it is rather the role of illitic clays (e.g., Heald, 1959; Oelkers et al., 1992, 1996). If considerable overburden pressure was important for intergranular quartz dissolution and stylolitization, it would then be expected that the intergranular pores are eliminated substantially by compaction in consequence of reduction of sandstone bulk volume. (ii) The dissolved silica does not always re-precipitate as quartz overgrowths in the immediate vicinity of the stylolites via diffusional transport. (iii) The pores have resulted from dissolution of cement (e.g., carbonates) owing to the flux of organic acids derived from maturation of organic matter (Surdam et al., 1984) along the stylolites (Stone and Siever, 1996; Marfil et al., 2000). However, we have not detected evidence of such dissolution events in the sandstones. 7.2.3. Clay mineral transformation Clay mineral transformation in the studied sandstones includes conversion of kaolinite into dickite and illitization of smectite. The thin

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1000

very small illite crystals, resemble fecal pellets. Fecal pellets are composed of highly reactive cryptocrystalline smectite and/or Si-Al gels, which can be transformed into illite and chlorite during burial diagenesis (Worden and Morad, 2003). The extremely limited amount of detrital K-silicates in the sandstones suggests that potassium ions were probably derived externally from basinal fluids (Gaupp et al., 1993). Most of the potassium was consumed in the illitization of smectite, and smaller amounts have precipitated as authigenic illite on illite substrates.

Medium- to coarse-grained sandstone (Upper shoreface) Medium- to fine-grained sandstones (Middle shoreface)

100

Permeability (mD)

Fine- to very fine-grained bioturbated sandstones (Lower shoreface)

10

1

0.1

A 0.01 5

0.0

10

15

Porosity % 1000

Vertical Permeability (mD)

Medium- to fine-grained Middle shoreface sandstones Fne- to very fine-grained Lower shoreface sandstones

10

1

0.1

B 0.01 0.01

0.1

1

7.2.4. Albitization of feldspar The albitization of feldspar grains is a mesogenetic processes that takes place at temperatures greater than about 90–100 °C by a dissolution–precipitation mechanism, which is evidenced by the formation of tiny parallel-aligned albite crystals (Morad, 1986). Thus, the temperatures of albitization are similar to those of illitization of smectite (~100 °C) (Morad et al., 2000). The albitization of K-feldspar may have acted as a minor source of K ions needed for illitization of smectite and precipitation of filamentous illite (Morad, 1986; Morad et al., 1990). 8. Impact of depositional facies and diagenetic alterations on reservoir quality

Medium- to coarse-grained Upper shoreface sandstones

100

75

10

100

1000

Horizontal Permeability (mD) Fig. 13. (A) Cross plot of horizontal permeability versus vertical permeability of shoreface depositional facies displaying strong positive correlation. (B) Cross plot of porosity versus horizontal permeability displaying very weak positive correlation.

pseudohexagonal kaolinite crystals were intensively transformed into blocky crystals with smooth surfaces, which resemble dickite (Morad et al., 1994; Beaufort et al., 1998; Lanson et al., 2002). This kaolin conversion process has been suggested to occur during mesodiagenesis at + higher temperatures ≥ 100 °C by pore waters with low α+ K /αH ratio (cf., Ehrenberg et al., 1993; McAulay et al., 1994; Morad et al., 1994, + ratio was probably 2000; Beaufort et al., 1998). The low α+ K /αH enhanced by flux of organic acids to the pore fluids (Morad et al., 1994: Lanson et al., 1996, 2002) and/or lack of detrital K-feldspar in the middle Devonian sandstones (cf. Ehrenberg et al., 1993; Morad et al., 2000; El-ghali et al., 2006). The various textural habits of illite suggest variations in formation processes and origins (Worden and Morad, 2003; Abouessa and Morad, 2009). The flaky and honeycomb-like illites have possibly resulted from illitization of smectite (Moraes and De Ros, 1990; Morad et al., 2000), whereas the filamentous and the hair-like illite, which grown as termination on the honeycombs and flakes, have possibly precipitated directly from pore waters (Güven, 2001). The presence of illite patches, which have sizes similar to the framework grains are suggested to indicate formation by illitization of K-rich grains, such as K-feldspar and mud intraclasts (Morad et al., 2000; Worden and Morad, 2003). The small rounded grains, which are concentrated locally and composed of

A summary model of the diagenetic evolution pathways in shoreface sandstone facies (Fig. 16) reveals that the upper shoreface sandstones have better reservoir quality mainly owing to coarser grain size related to their higher depositional energy and to oil emplacement, which inhibited precipitation of quartz overgrowths. Conversely, abundant quartz overgrowths in the water saturated middle and lower shoreface sandstones deteriorated the reservoir quality. More extensive cementation of the latter sandstones by quartz overgrowths is attributed to the presence of illite along laminae, which promoted intergranular quartz dissolution and stylolitization. Mechanical and chemical compaction processes were more efficient (55%) in reservoir quality destruction than cementation (27.6%) (Fig. 17). The lower shoreface sandstones suffered more effective mechanical and chemical compaction than the upper shoreface owing to their fine grain size and more common presence of illite. A strong positive correlation (r2 = +0.9) between the horizontal permeability and vertical permeability (Fig. 13A) is attributed to the scarce presence of illite laminae in the middle and lower shoreface sandstones and complete absence in the upper shoreface sandstones. The presence of such laminae may act as barriers for vertical flow. Conversely, the very weak positive correlation (r2 = + 0.2) between horizontal permeability and porosity of the sandstones (Fig. 13B) is attributed to the poor connectivity of the primary intergranular pores owing primarily to precipitation of quartz overgrowths and chemical compaction, which resulted in occlusion of pore throats (Al-Ramadan et al., 2012b) (Figs. 6A and 12A, C, D). To summarize, the poorer reservoir quality of the lower shoreface sandstones than the upper shoreface sandstones is attributed to the fine grain size, more intensive chemical compactions, more abundant quartz overgrowths, and presence of illitic pseudomatrix formed by deformation of mud intraclasts. 9. Conclusions This study of the middle Devonian reservoirs in the Ghadamis Basin, Libya, yields important clues to the impact of depositional facies on distribution and evolution of the reservoir quality and diagenetic alterations in shoreface sandstones, including the following: • Better reservoir quality in the upper shoreface sandstones is attributed to (i) coarser grain size, and hence higher depositional permeability, and (ii) the smaller amounts of poorly coalesced quartz overgrowths possibly owing to oil saturation and limited stylolitization. Conversely, the finer grain size combined with more extensive cementation by well-coalesced quartz overgrowths of the middle and lower shoreface

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Diagenetic minerals

Eodiagenesis

Mesodiagensis

<2 km; <70°C

> 2 km; >70°C

Shoreface sandstones Upper I

Middle

Lower

I

I

Pyrite Mechanical Compaction Grain kaolinitization Feldspar grain dissolution Albitization of feldspar Illite Kaolinite Dickitization Chemical compaction Quartz overgrowths Fe-dolomite/ankerite

Fig. 14. Paragenetic sequence showing the relative timing of the diagenetic alterations in the shoreface sandstones based on petrographic relationships and fluid inclusion microthermometry.

Ord

Cam

S

Dev

Carb

Pre

Tri

Jur

Cret

Cen

Bottom sediment heat flow

70

A

Heat Flow (mW/m2)

60

B

Bottom basement heat flow

50

C

40

A: Heat flow over the southern margin of the basin

D

30

B: Heat flow over the western margin of the basin C: Heat flow over the central of the basin

20

E

D: Heat flow over the western part of the basin E: Heat flow over the eastern flank of the basin (Libya) using pre-Hercynian maximum burial

10 600

500

400

300

200

100

0

Age (Million years)

Cam = Cambrian

Carb = Carboniferous

Ord = Ordovician

Pre = Premian

S = Silurian

Tri = Triassic

Dev = Devonian

Jur = Jurassic

Cret = Cretaceous Cen = Cenozoic

Fig. 15. Predicted heat flow history in the Ghadamis Basin. Note the first event of high heat flow occurred during the Paleozoic (Triassic rifting), subsequent to cooling event during Jurassic– Cretaceous thermal sag phase, and a second event of higher heat flow recorded during Cenozoic, particularly over the southern and western margins of the basin (after Underdown and Redfern, 2007, 2008).

Lower shoreface

Fluid saturation Formation water

Middle shoreface

Hydrocarbon (oil and/or gas)

Upper shoreface

> 70˚C; > 2 km

Reservoir quality

< 70˚C; < 2 km

Cements

Kaolin

Mesodiagenesis

Illite

Eodiagenesis

77

Quartz

Facies

Grain size/porosity

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Quartz grain

Dickite

Intergranular pores

Illitized pseudomayrix along stylolites

Intragranular pores

Kaolinite

Quartz overgrowths

Increase upward

Feldspar albitization

Ferroan dolomite/ankerite

Increase downward

Feldspar grain

Squeezed mud intraclast

Fig. 16. A conceptual model summarizing the distribution of diagenetic alterations within the shoreface sandstone depositional facies of the Aouinet Ouenine Formation. The model indicates that facies control the distribution of cements, grain size, and reservoir quality evolution.

sandstones, formed by continued diagenesis in the water zone, resulted in fair to poor reservoir quality. Furthermore, the greater abundance of quartz overgrowths in the latter sandstones is attributed to the presence of illitic laminae, which promoted intergranular quartz dissolution and stylolitization, and hence acted as a local source of dissolved silica. • Kaolinitization of feldspars, which has affected the upper and middle shoreface sandstone facies, is attributed to greater flux of meteoric waters into relatively coarser sands during fall in relative sea level. • The presence of illite laminae in the sandstones is attributed to illitization of smectitic, ductile mud intraclasts squeezed between the rigid quartz grains by mechanical compaction. These laminae may act as sites for intergranular quartz grain dissolution and stylolite development.

• The presence of stylolites in porous sandstones suggests that compaction is not a major factor in their formation of stylolites, and instead confirms the role of illitic clays in quartz dissolution. • The presence of bitumen and saddle dolomite crystals along stylolites suggests that they have acted as conduits for flux of basinal fluids and hydrocarbons during tectonic compression in the basin.

Acknowledgments Muftah Khalifa would like to thank Dr. Mansour Emtir (The Chairman of Management Committee of the Libyan Petroleum Institute) and Dr.

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Cement (%) 0

10

20

30

40

50

25

%

40

30 %

Intergranular Volume (%)

40

50

30

%

20

Medium- to course-grained sandstones (upper shoreface)

20

Fine- to medium-grained sandstones (middle shoreface)

%

10

75

Very fine- to fine-grained sandstones (lower shoreface)

10

90

Original porosity destroyed by mechanical compaction and intergranular pressure solution (%)

0 10

50

Mean intergranular volume (31%) mean cement (19%)

0%

100

0 0

25

50

75

100

Original porosity destroyed by cementation (%) Fig. 17. Plot of cement volume versus intergranular volume (intergranular porosity + cement) (after Houseknecht, 1987; modified by Ehrenberg, 1989) of the Middle Devonian sandstones in the southern Ghadamis Basin, showing that the original intergranular porosity has been reduced mainly by compaction.

Salem Al Khader (The Management Committee Member for Technical Affairs) for the their encouragement and financial support, and to the staff of Engineering and Exploration Departments for their assessment in the laboratories works. Great thanks go to Dr. Abu-Rema AbuAlgasim (The Ex-General Manager of the Libyan Petroleum Institute) for the part time study program. Thanks also go to the Geological Staff of the Mellitah Oil and Gas Company for providing core samples and the necessary data for the study. References Abouessa, A., Morad, S., 2009. An integrated study of diagenesis and depositional facies in tidal sandstones: Hawaz Formation (Middle Ordovician), Murzuq Basin, Libya. Journal of Petroleum Geology 32, 39–66. Al-Aasm, I.S., Taylor, B.E., South, B., 1990. Stable isotope analysis of multiple carbonate samples using selective acid extraction. Chemical Geology 80, 119–125. Al-Ramadan, K., Morad, S., Proust, J.N., Al-Aasm, I., 2005. Distribution of diagenetic alterations in siliciclastic shoreface deposits within a sequence stratigraphy framework: evidence from the Upper Jurassic, Boulonnais, NW France. Journal of Sedimentary Research 75, 943–959. Al-Ramadan, K., Morad, S., Bjӧrklund, P.P., 2012a. Distribution of diagenetic alterations in relationship to depositional facies and sequences stratigraphy of a wave and tidaldominated siliciclastic shoreline complex: Upper Cretaceous Chimney Rock sandstones, Wyoming and Utah, USA. In: Morad, S., Ketzer, J.M., De Ros, L.F. (Eds.), Linking diagenesis to sequences stratigraphy. International Association of Sedimentologists Special Publication 45, pp. 271–296. Al-Ramadan, K., Morad, S., Norton, A.K., Hulver, M., 2012b. Linking diagenesis and porosity preservation versus destruction to sequences stratigraphy of gas condensate reservoir sandstones: the Jauf Formation (Lower to Middle Devonian), Eastern Saudi Arabia. In: Morad, S., Ketzer, J.M., De Ros, L.F. (Eds.), Linking diagenesis to sequences stratigraphy. International Association of Sedimentologists Special Publication 45, pp. 297–336. Ambrose, W.A., Mendez, M., Saleem Akhter, M., Wang, F.P., Alvarez, R., 1998. Geological controls on remaining oil in Miocene fluvial and shoreface reservoirs in the Miocene Norte area, Lake Maracaibo, Venezuela. Petroleum Geoscience 4, 363–376. Arduini, M., Barassi, M., Golfetto, A., Ortenzi, A., Serafini, E., Tebaldi, E., Trincianti, E., Visentin, C., 2003. Silurian–Devonian sedimentary geology of the Libyan Ghadamis Basin: example of an Integrated Approach to the Acacus Formation Study. In: Salem, M.J., Oun, K.M. (Eds.), Symposium on the Sedimentary Basins of Libya II.

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