Organic facies and thermal maturity of Late Aptian to Early Cenomanian shelf deposits, Northern Sinai (Egypt)

Ž .International Journal of Coal Geology 39 1999 251–278

Organic facies and thermal maturity of Late Aptianto Early Cenomanian shelf deposits, Northern Sinai

ž /Egypt

Junghyun Kim, Thomas Wagner ), Martina Bachmann,Jochen Kuss

Fachbereich 5-Geowissenschaften, UniÕersitat Bremen, Postfach 330440, 28334 Bremen, Germany¨

Received 15 December 1997; revised 23 April 1998

Abstract

Organic petrologic and geochemical investigations were carried out on Late Aptian to EarlyŽ .Cenomanian Mid-Cretaceous sediments recovered from the paleoshelf of the Northern Sinai

Ž .Egypt . The organic facies and thermal maturity of sedimentary organic matter were evaluatedŽwith regard to maximum burial depth. Results from organic geochemical analyses C rN ,org tot

.Rock–Eval Pyrolysis performed on kerogen concentrates indicate the dominance of hydrogen-de-pleted organic matter of either terrestrial or degraded marine origin. Kerogen typing reveals amixture of type III and IV. Maceral analyses conducted on concentrates support geochemicalresults showing variable proportions of vitrinite, migrabitumen, graphite and traces of inertinite.Vitrinite and inertinite are considered to be of autochthonous or parautochthonous origin indicat-ing at least partly humid paleoclimatic conditions in the hinterland. This assumption is alsosupported by the occurrence of graphite, which points to aquatic transport from the southerlyexposed Precambrian basement. The presence of migrabitumen presumably relates to migratedhydrocarbons from underlying organic carbon-rich Jurassic strata. The organic facies of Mid-Cretaceous deposits from the Northern Sinai indicates a proximal fluvio-deltaic or oxic shelf

wenvironment equivalent to organic facies types C–D according to Jones Jones, R.W., 1987.Ž .Organic facies. In: Brooks, J., Welte, D.H. Eds. , Advances in Petroleum Geochemistry.

xAcademic Press, London, pp. 1–80 . T data obtained from Rock–Eval pyrolysis reveal anmaxŽ .immature level for organic matter 3508C to 4258C , although these values are probably depressed

due to the presence of migrabitumen. Since vitrinite is very scarce in kerogen concentrates,reflectance measurements were also performed on migrabitumen. The best correlation of both

) Corresponding author. Tel.: q 49-421-218-7137; Fax: q 49-421-218-7431; E-mail:[email protected]

0166-5162r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0166-5162 98 00048-2

( )J. Kim et al.r International Journal of Coal Geology 39 1999 251–278252

types of reflectance data was obtained when migrabitumen data were corrected following thewapproach of Jacob Jacob, H., 1989. Classification, structure, genesis and practical importance of

xnatural solid oil bitumen. Int. J. Coal Geology, 11, pp. 65–79 . Reflectance values range between0.6% to 0.7% Rm, equivalent to high volatile bituminous coalification stages. Since there is noindication for an abnormal geothermal gradient along the NE-African margin, a maximum burialdepth between 1.2 km and 2.0 km is estimated for the Mid-Cretaceous strata of the NorthernSinai. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: organic facies; migrabitumen reflectance; vitrinite reflectance; maximum burial depth; Mid-Creta-ceous; Northern Sinai

1. Introduction and regional geological setting

Organic geochemical studies in the Mid-Cretaceous western Neotethys and theadjacent Atlantic Ocean have a long tradition but are mainly concentrated on organic

Žcarbon-rich marine strata e.g., Einsele and Wiedmann, 1982; De Graciansky et al.,1986; Herbert and Fischer, 1986; Herbin et al., 1986; Stein et al., 1986; Thurow et al.,

.1992; see actual compilation in Huc, 1995 . These studies essentially contributed todevelop comprehensive paleoenvironmental models which improved to understand theclimatic, paleoceanographic and sedimentological controls of black shale formation. Incontrast, only few studies on depositional processes controlling organic carbon accumu-lation and preservation in non-black shale environments of that time interval have beencarried out.

This study is focused on Late Aptian to Cenomanian carbonate ramp deposits whichdeveloped along the paleoshelf of northeastern Africa situated in the Northern SinaiŽ .Fig. 1 . Here, nearshore littoral sediments formed on a northward-dipping paleoramp,where marginal continental sediments interfinger with shallow marine depositionalsettings. The lateral and vertical distribution of various sedimentary units reflects theinterplay between marine, marginal marine, and fluvio-deltaic depositional processes,

Ž .studied in detail by Bachmann and Kuss 1998 .Two major regional tectonic units are distinguished at the Sinai Peninsula, the stable

Arabo-Nubian Shield, composed of mainly Precambrian magmatic units in the south,and the mobile orogenic belt, composed of Cambrian to Neogene sediments further

Ž .north Fig. 1 . They represent two major tectonic domains: the stable and the unstableŽ .shelf Lovelock, 1984; Cohen et al., 1990; Kuss and Bachmann, 1996 . The unstable

shelf in the north, comprising the field area of the present study, documents a regiondominated by northward increasing thicknesses of Phanerozoic sediments, explained byincreased subsidence rates during the Mesozoic rifting phase along the southern edge of

Ž .the Neotethys Ocean Hirsch et al., 1995 . As a consequence, Triassic, Jurassic, andpre-late Turonian depocenters were formed within an elongated basin revealing anENE–WSW strike and a northward dip. Halfgrabens dominate the southern boundaryincluding the unstable shelf of the Sinai. From Senonian onwards, inversion of thesehalfgrabens occurred resulting in the formation of the Syrian Arc intraplate orogenŽ .Moustafa and Khalil, 1995; Ayyad and Darwish, 1996 . Nevertheless, regarding the

( )J. Kim et al.r International Journal of Coal Geology 39 1999 251–278 253

Ž .Fig. 1. Regional tectonic map from the Sinai and neighbouring areas with location of the studied region box .The Sinai Peninsula is defined by two graben systems of Miocene age, the Gulf of Suez rift to the west and theAqafa-Dead Sea rift to the east. Late Cretaceous compressive tectonics resulted in E–W striking fold axes in

Ž .the northern Sinai modified from Kuss and Bachmann, 1996 .

time interval studied here, sedimentation took place during a phase of tectonic quies-Ž .cence Bachmann and Kuss, 1998 .

ŽThe study area is separated into a lower delta-dominated ramp Late Aptian to Early. ŽAlbian and an upper carbonate-dominated ramp Middle Albian to Cenomanian;

.Bachmann and Kuss, 1998 . The transition between the two settings took place during a


ŽFig. 2. Simplified geological map of the Northern Sinai and location of sections studied modified from.Bachmann et al., 1996 ; G.sGebel.

Ž .period of second order sea-level rise Kuss, 1992 in conjunction with major paleocli-Ž .matic changes from humid to less-humidrarid conditions Abdel-Kireem et al., 1996 .

Sedimentological and micropaleontological investigations allowed the detailed recon-

Fig. 3. Lithologic columns and biostratigraphic ranges derived from investigations carried out by Bachmann etŽ . Ž .al. 1996 and Bachmann and Kuss 1998 . Location of the samples selected for organic geochemical analyses

Ž .are indicated by arrows open arrows mark samples used for maceral analyses .



struction of several depositional environments that formed along a north–southheadingŽ .transect of the north-dipping ramp e.g., Bachmann et al., 1996; Bauer et al., 1997 .

High resolution facies studies on a dm-scale and semiquantitative distribution patterns ofthe microfacies were conducted at the excellently exposed sections in the NorthernSinai. They determined the main factors controlling the proximal ramp-sedimentation at

Ž .this location Bachmann and Kuss, 1998 . Based on a sequence stratigraphic andcyclostratigraphic approach a composite paleodepositional model for the Mid-Creta-ceous sedimentary successions was established. It was concluded that deposition duringthe Late Aptian to Early Cenomanian was mainly influenced by relative sea-level

Ž .fluctuations Bachmann et al., 1996 .Despite the fact that several paleoenvironmental studies have been carried on

Mesozoic deposits of the Northern Sinai, only few have used organic geochemical andorganic petrologic techniques in order to assess the type and thermal maturation ofsedimentary organic matter. Furthermore, there are still some unresolved questionsconcerning the reconstruction of the depositional environment and the maximum burialdepth of the Mid-Cretaceous sediment section. We therefore chose a combined organic

Fig. 4. Linear relationships between the reflectance values of vitrinite and migrabitumen according to threeŽ . Ž . Ž .different studies. 1 %Rm s0.618P%Rm q0.4 according to Jacob 1989 . 2 %Rm s1.150P%Rm qv mb v mb

Ž . Ž .0.114 for limestone and %Rm s0.858P%Rm q0.452 for shale according to Bertrand 1993 . 3v mbŽ .%Rm s0.897P%Rm q0.415 according to Landis and Castano 1995 .˜v mb


petrologic and geochemical approach to contribute to these remaining issues. The mainŽ . Ž .aims of this study were i to characterize the organic facies, and ii to define the

thermal maturity of the organic matter with regard to maximum burial depth.

2. Samples and methods

The 69 clayey-silty or marly deposits investigated were selected from six surfaceŽ .sections spanning a N–S transect across the Northern Sinai Fig. 2 . The sections studied

ŽŽ . Ž .. ŽŽ .are located in the southern 1 Gebel El Minshera GM , central 2 Gebel MagharaŽ . Ž . Ž . Ž . Ž . Ž .South MS , 3 Gebel Mansoura North M , 4 Gebel Mansoura East ME , 5 Gebel

Ž .. ŽŽ . Ž ..Raghawi R , and northern part 6 Gebel Rizan Aneiza RN of the Northern Sinai.The profile Gebel El Minshera in the southern area covers a stratigraphic range from theMiddle Albian to Early Cenomanian, whereas the other profiles in the central and

Ž .northern areas chiefly range in age from the Late Aptian to Albian Fig. 3 .Ž .The total amount of organic carbon TOC , measured in bulk sediment samples using

a Leco CS-300 Carbon–Sulphur analyser, ranged from 0.02–0.8 wt.%. Because of theselow TOC contents, sedimentary organic matter was concentrated by demineralisationusing the nonoxidative acids HCl and HF without heavy liquid separation to obtain thekerogen fraction before further processing. Further investigations were exclusively

Ž .Fig. 5. Correlation of total organic carbon TOC measured in bulk sediments and kerogen concentrates.Stippled lines indicate various concentration factors due to acid treatment.



.



carried out on kerogen concentrates using a combination of organic petrologic andŽ .geochemical methods. C rN ratios were calculated using TOC wt.% and totalorg tot

Ž .nitrogen TN in wt.% data determined on a Heraeus CHN–O-Rapid elemental analyser.Ž .Rock–Eval pyrolysis Hydrogen Index, Oxygen Index and T was performed using amax

Ž .Rock–Eval II, following the analytical procedure outlined by Espitalie et al. 1977 .´Evaluation of maceral composition was conducted on polished blocks from 30 kerogenconcentrate and two bulk sediment samples using a Zeiss Axiophot equipped withincident white and ultraviolet light.

Information on the thermal maturity of sedimentary organic matter was derived fromT values obtained by Rock–Eval pyrolysis and reflectance values measured onmax

dispersed macerals in 13 kerogen concentrate samples. Due to the deficient amount ofvitrinite in most concentrates, reflectance measurements were also carried out onmigrabitumen. Migrabitumen, also known as exsudatinite, solid or solified bitumen,pre-oil bitumen and natural tar, is defined as an allochthonous, secondary maceral

Žrelated to migrated hydrocarbons Teichmuller, 1974; Stach et al., 1982; Jacob and¨.Hiltman, 1985; Curiale, 1986; Jacob, 1989 . Migrabitumen is amorphous, taking over

the shape of pore spaces or fractures into which it migrated. Formation of migrabitumenŽbegins during late diagenesis, i.e., at the top of the oil window Teichmuller, 1974;¨

.Stach et al., 1982; Jacob, 1989 . To overcome difficulties caused by the lack of vitrinitein some sediments, various authors have suggested to use migrabitumen reflectance dataŽ .%Rm as an alternative measure to assess information on the thermal maturity levelmb

Žof sedimentary organic matter e.g., Jacob and Hiltman, 1985; Jacob, 1989; Bertrand,.1993; Riediger, 1993; Landis and Castano, 1995 . A comparison of three different˜

methodical approaches to convert migrabitumen reflectance to vitrinite reflectance isŽ .illustrated in Fig. 4. The relationship of Jacob 1989 reveals that migrabitumen has a

lower reflectance than corresponding vitrinite below 1.0% Rm and a higher reflectancev

than corresponding vitrinite above 1.0% Rm . Other authors provided slightly differentvŽ .linear relationships. Following Bertrand 1993 , reflectance values of migrabitumen

determined on limestones show more consistent relationships with those of correspond-ing vitrinite if his approach is applied. In our study, the best fit between migrabitumen

Ž . Žand vitrinite reflectance was achieved when the approach by Jacob 1989 see presenta-.tion of results was used. A total of 30 to 50 reflectance measurements per sample were

Ž .performed at 546 nm, following the standard procedure described by Stach et al. 1982 .

3. Results

3.1. Elemental analysis and Rock–EÕal pyrolysis

The correlation of TOC contents measured on bulk sediments and on kerogenconcentrates is presented in Fig. 5. Overall, concentration of sedimentary organic matter

Ž . Ž .Fig. 6. Total organic carbon TOC , C rN , Hydrogen Index HI , T and reflectance values obtainedorg tot maxŽ . Ž . Ž .from kerogen concentrates of the profiles a Gebel El Minshera, b Gebel Mansoura East, and c Gebel

Ž .Rizan Aneiza compare Fig. 3 .


Fig. 7. CrN ratios vs. Hydrogen Indices as derived from kerogen analyses indicating a dominance of kerogenŽtypes III and IV kerogen types: IIsmarinerterrestrial, IIIs terrestrialrmarine, and IVs terrestrialrhighly

.oxidized .

by acid treatment was successful, taking an up to 50-fold enrichment of organic carbonin kerogen concentrates into account. TOC in kerogen concentrates ranges from 2–53wt.% allowing further analytical processing.

Data obtained by elemental analyses and Rock–Eval pyrolysis are presented for threerepresentative outcrop profiles in Fig. 5a–c. CrN ratios show the highest values of up to80 in the southern area, whereas they never exceed 35 in the central and northern sectorŽ .Fig. 6a–c Fig. 7 . Consistently high CrN ratios suggest a dominance of terrigenous

Žorganic matter, although diagenetic effects must also be considered Muller, 1977;¨.Emery and Uchpy, 1984; Tyson, 1995 .

Ž .Despite the high TOC contents in kerogen concentrates, hydrogen indices HIobtained by Rock–Eval pyrolysis generally remain very low. HI are below 50

Ž .mgHCrgTOC in the southern area with one exception , between 5 mgHCrgTOC and

Ž .Fig. 8. Compilation of results obtained from Rock–Eval pyrolysis. a Hydrogen Index vs. Oxygen Index, andŽ . Žb Hydrogen Index vs. T modified from Tyson, 1995; kerogen types: IIsmarinerterrestrial, IIIsmax

.terrestrialrmarine, and IVs terrestrialrhighly oxidized .



Fig. 9. Characterization of optical and reflectance properties of macerals observed in kerogen concentrates ofMid-Cretaceous deposits of the Northern Sinai.

120 mgHCrgTOC in the northern area, and reach the highest values of up to 250Ž .mgHCrgTOC in the central area Fig. 6a–c, Fig. 7 . Such low indices indicate a

dominance of hydrogen-depleted organic matter, probably of terrestrial or stronglyŽ .degraded marine origin Tissot and Welte, 1984 . Corresponding T values scattermax

from 3308C to 4258C, suggesting an immature to very low maturity level for the organicŽ .matter Espitalie et al., 1977; Tissot and Welte, 1984 . Highest HI and T values are´ max

observed in profiles from the central part of the study area. Kerogen typing based onRock–Eval pyrolysis data is deduced from a ‘van Krevelen’ graph and a plot showing

Ž .T values vs. HI modified according to Tyson, 1995; Fig. 8 . Due to the persistentmax

low hydrogen yields measured in most concentrates, kerogen types III and IV, represent-ing terrigenous and highly oxidized organic matter, prevail throughout the profiles.However, some samples from the central area are plotted on the field of kerogen typeIIrIII, suggesting a stronger influence hydrogen-enriched organic matter. Optical identi-fication of this type of organic matter remains an open target for future work, sincefluorescent liptinite was not encountered in any sample studied. We therefore favour theinterpretation that elevated HI values attest to the presence of hydrogen-enrichedmigrabitumen.

3.2. Organic petrology

The maceral composition of dispersed organic matter in kerogen concentrates wasŽ .characterized following the nomenclature established by Stach et al. 1982 . In Fig. 9,

the typical inventory of macerals, including their reflectance characteristics, observed in

Ž .Plate 1. Photographs illustrating vitrinite and graphite in Cretaceous sediments from the Northern Sinai. 1–2Ž .Grey to dark grey vitrinite V and vitrodetrinite showing predominantly subangular to subrounded particle

Ž . Žshapes, inertodetrinite I with oxidized rim Gebel El Minshera, scale bars50 mm and 150 mm, respectively;. Ž .reflected white light . 3–4 Elongated graphite characterized by minute laminar and internal irregular granular

Žstructures and a very high yellowish to white reflectance Gebel Maghara South, scale bars150 mm; reflected.white light .



kerogens from the Mid-Cretaceous Northern Sinai is presented. In total, three differentŽ .maceral groups — huminitervitrinite textinite, vitrodetrinite , inertinite

Ž . Ž .semifusiniterfusinite, inertodetrinite , and liptinite migrabitumen —as well as graphitewere identified. Fluorescent macerals were not observed. Plates 1–3 give an illustrationof the maceral spectrum present.

ŽDark grey and angular shaped textinite, clearly attributable to woody tissues Stach et.al., 1982 , rarely occurs. Typically, textinite is larger than 50 mm in particle size and

Žreveals open, partially gelified cell-lumina, indicating a transition to ulminite Fig. 3 in.Plate 2 . From its optical appearance, this diagenetic level would correspond to a high

Ž .volatile bituminous coalification stage Stach et al., 1982 . Macerals of the vitriniteŽ .group are identified as grey to dark grey vitrinite larger than 10 mm particle size and

Ž .vitrodetrinite smaller 10 mm particle size . The predominantly subrounded to well-rounded shape of these macerals suggests long-range or high-energy transport prior to

Ždeposition, indicating at least a parautochthonous origin Littke et al., 1997; Fig. 1 in.Plate 1 . Subangular textinites, in contrast, rather exclude extended transport distances,

thus probably representing the autochthonous or parautochthonous fraction of bulkorganic matter.

ŽInertinite is rarely identified as white grey to white inertodetrinite smaller 10 mm. Ž .particle size or semifusiniterfusinite larger 10 mm particle size . Inertodetrinite often

occurs as subrounded to rounded organic matter frequently showing oxidation rimsŽ .Figs. 1–2 in Plate 1 , whereas semifusiniterfusinite exhibits angular to subangular

Ž .shapes with characteristic bogen structures Figs. 1–2, and 4 in Plate 2 . Regarding thesefeatures of larger inertinitic particles, an autochthonous or parautochthonous origin isdeduced.

Dark grey to opaque migrabitumen, generally exceeding 20 mm in size, representsthe only maceral of the liptinite group. The shape of migrabitumen apparently is

Ž .structureless and adapted to surrounding cavities Figs. 1–2 in Plate 3 , often showingŽ .degassing pores Figs. 3–4 in Plate 3 . Migrated hydrocarbons, leading to the formation

of migrabitumen in Mid-Cretaceous sediments of the Northern Sinai, presumably relateto the presence of stratigraphically older, TOC rich strata occurring adjacent to the studyarea.

Elongated, large graphitic particles, usually exceeding 50 mm in particle length, areŽ .rarely observed in the southern areas but are more common further north Fig. 10 .

Graphite is characterized by a minute laminar and internal irregular granular structure,Ž .typically showing a yellowish to white-yellowish reflectance Figs. 3–4 in Plate 1 .

Formation of graphite starts in the transitional zone from metagenesis to metamorphismŽat temperatures of 2508C to 3308C Diessel et al., 1978; Okuyama-Kusunose and Itaya,

Ž .Plate 2. Photographs illustrating inertinite and textinite in Cretaceous sediments from the Northern Sinai. 1Ž . ŽLight grey semifusinite S with characteristic bogen structures Gebel El Minshera, scale bars50 mm;

. Ž . Ž . Žreflected white light . 2 White Fusinite with characteristic bogen structures S Gebel El Minshera, scale. Ž . Ž .bars50 mm; reflected white light . 3 Dark grey textinite T characterized by wide open and partially

Ž . Ž .gelified cell-lumina Gebel El Minshera, scale bars150 mm; reflected white light . 4 Semifusinite orŽfusinite distinguished by high reflectance and open cell lumina Gebel Mansoura North, scale bars50 mm;

.reflected white light .




Fig. 10. Triangle plot showing semiquantitative estimates of relative abundance of the three main maceraltypes with regard to type of lithology. Macerals of the inertinite group were not considered due to their rareoccurrences.

.1987; Rantitsch, 1995 . Hence, graphite must be of allochthonous origin, being erodedfrom surrounding Precambrian basement outcrops and subsequently transported byrivers to the Northern Sinai area.

Regional differences in maceral composition between the southern, central andnorthern part of the study area are illustrated in Fig. 10. Inertinites were not considereddue to the general scarcity of their occurrence. It is striking that macerals preserved inthe south are different from those observed in central and northern areas. It has to benoted, however, that variations in maceral composition may also be related to changes inlithology, e.g., marls are dominated by equivalent proportions of migrabitumen and

Ž .Plate 3. Photographs illustrating migrabitumen in Cretaceous sediments from the Northern Sinai. 1–2Ž .Migrabitumen M showing a dark grey to opaque reflectance, filling surrounded cavities and occurring in bulk

Ž . Ž . Žsediments Gebel El Minshera, scale bars150 mm; reflected white light . 3 Structureless migrabitumen M;. Ž . ŽGebel Maghara South, scale bars50 mm; reflected white light . 4 Migrabitumen with degassing pores Dp;

.Gebel El Minshera, scale bars50 mm; reflected white light .

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Table 1Reflectance data obtained on vitrinite and migrabitumen of Cretaceous deposits from the Northern Sinai

Samples Type of sediment %Rm %Rm %Rm %Rm %Rm Maceralmb v v v vŽ . Ž . Ž . Ž . Ž Ž .measured measured by Jacob, 1989 by Bertrand, 1993 by Landis and measured

.Castano, 1995˜

GM I 4 dolomitic marl 0.44 0.67 0.62 0.81 migrabitumenGM I 17 marl 0.54 0.68 0.73 0.74 0.90 vitrinitermigrabitumenGM I 24 marl 0.67 vitriniteGM II 2 dolomitic marl 0.57 vitriniteMS I 1 claystone 0.46 0.68 0.85 0.83 migrabitumenMS I 12 claystone 0.38 0.60 0.63 0.78 0.76 vitrinitermigrabitumenM I 8 claystone 0.49 0.63 0.70 0.80 0.85 migrabitumenM IV 2 dolomitic marl 0.46 0.68 0.64 0.83 migrabitumenME I 8 siltstone 0.44 0.67 0.83 0.81 migrabitumenME I 16 siltstone 0.39 0.64 0.79 0.76 migrabitumenR I 53d claystone 0.44 0.59 0.67 0.83 0.81 migrabitumenrvitriniteRN I 29 claystone 0.54 0.73 0.92 0.90 migrabitumenRN III 4 claystone 0.54 0.73 0.92 0.90 migrabitumen

Ž . Ž .Raw data are supplemented by corrected migrabitumen values following the three different methodological approaches by Jacob 1989 , Bertrand 1993 , and LandisŽ .and Castano 1995 .˜

Ž . Ž .1 %Rm s0.618P%Rm q0.4 according to Jacob 1989 .v mbŽ . Ž .2 %Rm s1.150P%Rm q0.114 for limestone and %Rm s0.858P%Rm q0.452 for shale according to Bertrand 1993 .v mb v mbŽ . Ž .3 %Rm s0.897P%Rm q0.415 according to Landis and Castano 1995 .˜v mb

%Rm sMean reflectance value of vitrinite.v

%Rm sMean reflectance value of migrabitumen.mb


vitrinite, whereas clay- and sandstones typically reveal some admixture of graphite.Whatever mechanism controlled the deposition of organic matter in Northern Sinaideposits, a mixture of vitrinite, migrabitumen and traces of graphite and inertinite wasfound in the southern area, whereas central and northern areas reveal considerablyhigher proportions of graphite with traces of vitrinite and inertinite. Distinguishingbetween the central and northern areas is hardly possible based on optical data only,although there appears to be a general trend to lower vitrinite contents towards the north.Moreover, graphite particles in the central area are generally larger than those observedin the northern area.

Mean reflectance values determined on disperse vitrinite and migrabitumen particlesare listed together with converted migrabitumen reflectance values in Table 1. Forillustration, results from two samples are presented in Fig. 11. Vitrinite reflectancevalues consistently range from 0.6% to 0.7% Rm , indicating a high volatile bituminousv

Ž .coalification stage Stach et al., 1982 . Corresponding migrabitumen reflectance valuesvary from 0.4% to 0.5% Rm , revealing lower values than obtained on vitrinitemb

particles. For comparison, migrabitumen reflectance data converted according to JacobŽ . Ž . Ž .1989 , Bertrand 1993 and Landis and Castano 1995 are presented. Converted values˜range from 0.6% to 0.7% Rm , 0.6% to 0.9% Rm , and 0.8% to 0.9%mb-corr mb-corr

Ž .Rm , respectively. Best fit to vitrinite reflectance data %Rm was observed usingmb-corr vŽ .the approach presented by Jacob 1989 . However, a comparably good correlation was

Ž . Žachieved for marly samples after using the conversion according to Bertrand 1993 see.sample GM I 17 in Fig. 11 .

4. Discussion

4.1. Distribution of organic facies along the Mid-Cretaceous continental shelf ofNorthern Sinai

ŽVariations in the spatial distribution of organic components vitrinite, migrabitumen.and graphite observed in Mid-Cretaceous sediments of Northern Sinai are mainly

attributed to differences in paleoenvironmental settings, i.e., the southernmost profileŽ .Gebel El Minshera is situated closer to the hinterland compared to the other profilesfurther north. However, time-transgressive changes in the depositional facies cannot be

Ž .excluded, considering that the southern section Late Albian to Early Cenomaniancomprises a stratigraphically younger range than those from the center and the northŽ .Late Aptian to Albian; see Fig. 3 . Enhanced deposition of graphite in the central andnorthern part of the study area may indicate a strongly fluvial- to deltaic-influencedsedimentation, whereas dispersion of vitrinite was overall concentrated to the southernsector. The occurrence of well-preserved vitrinite and textinite in transitional marinedeposits requires the presence of a fairly dense vegetation cover close to the site ofdeposition. Hence, humid paleoclimatic conditions must have persisted along thehinterland. This assumption is also supported by long-range aquatic supply of graphiteparticles, which most likely were eroded from the surrounding basement.



Middle Jurassic strata, underlying the studied units and outcropping west of theŽ .central part of our study area Fig. 2 , are known to contain rich potential source rocks,

which usually reveal TOC contents up to 6 wt.%, kerogen types II and III, and anŽimmature to low mature level Aal and Lelek, 1994; Alsharhan and Salah, 1996; Ayyad

.and Darwish, 1996 . In addition, these strata include sub-bituminous coals, deposited inŽ .coal swamp environments adjacent to the coastline Jenkins, 1990; Keeley, 1994 and in

the oxygen-depleted restricted depocentres developed along the segmented rift faultsŽ .Ayyad and Darwish, 1996 . Regarding the formation of migrabitumen in Mid-Creta-ceous sediments of Northern Sinai, these Jurassic strata are expected to be the sourcerocks of migrated hydrocarbons.

A generalized organic facies model of the Mid-Cretaceous Northern Sinai wasdeveloped based on geochemical and optical characteristics of sedimentary organic

Ž .matter Fig. 12 . Typical organic geochemical and petrologic features for the southern,central and northern sections are indicated. Following the comprehensive concept

Ž .presented by Jones 1987 , organic facies deposited along the study area are classified asŽ .types C to D. According to Jones 1987 , deposition of these types of organic facies is

restricted to highly oxygenated environments. In particular, organic facies C occurs onshelves and slopes, whereas organic facies CD is deposited along inner shelf environ-ments, and organic facies D in nearshore settings as well as deep sea basins. Applicationof this concept to the study area indicates a proximal fluvio-deltaic to prodeltaic, oxicshelf environment. A more landward highly oxic area of deposition is suggested fororganic facies D deposits. This facies model is in good agreement with interpretations

Žbased on sedimentary structures, depositional geometries, and microfacies Bachmann.and Kuss, 1998 . Inner ramp sediments prevail in the study area, and only few mid-ramp

incursions have been evidenced from the northern area. In other words, the organicfacies of the Northern Sinai reflect deposition in littoral shallow water environments.

4.2. Estimation of maximum burial depth of Late Aptian to Early Cenomanian depositsof the Northern Sinai

In Northern Sinai, the depth of peak hydrocarbon generation in Jurassic strata wasŽestimated to be approximately 1.7 km to 1.8 km based on vitrinite reflectance data Aal

. Ž .and Lelek, 1994; Alsharhan and Salah, 1996 and in Lower Cretaceous Pre-Cenomanianstrata deposited onshore to be about 1.5 km by means of seismic prospecting and

Ž .computer modelling Ayyad and Darwish, 1996 . Regarding these estimates, T valuesmax

and vitrinite reflectance values obtained in this study add further information. Appar-ently, T values ranging from 3508C to 4258C are too low if compared to vitrinitemax

Ž .reflectance data 0.6% to 0.7% Rm . Since reflectance data are more consistent andv

Ž .Fig. 11. Frequency histograms comparing raw reflectance values measured on vitrinite a and migrabitumenŽ . Ž . Ž . Ž .b with converted reflectance values of migrabitumen c–e according to Jacob 1989 , Bertrand 1993 , and

Ž .Landis and Castano 1995 .˜


Fig. 12. Organic facies model of Mid-Cretaceous depositional settings in the Northern Sinai as deduced fromŽ .organic geochemical and petrological results modified from Jones, 1987; Bachmann et al., 1996 .

therefore are considered to be more reliable than Rock–Eval data, it is concluded thatthe occurrence of migrabitumen may have caused a depression of T values during themax

Ž .analytical cycle Clementz, 1979; Peters, 1986; Snowdon, 1995 . Assuming a normalgeothermal gradient of about 308Crkm for the study area, we conclude that themaximum burial depth of Mid-Cretaceous strata in the Northern Sinai was between 1.2


Fig. 13. Estimated range of paleotemperature and equivalent paleodepth of Mid-Cretaceous sediments of theŽNorthern Sinai, assuming a normal geothermal gradient of about 308Crkm modified from Tissot and Welte,

.1984 .

Ž .km and 2.0 km Fig. 13 . This interpretation corresponds very well with other data fromthe literature and new evidence from illite crystallinity, which was determined on a split

Ž .of our sample material Bauer, 1997 . Clay mineralogical data indicate a burial depth forMid-Cretaceous sediments which was not sufficient for the transformation of montmoril-lonite to illite, based on the absence of alternating bedding minerals as well as

Ž .hydroscopic micas e.g., illite . However, uplift tectonics during the Late Cretaceousmust also be taken into account, and may potentially have caused repeated changes inthe paleo-heat flow and consequently might have affected the maturation of thesediments investigated. This aspect, however, cannot be fully discussed based on thepresent information but requires further work.

5. Conclusion

Organic petrologic and geochemical investigations were carried out on Late Aptian toEarly Cenomanian sediments recovered from the paleoshelf of the Northern Sinai


Ž .Egypt . Characterization of organic facies and evaluation of thermal maturity ofsedimentary organic matter with regard to maximum burial depth were the main scopesof this study.

Ž .Results from organic geochemical analyses C rN , Rock–Eval Pyrolysis per-org tot

formed on kerogen concentrates indicate the dominance of hydrogen-depleted organicmatter of probably terrestrial or degraded marine origin. Kerogen typing reveals amixture of types III and IV. Kerogen type IIrIII was encountered in few samples fromthe central part of the study area, probably related to the presence of hydrogen-enrichedmigrabitumen. Maceral analyses conducted on concentrates support geochemical resultsshowing variable proportions of vitrinite, migrabitumen, graphite and traces of inertinite.Vitrinite and inertinite are considered to be of autochthonous or parautochthonous originsuggesting at least partly humid paleoclimatic conditions in the hinterland. This assump-tion is also supported by the occurrence of graphite, which points to erosion andsubsequent aquatic transport from the surrounding Precambrian basement. The presenceof migrabitumen presumably relates to migrated hydrocarbons from underlying organic

Žcarbon-rich Jurassic strata which outcrop along several anticlines of the study area Fig..2 . The organic facies of Mid-Cretaceous deposits from the Northern Sinai indicates a

proximal fluvio-deltaic or oxic shelf environment equivalent to organic facies C–DŽ .according to Jones 1987 .

T obtained from Rock–Eval Pyrolysis suggests an immature level for the organicmaxŽ .matter 3508C to 4258C , although these values are probably depressed due to the

presence of migrabitumen. Due to the scarcity of vitrinite in kerogen concentrates,reflectance measurements were also performed on migrabitumen. Migrabitumen re-

Ž . Ž .flectance, converted according to Jacob 1989 , Bertrand 1993 and Landis and Castano˜Ž .1995 , ranges from 0.6% to 0.7% Rm , 0.6% to 0.9% Rm , and 0.8% tomb-corr mb-corr

0.9% Rm , respectively. Correlation of both types of reflectance data fits best aftermb-corrŽ .correction of migrabitumen data following the approach of Jacob 1989 . Reflectance

values range between 0.6% to 0.7% Rm, equivalent to high volatile bituminouscoalification stages. Since there is no indication for an abnormal geothermal gradientalong the NE-African margin, a maximum burial depth between 1.2 km and 2.0 km isestimated for the Mid-Cretaceous strata of the Northern Sinai.

Acknowledgements

We would like to thank J. Hower and one unknown referee for critical and instructivecomments on a former version of the manuscript. We are grateful to J. Quick and R.Littke for their readiness for critical discussions and comments. Kerogen concentrationand Rock–Eval measurements, performed at the Institute of Alfred Wegener in Bremer-haven, were supported by U. Mann, B. Boucsein and R. Stein. Nitrogen data wereprovided by P. Muller at Bremen University. Technical assistance during sample¨preparations was provided by M. Brinkmann. We finally thank Prof. Dr. A.M. BassiouniŽ .Ain Shams University, Cairo for various kinds of assistance during the course of fieldwork. Funding for M. Bachmann and J. Kuss was generously provided by German

Ž .Research Foundation DFG-grant Ku 642r10-1,2 .


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Organic facies and thermal maturity of Late Aptian to Early Cenomanian shelf deposits, Northern Sinai (Egypt)