Cretaceous petroleum-bearing rock types—their diagenesis and significance in the Gulf of Suez area, Egypt

Sedimentary Geology, 53 (1987) 269-304 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

269

CRETACEOUS PETROLEUM-BEARING ROCK TYPES--THEIR DIAGENESIS AND SIGNIFICANCE IN THE GULF OF SUEZ AREA, EGYPT

M. H A M E D METWALLI 1, A.M.A. WALl 1 and A. ABD EL-SHAFY 2

1 Department of Geology, Faculty of Science, Cairo University, Giza (EgYpO 2 Gulf of Suez Petroleum Company (GUPCO), Maadi, Cairo (EgypO (Received May 12, 1986; revised and accepted February 2, 1987)

ABSTRACT Metwalli, M.H., Wali, A.M.A. and Abd E1-Shafy, A., 1987. Cretaceous petroleum-bearing rock types--Their diagenesis and significance in the Gulf of Suez area, Egypt. Sediment. Geol., 53: 269-304. Detailed petrographic studies of cores of Cretaceous sediments from the Gulf of Suez area, from both productive and non-productive fault blocks, reveal the rock types, their petrophysics, the nature and distribution of their cements and the effect of diagenesis on hydrocarbon migration and accumulation. An understanding of the petrographic and sedimentologic characteristics of these clastic and non-clastic sediments helps in a valid interpretation of their depositional history and their significance in petroleum exploration. This study is important because of the observed complexity of the virtually unexplored areas and sections of buried Cretaceous rocks in the Gulf of Suez petroleum province, caused by the combined effects of sedimentation and tectonic movements that shaped the Gulf as an intercontinental juncture. There are four petrographic groups, arenite, micrite, wackestone, and claystone. Each group is subdivided into main types and subtypes that reflect the environmental changes within the shallow, fluctuating basin of deposition in the Gulf and related areas. In order to elucidate the porosity changes, the dolomitization process and the clay fractions were investigated. It appears that a post-compactional mechanism is responsible for providing the medium with the Fe 2+, Mg 2+ and K + needed for dolomite formation. Consequently, a porosity decrease may arise from the effect of argillaceous-rich carbonate portions. Also, the decrease of porosity of the Matulla Formation (Lower Senonian sandstone) as compared to the pre-Cenomanian sandstone is the result of increasing overgrowth of secondary silica. Significant dolomitization has increased the porosity of the Lower Senonian--Turonian sediments. This is due to the difference in ionic radii between calcite and dolomite crystal packing. Thus, the petrographic variability of the studied sediments and their diagenesis reflect the variability of their capacity as petroleum-bearing rocks, both laterally and vertically.

INTRODUCTION

This paper deals with a detailed petrographic study of the Cretaceous petroleumbearing rocks in the October oilfield area. 0037-0738/87/$03.50

© 1987 Elsevier Science Publishers B.V.

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The study area is in the Gulf of Suez offshore concessions of Egypt, between 29010 ' and 29°20'N, and 33 ° and 33°10'E. covering an area of about 129 km 2. It lies on the western coast of the Sinai peninsula, where Cretaceous subcrops were penetrated in all drill sites, as well as in many wells in the Gulf of Suez petroleum province (Fig. 1). The geologic sections include post-Miocene, Miocene, Eocene and Paleocene rocks which unconformably overly the Cretaceous successions. The Cretaceous section overlies Paleozoic clastic rocks (Devonian a n d / o r Carboniferous?) with a major unconformity in the Gulf of Suez region. The subcropping sections can be correlated with surface sections in southern and central Sinai. The Lower Miocene (Nukhul Formation), Lower Senonian (Matulla Formation), and sandstone of the pre-Cenomanian clastic unit (Nubia facies) are oil-bearing zones. Petrographic studies of the Cretaceous sediments in this regton are of great importance (Metwalli et al.. 1978; Metwalli et al., 1981), both because of the oil production up to now and for future drilling programs. The October oilfield area is a fault-block structure. It is an example of petroleum production from a fairly young rift basin ("aulacogen") situated in the Gulf of Suez area where continental separation is incomplete, being related to the continuing "break-up" of the African plate. The detailed lithe- and biostratigraphic zonation and tectonic setting of these oil-bearing rocks and their chronostratigraphy are reported by Metwalli et al. (1987). Accordingly, this paper is concerned only with a detailed petrographic and sedimentologic study of the Upper and Lower Cretaceous oil-bearing rocks.

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Upper Cretaceous rocks cover the greater part of northern and central Sinai, extending southward to E1-Tor. These rocks are well represented on the Tih plateau, and form the major part of the structures developed. Lower Cretaceous deposits are widely developed in the Gulf of Suez region, mainly formed of non-fossiliferous clastic sands and sandstones that are overlain by well-developed, shallow marine Cenomanian rocks, and underlain unconformably by black shale series of Carboniferous age. The petrographic study included sixteen cores from well GS 195-B2 (Oct. B2), seventeen cores from well GS 197-B2, with ditch samples from other wells (Fig. 2).

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273 The samples represent the Matulla (Lower Senonian), Wata (Turonian), Abu Qada (Turonian-Cenomanian) and Raha (Cenomanian) formations, and the preCenomanian (Lower Cretaceous) sandstone unit (Fig. 3). MATERIALSTUDIED AND TECHNIQUES Two hundred thin sections were studied using the polarizing light microscope in order to deduce their petrographic clans and their diagenetic features. Contacts between quartz grains were also studied using the universal stage (Glover, 1964). Thin sections representing the dolomite-bearing intervals were stained, using the staining technique of Friedman (1959), in order to differentiate between calcite and dolomite. Six samples (bulk) were submitted to X-ray analysis using an XRD unit Pw/1050 with Ni-filter, Cu-radiation, N = 1.54418 ]k at 30 kV, 18 rnA and abnormal scanning speed of 2 0 min-1 min. Two selected samples of the clay-size fraction have also been analysed using the XRD method for clay analysis after treatment with KC1 (heated to 300 and 500 ° C, glycolated and oriented). PETROGRAPHIC CHARACTERISTICSOF THE CRETACEOUSROCKTYPES The lithologic association could be grouped petrographically into four main groups arranged according to their population (Fig. 3), with a few derivative subtypes reflecting environmental changes in the Cretaceous depositional basin. Arenite group

This group is characterized by the suffix "arenite", indicating that quartz grains form more than 95% of the bulk. It is represented by a single main type, quartz arenite, and five types (types 1-5). Quartz arenite This type represents the highest population within the arenite group which is characterized by more than 95% quartz, as used by Gilbert (1954), McBride (1963), and Pettijohn et al. (1972). The quartz arenite is commonest in the pre-Cenomanian clastic unit (well GS 195-B2, cores 14-16) where quartz grains are single (rarely composite) showing straight to slightly undulose extinction (Fig. 4). The quartz grains are more or less equigranular to subrounded, showing overgrowth in optical continuity (Fig. 5) suggesting a plutonic origin (Krynine, 1940). The average size is 0.53 mm according to the Wentworth scale (1922), matching with coarse sands. Inclusions are recorded as prismatic zircon crystals, rutile needles in parallel or intersecting trains, which support the plutonic origin. Secondary silica cement leads to different contacts, mainly of slightly curved type, indicating a moderate- to low-pressure solution effect (Pettijohn et al., 1972). Calcite is recorded as a minor

274

Fig. 4. Equigranular quartz arenite (oil pay-zone), showing overgrowth of secondary silica in optical continuity (well GS 195-B2, Core 15, depth 3830.5 m). Crossed polarizers, bar is 100 #m.

cementing material, scarcely filling free spaces between grains. This mode of cementation is only recorded at the top of the pre-Cenomanian clastic unit (core 12 in well GS 195-B2). This type is mainly restricted to the Lower Cretaceous sandstone, which is considered as the main reservoir in the October oilfield, as welt as in the Ramadan oilfield and in the B-trend area, both also in the Gulf of Suez (Fig. 1). The quartz arenite includes five types: (1) Calcareous quartz arenite. This type is recorded in two horizons, the upper in cores 2 and 8 (Lower Senonian/Matulla Formation), the lower in core 12 in well GS 195-B2 ( C e n o m a n i a n / R a h a Formation). The sizes of quartz grains range between 0.42 and 0.28 ram, which is the abundant size. The quartz grains are single, and show straight to slightly undulose extinction. The pressure-solution effect is clear, resulting in different contacts. The framework is cemented by calcite as random patches (Fig. 6) with floating quartz relicts, Going downward (core 12), the quartz grains show a remarkable decrease in the grain size (0.105 ram) and increase

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Fig. 5. Quartz arenite (oil pay-zone)with secondary overgrowthsof silica. Contacts between grains are straight to gulfing (well GS 195-B2,core t5, depth 3831 m). Crossed polarizers, bar is 100/~m.

275

Fig. 6. Calcareous quartz arenite; calcite is corroding most of quartz grains (well GS 195-B2, core 12, depth 3642 m). Crossed polarizers, bar is 200/~m.

in the calcareous cement. This is observed until the cementing material reaches 35%. This change in size might indicate a multisource of the sands. Some quartz grains tend to be ill-sorted, others are brecciated. (la) Fossiliferous calcareous quartz arenite. This subtype is recorded in cores 12 and 13 in well GS 195-B2 (Raha Formation) where shell fragments constitute 5% of the framework. In core 12, at 3680 m, the quartz grains are similar to those previously discussed, while the calcareous cementing material is made up of recrystallized micritic material reaching the size of microsparite (Fig. 7). In core 13, at 3700 m, quartz grains are embedded within micritic lime mud showing slight corrosion of quartz grains. A clear sign of aggrading recrystallization is also detected, indicated by the presence of a bird's-eye texture in thin section. Secondary calcite is recorded as additional cementing material which shows intensive corrosion in quartz grains and a lesser degree of corrosion in the recorded bioclasts. (lb) Glauconitic calcareous quartz arenite. This subtype is recorded in cores 1 and 8 in well GS 195-B2 (Matulla Formation). The quartz grains are similar to subtype la except for the intense corrosion by calcite cement in the form of random patches of different optical orientation. Pyrite is the only opaque mineral observed. It appears as discrete crystals or clusters of tiny crystals (framboids). Early di-

Fig. 7. Fossiliferous glauconitic micritic quartz arenite. Notice the aggrading crystallization within the micrite (well GS 195-B2, core 7, depth 3403.5 m). Crossed polarizers, bar is 200/.tm.

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Fig. 8. Glauconitic dolomitic fossiliferous quartz arenite (well GS 195-B2, core 8, depth 3421 m). Crossed polarizers, bar is 200/~m.

agenetic pyrite tends to occur dispersed in the matrix as framboids. Late diagenetic pyrite occurs as concentrations, and is tran~essive on other minerals. Glauconite is a major part of the framework, attaining 30%. It is equigranular, rounded, ovoidal and discoidal. The colour ranges from green to brownish-green, reflecting different degrees of oxidation (Fig. 8). Pyrite framboids are a common minor constituent, always associated with glauconite at the center and the peripheries, indicating prevailing reducing conditions. Detrital zircon is rarely present. In addition to the previously mentioned minor constituent, phosphatic rock fragments (collophane) are also recorded. (lc) Calcareous glauconitic quartz arenite. This subtype is recorded in core 10 in well GS 195,B2 (Matulla Formation), where there is a remarkable increase of glauconite (> 30%) showing different degrees of oxidation (Fig. 9). The most oxidized are those which correlate with periods of reworking. Quartz is similar to the previously mentioned subtypes, except in the unusual increase of their size. suggesting the possibility of different sources. A few grains of K-feldspars (microcline) and plagioclase are recorded. The cementing material is mainly calcite which corrodes in glauconite. Minor argillaceous matrix is also recorded (5%).

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l Fig. 9. Calcareous glauconitic quartz arenite. Notice the etching and corrosion of calcite in the different components (well GS 195-B2, core 10, depth 3455 m). Crossed polarizers, bar is 200/~m.

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Fig. 10. Glauconitic quartz arenite. Notice effect of pressure on glauconite G (well GS 195-B2, core 8, depth 3421.5 m). Crossed polarizers, bar is 200/tin.

(ld) Glauconite quartz arenite. This subtype is recorded in cores 2, 8, 9 and 10 in well GS 195-B2 (Matulla Formation). Glauconite grades up to 40%, allowing the rock to be termed "green-sand" (Fig. 10). Quartz grains are similar to those previously described, except for their secondary precipitated silica cementing material, resulting in different types of contacts (straight and curved), indicating low to moderate pressure-solution effects. Argillaceous constituents increase to 10% of the rock. Pyrite framboids are also found. (le) Glauconitic fossiliferous calcareous quartz arenite. This subtype is recorded in cores 6, 8 and 9 in well GS 195-B2 (Matulla Formation), and in core 12 (Raha Formation). The quartz grains decrease in size from 0.57 to 0.18 mm, They are of single type, angular to subangular, and show straight extinction. Inclusions are rare, mainly zircon and futile needles. Up to 5% interstitial glauconite occurs between quartz grains. The pressure-solution effect is seen clearly on the glauconite. Bioclasts are arranged in parallel, reflecting calm deposition conditions (Fig. 11). Calcite cements corrode framework grains. Micrite matrix is also recorded. In core 12, calcareous cement increases downwards until it completely replaces quartz grains and highly corroded shell fragments.

Fig. 11. Glauconitic fossiliferous calcareous quartz arenite. Load effect on glauconite is helpful in top and bottom determinations (well GS 195-B2, core 8, depth 3421.5 m). Crossed polarizers, bar is 100/xm.

278

(2) Dolomitic quartz arenite. This type is recorded in core 7 in welt GS 195-B2 at 3999.3 and 3400 m (Matulla Formation), and in core 13 at 3707 and 3707:5 m (Raha Formation). The quartz grains are identical to those previously described. Dolomite rhombs range in size from 0.06 to 0.09 mm. with subedutopic to idiotopic habits. Rhomb faces are best developed in the largest crystals, where the rich argillaceous nests contain the highest concentration of the first-formed dolomite. Dolomite rhombs of varying sizes are encountered as nests within the argillaceous-rich carbonate portions. The minimum size of 0.06 mm corresponds to minute dolorhombs taking the form of condensed rhombs embedded within the micrlte. indicating the latest generation of dolomite. The larger sizes are always found further from the argillaceous-rich nests, indicating the continued disappearance of these nests by continued creation of dolorhornbs. Accordingly, each phase could be interpreted as a result of deposition. This observation suggests the argillaceous-rich medium as a source of Mg 2÷ and Fe 2÷ ',.

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Sample 2: Core 71 3 4 1 8 m depth ; Sample 5 : C o r e 12, 1 3 , 3 7 0 7 m depth .

Fig. 12. X R D p a t t e r n s o f s o m e selected s a m p l e s (bulk). A b b r e v i a t i o n s : ch = chlorite; il = illite; m o n t = m o n t m o r i U o n i t e ; s m = s m e c d t e ; k a = k a o l i n i t e ; ca = calcite; A n = A n k e r i t e ; a n d q z = q u a r t z .

279

Fig. 13. Dolomitic quartz arenite, Rhombs of dolomite are seen growing up from argillaceous-rich carbonate nests (well GS 195-B2, core 12, depth 3647 m). Crossed polarizers, bar is 100/Lm.

The dolomite (Fig. 12) is of ferroan type (ankerite). Minute dolorhombs are recorded as cementing material of the portions previously filled with the argillaceous-rich carbonate matrix, while the larger sizes act as the cement and show clear signs of corrosion in quartz (Fig. 13). Zoned dolorhombs are restricted to these larger sizes, whereas minute zoned rhombs are not seen. Zoning is recorded as a dusty cloud or as framboidal pyrite regularly scattered as rhombs or as spotted pyrite, where both previously mentioned forms give the primary boundary shape and are defined by the zone itself (Fig. 14). This means that some of the dolorhombs took on an edutopic form (Friedman, 1965) i.e., those which grew continuously, reflecting favourable conditions of dolomitization, whereas the others were of xenotopic form, i.e., conditions favourable for dolomitization were not perfect. This is always found as concentrated dolomite in the rich nests of argillaceous carbonate. The dolorhombs range between 0.105 and 0.53 mm in diameter. Zoned dolorhombs show that zoning is mainly due to high concentrations of Fe 2+ and Mg 2+. This is probably due to substitution between Fe 2+ and Mg 2+ ions (Katz, 1971)

Fig. 14. Dolomitic quartz arenite. Notice the rounded zone of framboidal pyrite, also the close association between framboidal pyrite and dolomitized areas (well GS 195-B2, core 7, depth 3399 m). Crossed polarizers, bar is 100/~m.

280

Fig. 15. Dolomite is seen replacing lime-mud; note the non-susceptibility of bone material to replacement (well GS 197-2, core 16, depth 3814 m). Crossed polarizers, bar is 100/~m.

which means that the activity of Fe 2+ is increasing and is at least greater than that of Mg 2+. The increase in Fe 2+ would prevent the formation of dolomite. This would be in accordance with prevailing reducing conditions explaining the zoned pattern of Fe2+-rich clouds; it is also supported by the existence of pyrite framboids recorded in the previous boundary area within the dolorhombs (Fig. 14), i.e., the prevailing environmental condition is believed to be of reducing character (swamp, lagoonal, etc.). (2a) Dolomitic fossiliferous quartz arenite. This subtype is recorded in core 12, well GS 195-B2, at 3647 m (Raha Formation), and in core 16, well GS 197-2 at 38t4 m. The quartz grains tend to be larger in size, the maximum size reaching 0.53 mm and the minimum 0.105 ram. These sizes correspond to coarse sand and very fine sand, respectively (Wentworth scale, 1922). Dolorhombs are associated with argillaceous-rich carbonate portions, where dolomite crystals are seen replacing quartz. Some bioclasts are not replaced by growing dolomite. This indicates that the composition of these bioclasts controlled their susceptibility to replacement. It is clearly detected in the embedded shell fragments within the argillaceous-rich carbonate nests in which dolomite is formed and replaces quartz grains. On the other hand, the shell fragments are neither replaced nor show any sign o f corrosion (Fig. 15). (2b) Glauconitic dolomitic quartz arenite. This subtype occurs in core 7 at 3400 m, and in core 8 at 3418 m in well GS I95-B2. This subtype is differentiated from subtype 2a by the presence of green glauconite pellets with an average size of 0i026 mm. Dolomite is always associated with the ~ ~ u s ~ f i c h portion as v a r y i ~ sized rhombs, either zoned or not. Zoned rhombs are of double clouds a n d / o r spotted by pyrite, while the unzoned rhombs arealways minute. A ~ t e replaces both quartz and glauconite at almost the same rate (Fig. I6). (2c) Dolomitic glauconitic arenite. This subtype is recorded in core 8 at 3420 m in well GS 195-B2 (MatuUa Formation). It is similar to subtype 2b, except for the remarkable increase in glauconite, reaching about 30-40%, with an average size of

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Fig. 16. Glauconitic dolomitic quartz arenite. Notice the clear etching and corrosion of dolomite in glauconite (G) and quartz (Q), respectively (well GS 195-B2, core 8, depth 3418 m). Crossed polarizers, bar is 100 p~m.

0.28 mm. Framboidal pyrite is seen in association with glauconite as scattered centers or individuals, and at the boundary of the argillaceous-rich portions. This is also observed with dolomite, reflecting the prevalence of reducing conditions. (2d) Glauconitic fossiliferous dolomite quartz arenite. This subtype is recorded in core 7 at 3400 m in well GS 195-B2 (Matulla Formation). It differs from subtype 2c by the presence of up to 20% of elongated calcitic shell fragments. The parallel shell fragments alternate with rich bands of glauconite and quartz. This facies reflects calm conditions of deposition and a reducing environment, suggesting that deposition was below base level in relatively deep water far from oxygenation (Figs. 11 and 17). Ankerite is still recorded in association with argillaceous carbonate nests which are associated with a green band (rich in glauconite and quartz). Glauconite plays an important role in determining the top and bottom features, as well as the direction affecting the stress and even its amplitude, due to its immaturity softness (Fig. 11).

Fig. 17. Glauconitic fossiliferous dolomitic quartz arenite. Notice the preferred direction due to load effect (well GS 195-B2, core 8, depth 3421 m). Crossed polarizers, bar is 100 ~tm.

282

Fig. 18. Ooidal gtaucortitic biomicrite (well GS 195-B2, core 7, depth 3404 m). Crossed polarizers, bar is 200/am.

(2e) Dolomitic calcareous quartz arenite. This subtype is similar to subtype 2b, except for the absence of glauconite and the presence of calcite as a cement and as disseminated patches.

(3) Ooidal glauconitic fossiliferous micritic quartz arenite. There are two prominent ooid-bearing horizons in the lower Senonian/Matulla Formation. The upper one is recorded within the blocks of wells GS-160, GS-173, GS-135 and GS-195, the lower one over the whole field area. These two horizons are of environmental significance. This petrographic type represents the upper horizon, it is recorded in core 7. at 3403.5, 3404 and 3405 m in well GS 195-B2. The framework is built up of quartz grains, shell fragments, forams and ooids. The ooids make up about 40% of the rock. They are equigranular (0.35 mm), subrounded to rounded, rarely in direct contact, embedded in recrystallized micrite matrix of microspar size, and show bird's-eye texture (Fig. 18). Quartz grains are sometimes found as ooid nuclei. indicating their derivation and accumulation in nearshore conditions. Shell fragments, forams and glauconite are, however, more common nuclei in the ooids. Petrographicalty, the quartz is similar to that previously described. The aggrading recrystallization character of micrite shows a clear sign of corrosion in quartz grains and slight etching in ooids (Fig. 19). Glauconite grains are green, equigranular and subrounded, with an average size of 0.21 mm. Framboidal pyrite is recorded as a minor constituent in concentrated portions and as disseminated framboids. This petrographic type corresponds to the near-shore e n ~ ¢ environmental conditions of Folk (1974). (4) Calcareous sublitharenite. This type conta~as 10-25% of sedimentary rock fragments. Accordingly, it is sublitharenite ( t ~ 1975; MeBride, 1963), or the protoquartzite of Krynine (1948). It is recorded in cores 1, 2 and 3 in well GS 195-B2 (MatuUa Formation). The quartz grains are fine, with an average size of 0.20 mm, subangular to subrounded, mostly single, rarely composite, and show straight

283

Fig. 19. Dolorhombs replacing the ooids in an outward-inward direction (well GS 195-B2, core 7, depth 3412 m). Crossed polarizers, bar is 100/~m.

Fig. 20. Calcareous sublitharenite (well GSY 195-B2, core 1, depth 3374.5 m). Crossed polarizers, bar is 100 ~*m. to slightly u n d u l o s e extinction. Inclusions are m a i n l y of rutile needles, zircon crystals a n d a b u n d a n t vacuoles. R o c k f r a g m e n t s are m a d e up of p h o s p h a t i c m a t e r i a l ( m a i n l y c o l l o p h a n e a n d apatite). Calcite c e m e n t shows intense c o r r o s i o n against q u a r t z grains a n d slightly c o r r o d e s the p h o s p h a t i c m a t e r i a l (Fig. 20).

Fig. 21. Sedarenite. Notice the high degree of compaction, mainly due to load effect (well GS 195-B2. core 1, depth 3370 m). Ordinary light, bar is 0.62 mm.

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Fig. 22. Glauconitic calcareous quartz sedarenite. Notice the pressure effect on glauconite by quartz grains (well GS 195-B2, core 9, depth 3436 m). Crossed polarizers, bar is 100/tm.

(4a) Calcareous biosublith, arenite. This subtype is similar to type 4~ except for the presence of about 20% of recrystallized calcitic shell fragments.

(5) Sedarenite. This type is recorded in core 8, well G S 195-B2 at 3425.5 m (Matulla Formation). It is mainly made up of more than 60% p h o s p h a t e (collophane). The framework is welded by tightly compacted shell fragments (Fig, 21). This type represents the highest population of phosphate rock fragments in the t ~ sections studied. (5a) Glauconitic calcareous quartz sedarenite. ~ s subtype is recorded in core 7 at 3413 m; and core 9 at 3436 m in well GS 195-B2. The framework is made u p of sedimentary rock fragments similar to the main type, except for an increase of quartz, glauconite and calcareous cement reflecting shallowing of the basin of deposition (Fig. 22). Minute dolorhombs are recorded within the argillaceous carbonate matrix. This subtype is less common than type 5. Micrite group This main type is recorded in core 1 at 3365 m, core 4 at 3388 m and core 9 at 3445 m, in well GS 195-B2 (Matulla Formation), and in core 15 at 3804 m in well

j Fig. 23. Dolornitized micrite (well GS 197-2, core 15, depth 3804 m). Crossed polarizers, bar is 200 ~m.

285 GS 197-2 (Abu Qada Formation). Micrite is of an average size of 1-4 ~m, equivalent to the very coarse crystalline of Folk (1974). The majority of micrite crystals are more or less of the same size. Others show aggrading recrystalhzation reaching the microspar size of 0.005-0.02 ram, with the bird's-eye texture of Wolf (1963). Framboidal pyrite is recorded as disseminated a n d / o r concentrated patches, indicating reducing conditions. Rare discrete dolomite rhombs are embedded in the micrite matrix (Fig. 23). The dolorhombs are clear, without zoning, subidiotopic to idiotopic, showing clear signs of replacing micrite crystals, though the manner of replacement is observed to be selective. Selectivity occurs where there is a high concentration of argillaceous carbonate. There are minor scattered quartz grains.

Biomicrite This type is recorded in core 1 at 3373 m in well GS 195-B2 (Matulla Formation), and core 16 at 3811 m in well GS 197-2 (Abu Qada Formation). The framework is built up of shell fragments and fossils embedded in a micritic matrix, with some scattered glauconitic pellets, quartz grains and pyrite framboids. Micrite tends to recrystallize with an aggrading character up to the microspar size, with bird's-eye texture. Dolomitization is absent, and the rock is almost free of terrigenous material. Quartz and glauconitic pellets are rarely present. Glauconite pellets are observed randomly scattered, but in association with framboidal pyrite, reflecting the prevalence of reducing conditions. Glauconite grains are elliptical-to-rounded green pellets with hazy boundaries due to corrosion by the aggrading recrystallization of micrite. There is a calcite cement (Fig. 24).

(1) Sandy biomicrite. This subtype is recorded in cores 1 and 5 at 3364.5 and 3397 m (Matulla Formation) in well GS 195-B2 and in core 16 at 3811 m in well GS 197-2 (Abu Qada Formation). The framework is made up of a relatively high percent of quartz grains (10%) embedded in a micritic matrix, in addition to shell fragments and microfossils which show no signs of reworking, supporting the prevalence of calm conditions. The relative increase of quartz grains is believed to result from derivation to the basin of deposition rather than shallowing. Some intraclasts are

Fig. 24. Biomicrite(well GS 197-2, core 16, depth 3811 m). Crossed polarizers, bar is 20(I /~m.

286

Fig. 25. Sandy biomicrite (well GS 197-2, core 10, depth 3741 m). Crossed polarizers, bar is 200/~m.

recorded as scattered portions, where their percentage grade them up to sandy intrabiomicrite (Fig. 25). Recrystallized mierite is observed corroding in shell fragments and quartz grains, while microfossils are replaced by calcite. Pyrite is also relatively high ( _ 8%); it is seen as scattered a n d / o r accumulated concentrates, and replaces the soft parts and shell walls pseudomorphology (Fig, 26). Dolomitic micrite

This type is recorded in core 4 at 3385.5 m in well GS 195-B2 (Matulla Formation) and in core 16 at 3805 m in well GS 197-2 (Abu Qada Formation). It is to some extent similar to the main type of micrite, except for the dolomitization process, which reaches a high degree of obliteration of the original fabric. Dolorhombs are widely spread within this type, and always associated with the argillaceous-rich nests from which dolorhombs are observed to be growing (Fig. 27). They have a roughly equal size of 0.043 ram, whereas some of the larger rhombs are zoned crystals, mostly with dark centers. Pyrite framboids are observed in concentrated laminae alternating with dolomitized micritic ones, indicating quiet, calm conditions alternating with a reducing environment. This type includes seven subtypes:

Fig. 26. Complete replacementof foraminiferaltest by pyrite as pseudomorphs (well OS 197-2, core 16, depth 3810.5 m): Crossed polarizers, bar is 100 gm.

287

Fig. 27. Nests of dolomite (well GS 197-2, core 16, depth 3805 m). Crossed polarizers, bar is 100 /tin.

(1) Glauconite dolomicrite. This subtype is recorded within core 9 at 3443 m in well GS 195-B2 (Matulla Formation). The glauconite reaches 5-20% as randomly scattered green pellets, mostly rounded (0.175 mm), associated with pyrite framboids which are either in concentrated patches within the glauconite or at the periphery, indicating the prevalence of reducing conditions (Fig. 28). The concentrated framboids within the glauconite pellets are believed to be due to degradation in glauconite leading to formation of FezS; the iron-rich spots on the glauconite surface are susceptible to pyrite formation rather than other factors (Fig. 28). This observation is in harmony with E1-Sharkawi and E1-Awadi's observation (1982) in the Burgan oilfield, Kuwait. (2) Sandy dolomitic oo-sparry-micrite. This subtype is recorded in core 7 at 3412 m in well GS 195-B2 (Matulla Formation). The framework is built up of scattered single quartz grains (average size 0.175 ram). They are occasionally in contact, and show low to moderate pressure-solution effects. This is indicated by the presence of straight to curved contacts. Dolorhombs are identical to those previously described. They are seen replacing quartz grains and deformed oolites (Fig. 29). Oolites are

Fig. 28. Glauconitic dolornicrite. Dolorhombs are replacing preferably the glauconite grains (G) (well GS 195-B2, core 7, depth 3412 in). Crossed polarizers, bar is 100 ~tm.

288

Fig. 29. Sandydolomitic oo-sparrymicrite. Oolite with quartz nucleus is replaced by growing dolorhombs (well GS 195-B2, core 7, depth 3412 m). Crossed polarizers, bar is 100 #m.

present as oval, elliptical or mostly rounded, in contact until they display embayment and gulfing contacts. This indicates intense degrees of deformation. Deformed oolites are partially a n d / o r completely replaced by siderite in an outward-inward manner. This forms circular radiating crystals, where ghosts of oolite nuclei are still detectable (Fig. 30). The periphery of the replaced oolites is clear and traceable, due to the size difference between dolomitized micrite and siderite. The interstitial pores which are not filled with dolomite are filled with secondary precipitated calcite.

(3) Glauconitic sandy biodolomicrite. This subtype is recorded within core 7 at 3413, 3414.5 and 3415 m in well GS 195-B2 (Matulla Formation). It is similar to subtype 1, except for the presence of a relatively high percentage of single quartz grains (10-15%), and shell fragments, whereas glauconite is decreased (Fig, 3:1). Quartz grains of average size (0.072 mm) and shell fragments are believed to be derived through transportation and deposition in a calm reducing environment: Framboidal pyrite occurs in concentrated patches a n d / o r trails, confirming the prevalence of

Fig. 30. Pseudomorphicreplacement of siderite after oohte (wcU GS 195-B2, core 8, depth 3433.5 m). Crossed polarizers, bar is 100/~m.

289

Fig. 31. Glauconitic sandy biodolomicrite (well GS 195-B2,core 8, depth 3420 m). Crossed polarizers, bar is 100 ~tm.

reducing conditions. Glauconitic pellets are similar to those previously described in subtype 1.

(4) Fossiliferous glauconitic sandy dolomicrite. This subtype is recorded in core 7 at 3415.5 m in well GS 195-B2 (Matulla Formation). Microfossils, glauconitic pellets and quartz grains are observed embedded in dolomitized micritic matrix. The dolorhombs are growing up from the argillaceous-rich portions as in the previously described conditions. Dolorhombs of average size (0.035 mm) are observed replacing glauconite (and quartz to a lesser extent), while the fossils encountered are not in contact with muddy portions and are not replaced. Some bioclasts are in contact with the dolomitized portions; these are not susceptible to dolomitization. This may be due to mineralogical composition rather than to other factors (Fig. 32). There are some pyrite framboids.

(5) Glauconitic sandy biodolomicrite. This subtype is recorded in core 7 at 3414 m in well GS 195-B2 (Matulla Formation) and in core 16 at 3808 m in well GS 197-2 (Abu Qada Formation). The framework is similar to that of subtype 3, except for the presence of intraclastic portions as reworked parts of biomicritic rock trans-

Fig. 32. Fossiliferous glauconitic sandy dolomicrite (well GS 195-B2, core 7, depth 3415 m). Crossed polarizers, bar is 100 ~tm.

290

• i ~ :,•¸¸ Fig. 33. Glauconitic sandy bio-intradolomicrite (well GS 197-2, core 16, depth 3808 m). Crossed polarizers, bar is 200/~m.

ported and embedded within the micritic matrix. The framework is replaced to differing degrees by dolomite (Fig. 33).

(6) Biodolomicrite. This subtype is recorded in core 9 at 3444 m in well GS 195-B2 (Matulla Formation) and in core 16 at 3804.5 m in well GS 197-2 (Abu Qada Formation), Dolorhombs are always associated with argillaceous-rich portions. They have average diameters of 0.01-0.07 m m (Fig. 34). There is a similar manner of replacement interaction between dolomicrite crystals and fossils to that found in subtype 5. (7) Glauconitic sandy oo,biodolomicrite. This subtype is recorded in core 7 at 3415.5 m in well GS 195-B2 (Matulta Formation) and in core 11 at 3766 m in well GS 197-2 (Abu Qada Formation). It is similar to subtype 3, except for the presence of a relative high percentage of ooids (20-30%) embedded within the dolomitized micritic matrix in concentrated patches and isolated individuals. All of them show clear signs of intense reworking, indicating a high-energy environment. Signs of reworking are easily detectable, varying from slight obliteration of the outer

Fig. 34. Dolomitized micrite. Notice the non-susceptibilityof shell fragments to dolomitization (well GS 197-2, core 16, depth 3804.5 m). Crossed polarizers, bar is 200 #m.

291

Fig. 35. Glauconitic sandy oo-biodolomicrite.Notice the degree of deformation which is connected with the degree of dolomitization (well GS 195-B2, core 7, depth 3405.6 m). Crossed polarizers, bar is 200 ~m.

periphery to complete fragmentation, Those which are highly deformed show a higher accessibility to dolomitization than those less deformed (Figs. 35 and 36).

Wackestone group Generally, greywackes are distinguished by their dark-grey appearance, and abundance of both feldspars and rock fragments without internal stratification. The matrix consists of a fine-grained intergrowth of sericite and chlorite, with some silt-sized quartz and feldspars. The problem of percentage was discussed by Gilbert (1954) and Dott (1964); they both accepted the figure of 10%. Okada (1971) noted that the matrix percentage ranges from 5 to 25%. Most workers have chosen 15%. The upper size limit varied from 20 /~m (Okada, 1971) to 30 /zm (Pettijohn et al., 1972).

Quartz waeke This term has been used to define a greywacke with more than 95% quartz (Folk, 1974). This type is recorded in cores 7, 8, 10, 14 and 15 in well GS 195-B2. The

Fig. 36. Less deformed oolite showing lesser affinity to dolomitization (well GS 197-2, core ll. depth 3766 m). Crossed polarizers, bar is 200 #m.

292

Fig. 37. Quartz wacke (well GS 195-B2, core 7, depth 3414.5 m). Crossed polarizers, bar is 200 gm.

quartz is fine to very fine grained (0.105 mm), subangular to subrounded, rarely angular. Most of the grains are single and show straight to slightly undulose extinction. Zircon is recorded as inclusions. Some portions of fresh feldspars (0.07 mm) and microcline (0.029 mm) are detected, The matrix is made up of argillaceous material and reaches 15-30% (Fig: 37). The rock is lithified by pressure solution and secondary quartz. This cement appears as overgrowth, and has different types of contacts, mostly of slightly curved type, indicating low to moderate pressure-solution effects. (1) Glauconitic quartz wacke. This type is recorded in cores 7, 8, 9 and 10 in well GS 195-B2 (Matulla Formation). The quartz grains are ill-sorted, sub~gular to sub, rounded, with an average size of 0,17 ram. The rocks are fissile and show graded bedding, indicating deposition under quiet,water conditions. Glauconite pellets. pale yellowish-green to yellowish,brown, are recorded, reflecting oxidizing conditions. These pellets mostly fill the interstitial spaces between quartz grains with an average size of 0.105 mm, minimizing the percentage of porosity (Fig. 38), There is a clay matrix. The rock is lithified by pressure-solution and secondary quartz,

!.

i

.

.

.

.

.

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Fig. 38. Olauconitic quartz wack¢ (well GS 195-B2, core 8, depth 3420 m). Ordinary light, bar is 0.46 nlrn.

293

Fig. 39. Calcareous glauconitic quartz wacke, the ground-mass is dolomitized (well GS 195-B2, core 7. depth 3416.5 m). Crossed polarizers, bar is 200 ~tm.

resulting in different types of contacts between quartz grains. Few phosphatic rock fragments of collophane are recorded. (la) Calcareous glauconitic quartz wacke. This subtype is recorded in cores 7, 9 and 10 in well GS 195-B2. The quartz is fine, ill-sorted and single, and shows straight extinction. Straight to curved contacts are predominant (Fig. 39). The groundmass is argillaceous. The rock is lithified by pressure-solution and secondary calcite (15%). The calcite cement shows clear signs of corrosion in quartz grains. Framboidal pyrite is recorded as a minor constituent in scattered patches. (lb) Glauconitic calcareous quartz wacke. This subtype is recorded in core 10 at 3468 m in well GS 195-B2. It is similar to subtype la, with some differences in the percentage of glauconite and calcareous cement. The glauconite decreases to 5%, whereas in subtype l a it reaches about 20%; calcite cement increases to 15%. (lc) Calcareous fossiliferous glauconitic quartz wacke. This subtype is recorded in core 10 at 3463.5 m, and core 8 at 3432 m in well GS 195-B2. The quartz grains are similar to those described in the former subtypes. It is characterized by the presence of glauconite in the oxidized state, showing staining with organic pigment

Fig. 40. Calcareous fossiliferous glauconitic quartz wacke (well GS 195-B2, core 8, depth 3435 m). Crossed polarizers, bar is 200 ~tm.

294 r

Fig. 41. Calcareous fossiliferousquartz wackeshowing load effect (well GS 195-B2,core 14, depth 3779:5 m). Ordinary light, bar is 200 #m.

(Fig. 40). The cement is made up of secondary calcite. Some scattered bioclasts are also observed in the groundmass. (ld) Calcareous fossiliferous quartz wacke. This subtype is recorded in core 10 at 3463 m and in core 14 at 3780 m in well GS 195-B2. It is similar to subtype lc, except for the complete absence of glauconite. Energetic conditions are easily detectable, confirming the prevalence of oxidiziug conditions. The quartz grains are ill-sorted and brecciated, with some wedge-like forms in a parallel manner (Fig. 41). The cementing material consists of scattered patches of secondary calcite, which corrodes m quartz grains. (le) Dolomitic quartz wacke. This subtype is recorded in core 8 at 3420 m and in core 11 at 3634 m in well GS 195-B2 (Raha Formation). The quartz is single, ill-sorted, subangular to subrounded, and shows straight to undulose extinction. Dolorhombs are observed in association with argillaceous-rich carbonate portions. They form the cementing material, and fill up all the interstitial pore spaces. reducing the porosity. Most of the larger dolorhombs are zoned. The replacive manner between dolorhombs and quartz is clearly detectable (Fig. 42).

m

k. Fig. 42, Dolomitic quartz waeke (well GS i95-B2, core 8, depth 3420 m). Crossed polarizers, bar is 100 /.tm.

295

Claystone group This group is recorded on the top of the pre-Cenomanian clastic unit (Lower Cretaceous), in core 10 at 3456, 3469 and 3470 m, also between 3827 and 3829 m in well GS 195-B2. It marks a major influx of fine to very fine terrigenous material to the basin of deposition. This group is represented by a single petrographic type containing more than 75% clay matrix. Accordingly, it is a claystone (Pettijohn, 1975). Very fine and even silt-size quartz grains are present as bands alternating with clay-rich ones. This is clear evidence of quiet-water conditions (Fig. 43). In some other cases, these bands are observed alternating with rich glauconitic layers (Fig. 44), indicating the prevalence of reducing media and calm conditions of deposition. Some convolute structures are well-displayed by quartz accumulations, which are attributed to load effect. The former examples are reported to indicate the top and bottom directions (Fig. 45). These claystone group examples are considered to form the sealing rocks for the

Fig. 43. Laminated claystone, indicating deposition in calm conditions (well GS 195-B2, core 10, depth 3469 m). Ordinary light, bar is 1.87 mm.

Fig. 44. Banded claystone with rhythmic pattern. Rich band of glauconite defines the rhythm (well GS 195-B2, core 10, depth 3456 m). Ordinary light, bar is 3.12 rnm.

296

Fig. 45. Claystone with quartz-rich band. Trough feature indicates top and bottom direction (well GS 195-B2, core 10, depth 3470 m). Ordinary light, bar is 3:21 ram.

oil-bearing zones in the pre-Cenomanian sandstone unit in the October oilfield area, as well as in similar concessions in the Gulf of Suez region. DIAGENESIS OF THE STUDIED ROCK TYPES

Diagenesis of the sandstone

Physical changes These include compaction, pseudomorphism, crossing relations, and frequent occurrence of oil and gas accumulations. (a) Compaction. The effect of load on the sediments is a strong evidence for top and bottom arrangements, and is a tool in studying disturbed sections (Figs. 45-47). Compaction has halved the porosity (Figs. 11, 37, 46 and 48). (b) Pseudomorphism. Pseudomorphic replacement of shells by sparry mosaic calcite is clear in Fig. 25, defining the altered outline of the shell fragments. (c) Cross-cutting. Cross-cutting relations are evidenced by the e t c ~ g and embayment of detrital grains of quartz by single crystals or mosaics of calcite (Fig. 6).

Fig, 46. Effect of load on glauconite grams (well GS 195-B2, core 8, depth 3421.5 m). Crossed polarizers, bar is 100 ~m.

297

Fig. 47. Load effect, helpful for top and bottom determination (well GS 195-B2, core 8, depth 3420 m). Crossed polarizers, bar is 200/zm.

Fig. 48. Deformation effect (load) on glauconite (well GS 195-B2, core 7, depth 3388 m) Crossed polarizers, bar is 100 Izm.

(d) F r e q u e n t o c c u r r e n c e of p e t r o l e u m . T h e frequent o c c u r r e n c e of oil a n d gas a c c u m u l a t i o n s indicates the activity of diagenetic processes (Figs. 4 9 - 5 1 ) which m i g h t have b e e n r e s p o n s i b l e for the m i g r a t i o n of oil into these s a n d s t o n e reservoir pay-zones.

Fig. 49. Overgrowth in optical continuity on quartz grains (well GS 195-B2, core 15, depth 3830.5 m). Crossed polarizers, bar is 100/~m.

298

Fig. 50. Pressure solution effect on quartz grains (well GS 195-B2, core 1, depth 3375 m). Crossed polarizers, bar is 200/tm.

Chemical changes These changes include cementation, dissolution and reerystallization~ (a) Cementation. The most common cementing ma~lu~ia]s i n these elastie ~ ments are secondary precipitated silica and caleitel The former is recorded as overgrowths on detrital quartz grains, the latter as w e l ~ m a t e ~ , commonly by corrosion of quartz, feldspars and glauconite. Silica is o ~ e d as welding material between quartz grains in all the thin sections studied, and increases in percentage with depth. The decrease in porosity of the Lower Senonian sandstone ( M ~ u ~ Formation) is more a result of increasing overgrowth of secondary silica than in the preCenomanian sandstone (Figs. 4, 5 and 50). This factor results in the increase of porosity with depth. Degrees of etching and corrosion vary from slight to intense, expressed by the remnants of quartz grains as ghosts (Fig. 6). Calcite is a result of more than one phase, appearing mostly as fissure fillings which could be a reflection of the different tectonic phases affecting the studied area. The different phases of calcite precipitation resulted in an increase in the porosity and permeability of the

Fig. 51. Sideritereplacing oolite. Ghost of the nucleus is visible(well GS 195-B2,core 8, depth 3433:5m). Crossed polarizers, bar is 100 #m.

299

rock. The amount of cement does not reflect any relation to the depth of the rocks. Dolomite is recorded as a minor additional diagenetic cementing material, and secondary dolorhombs are replacing quartz, glauconite and shell fragments (Figs. 16, 30, 33 and 42). The diagenetic dolomite rhombs resulted from the replacement of the secondary calcite cement; this is a factor significant in increasing the secondary porosity of these sandstones, which is due to the difference in ionic radii between calcite and dolomite crystal packing. The cementing material present could be attributed to the pressure-solution effect as a source of silica and calcite, while the degradation of argillaceous carbonate sediments resulted in the introduction of the Mg 2+ and Fe 2+ needed for the diagenetic dolomitization of secondary calcite cement. The principal factor in the decrease in porosity is overgrowth by secondary silica. (b) Dissolution. This term is used here to denote only a congruent dissolution process (Pettijohn et al., 1972) by which all the solid phase is gradually dissolved, leaving a fresh surface unaltered in composition. The dissolution effect is exemplified by the replacement of quartz and glauconite grains by calcite (Figs. 9 and

48). (c) Recrystallization. This term includes aggrading recrystallization resulting in neomorphism. It is clearly seen in the carbonates by the formation of single large sparry calcite crystals from smaller ones (Figs. 19 and 20). It is also exemplified by recrystallization into microsparite, mainly recorded as fillings (Figs. 30, 31 and 35).

Major diagenetic effects The most obvious diagenetic modification of these sandstones is the introduction of cement, mainly sihca and carbonate, in addition to the reduction in porosity due to cementation a n d / o r pronounced compaction. Silica as cementing material is believed by Waldschmidt (1941) to be derived from the points of contacts of grains, where it is dissolved and reprecipitated as overgrowths on the quartz grains. He also considered the interlocking boundaries of quartz grains as conclusive evidence for the existence of pressure to dissolve silica. The presence of long, concavo-convex and sutured types of contacts (Figs. 4, 50 and 51) is apparent evidence in harmony with his conclusion.

Accessory diagenetic minerals The amount of clay matrix recorded in the wackes of the Matulla and Raha formations shows a positive relation with depth. A maximum of 35% matrix is recorded (Figs. 10 and 38). Miscellaneous diagenetic minerals are represented by 50-t~m pyrite crystals. They are randomly scattered in the cement and are in direct contact with quartz and glauconite grains (Figs. 26 and 48). These may increase to form patches of framboids reaching about 4% of the rock; their maximum (up to

300 15%) recorded in complete or partially replaced shell fragments (Fig. 24). A distinct relationship between clay and amount of pyrite with depth is recorded. However, a relation between pyrite and glauconite is also clearly observed, indicating the prevalence of reducing conditions, especially in some spots of glauconite which are transferred to pyrite (Fig. 48). Authigenetic mica is recorded as a late burial feature characterized by the formation of diagenetic muscovite flakes.

Diagenesis of carbonate sediments The diagenesis of the carbonates includes lithification and cementation, and recrystalhzation and dolomitization. The cementing material is mainly composed of calcite as the primary form, in cases changing to dolomite and, rarely, to siderite (Figs. 30, 31 and 35, respectively). Two sources are postulated as providing the cementing material, pressure-solution taking place at grain contacts, and the dissolution of aragonite and hi~-magnesian calcite. Calcite as primary cementing material is believed to be the result of direct precipitation from waters rich in carbonate, in addition to pressure-solution effects on the grain contacts. Dolomite as cementing material is believed to be the result of diagenetic effects on the argillaceous-rich carbonate portions leading to the release of the Mg 2÷ and Fe 2+ needed for dolomitization of the nearby carbonates. Siderite is a minor replacive cementing material, and sphaerosiderite is seen replacing ooids, leaving some ghosts of the nucleus (Fig. 52), while each sphaerosiderite tends to weld the other nearly replaced ooids (Fig. 53). The aggrading recrystaltization of Folk (1965) is widely recorded in the thin sections, where most of the lime-mud (micrite) tends to aggrade into microspar with bird's-eye texture. Significant dolomitization has increased the porosity of the oil-bearing rocks, i.e., their capacity. The following observations can be reported: (1) there is a positive relation between increasing dolomite percent and shale; (2) most of the carbonate portions within shales are dolomitized (Figs. 13, 15, 23 and

Fig. 52. Sphaerosideritetends to weldwith the nearbyreplacedooids(wellGS 195-B2,core 8, depth 3433 m). Crossedpolarizers, bar is 200/~m.

301

Fig. 53. Sutured seam solution (well GS 197-2, core 8, depth 3721 m). Crossed polarizers, bar is 200/~m.

27); (3) most of the argillaceous carbonate zones are dolomitized (Figs. 16, 28 and 31); (4) dolomite associated with sutured and nonsutured seam solutions are believed to be a primary sedimentary feature modified by compaction (Figs. 54 and 55); and (5) the dolomite recorded within fractures is related and connected to shale or argillaceous zones. The dolomite-bearing sediments found are believed to be formed during burial (post-compactional) diagenesis for the following reasons: (1) Dolomite is the ferroan type, as indicated from the bulk XRD-analyzed samples (Fig. 12). (2) All the dolomite rhombs are associated with micritic clays (Figs. 13 and 27), and the percent of dolomitization decreases away from shaley or argillaceous carbonate portions. (3) A selected sample of the clay-fraction size was analyzed using the XRD method for clay analysis after treatment (glycolated, oriented and heated). The analysis showed that a smectite-illite mixed layer is present in all the samples. This indicates that the conversion of smectite to illite is a diagenetic process due to a post-compactional mechanism (Fig. 56). This observation is in harmony with McHargue and Price's mechanism (1982). (4) The source of CO 2- is believed to be either decay of organic matter, or a replacement crystal canabilizing pre-existing calcite for its CO 2-. Both of these conditions are confirmed; a first population supporting CO 2- from decay of organic matter is recorded within the studied petrographic types. In the dolomitized intrabiomicrite, many clear examples of

Fig. 54. Non-sutured seam solution (well GS 197-2, core 8, depth 3721 m). Crossed polarizers, bar is 200 /Lm.

302

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dolomite-growing rhombs are recorded with clear effect of preferred orientation of shell fragments due to compactional load effects. Replacement of calcite for its is considered to be the predominant condition in these samples, where

CO2-

Fig. 56. Preferred orientation effect on shell fragments (well GS 197-2, core 16, depth 3812.5 m). Crossed polarizers, bar is 200 #m.

303 dolomitization is seen to be restricted to the carbonate-(calcite) rich portions, taking the micritic (majority) or sparitic (rare) forms as can be seen in Figs. 13, 14, 32 and 42. The growing dolorhombs are seen replacing glauconite and quartz in Figs. 16, 28, 31 and 42. (5) All the dolomite recorded in these samples rarely replaces skeletal grains (Figs. 32 and 34); Figs. 37 and 39 further clarify this. This indicates that the only accessible part of dolomitization is that which filled with muddy carbonate matrix. The above points indicate that the source of dolomite is related to the compaction of clay burial, resulting in release to the medium of Mg 2+ and Fe 2+ which, combined with CO 2- and Ca 2+, form ankerite. SUMMARY AND CONCLUSIONS The petrographic investigations made reveal the presence of the following four petrographic groups: (1) arenite; (2) micrite; (3) wackestone; and (4) claystone groups. Each group can be subdivided into main types and a number of subtypes, reflecting the environmental changes within the Cretaceous basin of deposition. The arenite group consists petrographically of one main type, five types and twelve subtypes. The micrite group is represented by one main type, two types and eight subtypes. The wackestone group includes one main type, one type and five subtypes. The claystone group is represented by a single main type only. In order to elucidate the porosity changes on dolomitization, the bulk samples and the clay fraction of six samples were X-rayed. The results showed that post-compactional mechanism is responsible for providing the medium with the Fe 2÷, Mg 2÷ and K ÷ needed for dolomite formation. This mechanism explains the effect of argillaceous rich carbonate portions in decreasing the porosity of the Lower Senonian-Turonian sediments. Significant dolomitization has increased the porosity. Diagenetically, the sediments in the October oil-field area underwent intense changes, from redoxomorphic to phyllomorphic stages for the clastics, and lithification, recrystallization; and dolomitization for the non-clastic sediments. ACKNOWLEDGEMENT The authors are grateful to the Gulf of Suez Petroleum Company (GUPCO), Cairo, for providing the core and ditch samples, logs and data used in this study, and for permission to publish this paper. Special gratitude is due to Shawky Abdine, General Exploration Manager of GUPCO for his valuable support and continuous help during the progress of this work. REFERENCES Dott Jr., R.H., 1964. Wacke, greywackeand matrix--what approach to immature sandstone classification? J. Sediment.Petrol., 34: 625-632.

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