An overview of reservoir quality in producing Cretaceous strata of the Middle East

An overview of reservoir quality in producing Cretaceous strata of theMiddle East

Stephen N. Ehrenberg1,2,*, Adnan A. M. Aqrawi1 and Paul H. Nadeau1

1StatoilHydro ASA, Stavanger, Norway2Present address: PanTerra Geoconsultants BV, Weversbaan 1–3, 2352 BZ Leiderdorp, the Netherlands

*Corresponding author (e-mail: [email protected])

ABSTRACT: A compilation of average porosity and permeability data for Cretaceouspetroleum reservoirs of the Middle East reveals important differences between thetwo main tectonic provinces. The Arabian Platform is characterized by inversecorrelation of average porosity with burial depth in both carbonates and sandstones,whereas the Zagros Fold Belt (almost exclusively carbonates) has distinctly lowerporosity and no depth correlation. These contrasts are suggested to reflect the factthat Arabian Platform strata are mostly near their maximum burial depth, whereasZagros strata have experienced varying uplift and erosion following maximum burialin mid-Tertiary time. The carbonate reservoirs show no correlation between averageporosity and average permeability, probably because of wide differences in thedominant pore types present, and permeabilities tend to be much higher forsandstones than for carbonates.

Existence of the Arabian Platform porosity–depth correlation, despite an appar-ently wide diversity of depositional settings and early diagenetic porosity modifica-tions among the individual reservoirs, illustrates and confirms some fundamentalgeneralities about how burial diagenesis controls the overall porosity evolution ofreservoir rock bodies. Although porosity commonly shows enormous small-scaleheterogeneity in both carbonates and sandstones, the average pre-burial porosity oflarger stratigraphic intervals tends to be very high. Burial diagenesis progressivelydestroys this porosity by chemical compaction and associated (stylolite-sourced)cementation. Thus, all portions of the affected rock body move toward the zero limitas depth increases, although the rates of porosity occlusion vary greatly, dependingon rock fabric and early diagenesis. Average reservoir porosity therefore tends tocorrelate inversely with maximum burial depth, regardless of initial lithologicalheterogeneity.

KEYWORDS: porosity, permeability, Middle East, reservoir, diagenesis

INTRODUCTION

This study provides a unique perspective on reservoir qualitythroughout a major petroleum-producing region: a compilationof average values of key parameters for 435 producing reser-voirs of Cretaceous age from ten countries in the Middle East.No similarly comprehensive survey of quantitative, standard-ized reservoir quality data has been shown previously forMiddle East Cretaceous reservoirs, although similar compila-tions have been published for other major terranes (Ehrenberg1997; Ehrenberg et al. 2007; 2008) and many sources have listedaverage values, ranges and detailed core studies for much morelimited selections of Cretaceous reservoir examples (e.g.Alsharhan & Nairn 1997; Strohmenger et al. 2006a).

The focus is on the Cretaceous because this is the mostvolumetrically and numerically important period of reservoirdepositional age in this region. According to the database,Cretaceous strata contain 50% of the recoverable oil and 13%of the recoverable gas in the Middle East. Cretaceous reservoirs

are also far more numerous than reservoirs of any othergeological period in the Middle East. Of the 1814 MiddleEast reservoirs with reserves listed in the database, 49% areCretaceous, compared with 12% Tertiary, 14% Jurassic, 20%Permian/Triassic and 5% older.

The data examined in this paper are average values for theproducing zones of oil and gas fields. These data are acomponent of StatoilHydro’s global reservoir database, resultsfrom which have been reported in Bjørkum & Nadeau (1998),Ehrenberg & Nadeau (2005), Nadeau et al. (2005) andEhrenberg et al. (2007). For the Middle East, most of these dataoriginate from the EDIN-GIS database provided by IHS Inc.(http://energy.ihs.com/Products/Edin-Gis/index.htm), withadditional data originating from internal company files andscanned porosity–depth and porosity–permeability plots frompublished literature (Ehrenberg et al. 2007). The nature of theoriginal measurements upon which the average values are basedis commonly not specified, but each value is intended to bestrepresent the entire reservoir in question, and may thus be

Petroleum Geoscience, Vol. 14 2008, pp. 307–318 1354-0793/08/$15.00 � 2008 EAGE/Geological Society of LondonDOI 10.1144/1354-079308-783

derived from core and log data from representative wells. Thepermeability values are expected to reflect the contribution offractures where these are important for production. Numerousgaps exist in these data, such as reservoirs represented byporosity but not permeability values (242 out of the present 435reservoirs). The datum for top reservoir depth is the landsurface for onshore data and the sea floor for offshore data.Values for different zones within the same formation have beencombined so that each formation in each field is represented byonly a single set of average values, thus increasing internalconsistency between fields with different degrees of reservoirsubdivision.

Each data point in the compilation represents a largeand heterogeneous rock volume (as illustrated by Nadeau &Ehrenberg 2006, fig. 1). The main reason for examining theseaverage reservoir values is that this simplification has facilitatedthe present novel and concise portraiture of a very largeand complex geological terrane: the petroliferous part of theCretaceous sedimentary layer throughout almost the entireMiddle East. More detailed data, such as standard deviations,facies proportions and mineralogy are not available in stand-ardized, quantitative format for any similarly large selection ofreservoirs. Average reservoir values nevertheless provide validand valuable information because they are both fundamental toexploration risking and useful for indicating the general class ofreservoirs and associated production strategies to which aparticular example belongs. Furthermore, Aguilera (2006) haspointed out how average porosity and permeability data can beused for estimating the probability of encountering desired oilflow rates in a given area.

The present paper can be useful by providing a quantitativeframework within which detailed studies of individual MiddleEast Cretaceous reservoirs can be related to their larger contextby comparing them with the average parameter values. Themedian trends shown here can also be applied in probabilisticreservoir models and as input for reserve estimates for similarsettings in other parts of the world. Finally, both the correla-tions and ranges of values observed can be used to inferinformation about fundamental controlling processes.

GEOLOGICAL SETTING AND STRATIGRAPHY

Cretaceous Middle East reservoirs lie within a broad but sharplydefined geographical band (a series of foreland basins) that

stretches northwestwards from Oman to southeast Turkey(Fig. 1). Data for reservoirs in Yemen, Jordan and Israel havebeen excluded from the present consideration because theyrepresent different provinces lying well outside this maingeographical trend. The reservoirs include both carbonate andsandstone lithologies, each occupying distinct areas (Fig. 1), asdetermined by Cretaceous palaeogeography and depositionaldynamics (Aqrawi 1996; Christian 1997; Davies et al. 2002).This main petroleum-producing portion of the Middle East isdivisible into two principal tectonic provinces (Fig. 1): (1) theArabian Platform, where structures tend to be broad featurescaused by deep basement or salt movements; and (2) theZagros Fold Belt, where the dominant structures are elongateNeogene folds paralleling the Zagros suture.

From Late Permian to mid-Cretaceous time, the northern toeastern Arabian Plate was a gradually subsiding passive marginbordering the Neo-Tethys Ocean (Alsharhan & Nairn 1997;Sharland et al. 2001). Cretaceous palaeolatitudes were within 10�of the Equator, allowing prolific carbonate sedimentation.Beginning around 90 Ma, convergence and ophiolite obductionbegan, converting this margin into a foreland basin andenhancing the northeastward thickening of sediment accumu-lation. Eustatic and tectonically driven fluctuations in relativesea-level produced a hierarchy of depositional cyclicity withdurations ranging from <0.1–60 Ma.

At the largest scale, the Cretaceous strata are divided intotwo tectonostratigraphic megasequences (TSM) by Sharlandet al. (2001). TSM-8 comprises three main componentsequences, while TSM-9 comprises two component sequences(Fig. 2). The three subdivisions used in the present treatment,however, follow the traditional subdivision of Arabian Plate

Fig. 1. Map of Middle East Cretaceous reservoir locations for whichaverage porosity values are examined. Symbol colours indicatelithology.

Fig. 2. Cretaceous stages, ages of stage boundaries (Ma; fromGradstein et al. 2004), tectonostratigraphic megasequences (TSM;Sharland et al. 2001), sequences and groups.

S. N. Ehrenberg et al.308

Cretaceous strata into Thamama (TSM-8 lower and middlesequences), Wasia (TSM-8 upper sequence) and Aruma (TSM-9lower and upper sequences) groups, a scheme which seemsto provide a useful distinction between dissimilar reservoirassociations and is also widely familiar (Alsharhan & Nairn1986; 1988; 1990).

Table 1 lists the formations represented by the data shownin this paper, together with useful descriptive literature for eachcountry and stratigraphic interval. The upper Thamama and

Wasia siliciclastics represent prograding coastal-plain/delta sys-tems with quartz-rich sand sourced from uplift of the Arabianinterior (Alsharhan & Nairn 1997; Zeigler 2001; Davies et al.2002; Hohman et al. 2007).

FLUID PRESSURE

Reservoir fluid pressure for given depth can be described interms of the specific gravity (SG) of a hypothetical unconfined

Table 1. Formations included in the data compilation of the present study, with relevant references describing each

Sequence Lithology Country Formation References

upper Aruma carbonate Turkey Sinan, Garzan, Beloka Keskin & Can (1986); Salem et al. (1992)

Iraq, Syria Shiranish, Hartha Ahmed et al. (1986); Elzarka (1993); Sadooni (2004; 2005)

Neutral Zone Tayarat Dull et al. (2006)

lower Aruma carbonate Turkey Karabogaz, Karababa Celikdemir et al. (1991); Ozer (1993); Cater & Gillcrist (1994)

Syria Massive Limestone

Iraq Tanuma, Kasib, Komitan,Mushora, Sadi

Aqrawi (1996); Sadooni (1996; 2004; 2005)

Iran Ilam James & Wynd (1965); Setudehnia (1978); Motiei (1993); Farzadi &Hesthammer (2007)

Wasia carbonate Turkey Derdere Celikdemir et al. (1991)

Iraq, Syria Mishrif, Mauddud, Rumaila,upper Qamchuca

Elzarka (1993); Al-Shdidi et al. (1995); Aqrawi et al. (1998); Sadooni &Alsharhan (2003)

Iran Sarvak James & Wynd (1965); Setudehnia (1978); Habibnia & Javanbakht(1997); Farzadi (2006); Beiranvand (2007); Taghavi et al. (2007);Farzadi & Hesthammer (2007)

Kuwait, Bahrain Mishrif, Mauddud El Naggar & Al Rifaiy (1973); Al Shamlan et al. (1981); Strohmengeret al. (2006b)

UAE,Saudi Arabia

Mishrif Jordan et al. (1985); Videtich et al. (1988); Burchette (1993)

Oman Natih, Mauddud Harris & Frost (1984); Terken (1999); van Buchem et al. (2002);Smith et al. (2003); Droste & Van Steenwinkel (2004)

sandstone Syria Rutbah Ibrahim (1981)

Iraq, Kuwait,Neutral Zone,Bahrain, Iran

Burgan, Wara Brennan (1990); Alsharhan (1994); Kirby et al. (1998), Sadooni &Aqrawi (2000); Strohmenger et al. (2006b); Hohman et al. (2007)

Saudi Arabia Wasia Al-Sabti & Al-Bassam (1993); Hughes et al. (2006)

Qatar Nahr Umr Wells (1987)

upper Thamama carbonate Iran Dariyan, Gadvan Motiei (1993)

UAE, Oman,Qatar,Saudi Arabia

Shuaiba, Kharaib, Lekhwair Hassan & Wada (1981); Frost et al. (1983); Alsharhan (1985; 1990;1993); Litsey et al. (1986); Moshier (1989); Budd (1989); Vahrenkamp& Grötsch (1994); Oswald et al. (1995); Grötsch et al. (1998); Neilsonet al. (1998); Lucia (1998); Al-Awar & Humphrey (2000); Alsharhanet al. (2000); Saotome et al. (2000); Russell et al. (2002); Melville et al.(2004); Droste & Van Steenwinkel (2004); Strohmenger et al. (2006a);Yose et al. (2006)

sandstone Iraq, Kuwait,Neutral Zone,Saudi Arabia

Zubair Ali & Nasser (1989); Nemcsok et al. (1998); Sadooni & Aqrawi (2000)

lower Thamama carbonate Kuwait, NeutralZone, Iraq

Minagish, Yamama, Ratawi Longacre & Ginger (1988); Sadooni (1993); Davies et al. (2000)

Iran Khami, Fahiyan, Ratawi Avarjani et al. (2007); Soori & Bonab (2006)

UAE Habshan Alsharhan & Nairn (1997)

Reservoir quality in the Middle East Cretaceous 309

fluid column up to the land or sea surface, necessary to producethe observed pressure (Nadeau et al. 2005). A value of SG<1(fresh water) represents underpressure, which may result fromhydrocarbon production. SG>1.2 corresponds with over-pressure, while SG>1.8 is referred to as high overpressure. Forthe Middle East Cretaceous reservoirs, cases of high over-pressure occur mostly shallower than 1 km, whereas under-pressures occur mainly between 1 km and 2.6 km (Fig. 3). Mostoccurrences of both high overpressure and underpressureare located in the Zagros province (Fig. 4). Almost all of theunderpressured reservoirs are located in Turkey and northernIraq, whereas the reservoirs with high overpressure aredistributed more widely.

These data are relevant to the study of reservoir qualitybecause fluid overpressure has been suggested as a factor thatmay favour preservation of porosity during burial (Scholle1977; Bloch et al. 2002). Such an effect, however, is notapparent in the present data because there is no correlationbetween higher fluid pressure gradient and higher porosity forgiven depth (Fig. 4). The occurrence of most high overpressurevalues at burial depths less than 1 km contrasts markedly withthe situation in basins such as the Norwegian continental shelfand the US Gulf Coast, where overpressure is developed mainlybelow a certain minimum depth of several kilometres (Bjørkum& Nadeau 1998). This contrast and the occurrence of mosthigh overpressure in the Zagros tectonic province may indicatedevelopment of overpressure as the result of overburdenremoval during uplift and erosion.

POROSITY–DEPTH

Burial depth is relevant to porosity of both carbonates andsandstones because of the effects of increasing overburden andtemperature on promoting compaction and cementation.Figure 5 shows depth versus porosity data, with symbolsindicating lithology and country in (a) and lithology and

tectonic province in (b). The Arabian Platform carbonates andsandstones show broad correlation of decreasing porosity withincreasing depth, whereas the Zagros carbonates show nocorrelation (Fig. 5b). The Arabian Platform carbonates andsandstones also tend to have higher porosity than the Zagroscarbonates (Fig. 6).

Figure 7 examines the Arabian Platform porosity–depth datafor (a) carbonates and (b) sandstones. The lines compare themedian (P50) porosity values for each 0.5 km depth interval forthe present data (solid lines) with world-wide data for the samelithological group (dashed lines) reported by Ehrenberg &

Fig. 3. Depth versus fluid pressure for Middle East Cretaceousreservoirs. Symbols indicate ranges of fluid-pressure gradient definedin terms of the equivalent specific gravity of the fluid column to theland or sea surface required to produce the observed pressure: <1,underpressure; 1–1.2, normal pressure; 1.2–1.8, overpressure; >1.8,high overpressure (Nadeau et al. 2005).

Fig. 4. Depth versus porosity for Middle East Cretaceous reser-voirs: (a) Arabian Platform; (b) Zagros Fold Belt. Symbols indicateranges of fluid-pressure gradient defined in terms of the equivalentspecific gravity of the fluid column to the land or sea surfacerequired to produce the observed pressure: <1, underpressure; 1–1.2,normal pressure; 1.2–1.8, overpressure; >1.8, high overpressure(Nadeau et al. 2005).

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Nadeau (2005). The P50 lines shown in this paper have notbeen smoothed because there is no a priori requirement thatthese lines must necessarily be smooth or show correlation.

The Middle East Arabian Platform carbonates mostly havehigher average porosities than the global P50 trend, but the twotrends converge with increasing depth (Fig. 7a). The MiddleEast sandstones also define a trend with greater porosity losswith depth than the global P50 sandstone line, with shallowerreservoirs having higher porosity and deeper reservoirs com-monly having lower porosity than the global trend (Fig. 7b).The Middle East sandstone trend lies at higher porosity thanthe Middle East carbonate trend at depths shallower than2 km, but the two trends are nearly identical from 2.5–4 km.Maximum values of average porosities for given depth aredistinctly less for Middle East Cretaceous reservoirs of bothcarbonate and sandstone lithology than for correspondinglithologies from the global dataset (Fig. 7).

The same data and symbols as in Figure 5a are separatedaccording to major stratigraphic sequences in Figure 8. Thisfigure highlights a major geographical shift in stratigraphicoccurrence of petroleum in Middle East Cretaceous reservoirs.As discussed by Zeigler (2001), petroliferous Lower Cretaceousreservoirs occur mainly in the southeast to the middle part ofthe Middle East trend, while petroliferous Upper Cretaceousreservoirs occur almost exclusively in the northwest part of thetrend (in Turkey, Syria, Iraq and Iran).

POROSITY–PERMEABILITY

Figure 9a shows all permeability versus porosity data withsymbols indicating lithology and country. Figure 9b shows thesame data with symbols indicating lithology and tectonicprovince. Particularly striking is the presence of a cluster of datawith both high permeability (>100 mD) and high porosity(>13%) in the Arabian Platform carbonates but not in theZagros carbonates. Conversely, the Zagros carbonate reservoirs

are notable for a prominent group of data having moderatelyhigh permeabilities (>10 mD to several hundreds of mD)and low porosities (<13%), a combination of values that isuncommon for the Arabian Platform data.

As in Figure 5b, the lines in Figure 9b represent the mediantrends for world-wide carbonate and sandstone reservoirsreported by Ehrenberg & Nadeau (2005). The Middle Eastsandstones have much higher average permeabilities than theMiddle East carbonates, despite considerable overlap betweenthe lower range of the sandstones and the higher range of thecarbonates (150–500 mD). Most of the Middle East sandstonereservoirs have an average permeability greater than the globalmedian sandstone trend of Ehrenberg & Nadeau (2005),whereas the Middle East carbonates tend to have permeabilitieslower than the global median carbonate trend.

The same data and symbols as in Figure 9A are separatedaccording to major stratigraphic sequences in Figure 10. Thisseries of plots shows that all three age groups have similarlywide ranges of permeability for given porosity. Differencesbetween groups appear to be relatively subtle and are possiblenot significant at high levels of confidence.

DISCUSSION

Porosity

A principal finding from this compilation is that averagereservoir porosities from the Arabian Platform tectonic prov-ince show broad negative correlation with present burial depth(Fig. 7), whereas porosities from the Zagros Fold Belt aregenerally lower (Fig. 6) and show no depth correlation (Fig. 5b).To explain these contrasts, this paper first discusses the reasonsfor correlation in the Arabian Platform data and then considersthe ways in which the Zagros reservoirs are different.

Fundamental to this discussion is the recognition thatporosity of both carbonates and sandstones is reduced inproportion to the maximum depth to which the strata have

Fig. 5. Top reservoir depth versus average porosity for Middle East Cretaceous reservoirs. Symbols indicate (a) lithology (colour) and country(shape); (b) lithology (colour) and tectonic province (shape).

Reservoir quality in the Middle East Cretaceous 311

been buried (Maxwell 1964; Halley & Schmoker 1983). Thisextensively documented empirical generality is supported bymany petrographic studies showing that porosity loss duringburial below 1–2 km proceeds mainly by chemical compactionand associated cementation from the solutes thus supplied(Oswald et al. 1995; Walderhaug 1996; Ehrenberg 2004; 2006).Traditionally, chemical compaction is assumed to be driven byincreasing effective stress attending deeper burial, but thepreferred view here is that the key parameter is, in fact, ‘thermalexposure’ (temperature integrated over time; Schmoker 1984;Schmoker & Gautier 1988; Bjørkum 1996; Walderhaug 1996).In any case, the Arabian Platform strata are presently near their

maximum burial depth (Alsharhan & Nairn 1997), such thatthe deeper reservoirs in this province may be expected tohave experienced progressively greater degrees of stylolitedevelopment and consequent cementational porosity loss.

Although porosity–depth correlation is certainly nothingnew, it is nevertheless remarkable that this correlation shouldbe so clearly apparent in the Arabian Platform data, in viewof the wide geographical distribution of these reservoirs and

Fig. 6. Frequency distributions of average porosity values forMiddle East Cretaceous reservoirs. The limestone category includesa subordinate number of unspecified ‘carbonate’ lithologies.

Fig. 7. Top reservoir depth versus average porosity for Middle EastCretaceous reservoirs from the Arabian Platform tectonic province(Fig. 1): (a) carbonates (correlation coefficient ‘r’ = �0.59 for 207reservoirs); (b) sandstones (correlation coefficient ‘r’ = �0.71 for 67reservoirs). Solid lines show median (P50) trends for the data shown.Dashed lines show median trends for corresponding lithologies fromthe global dataset of Ehrenberg & Nadeau (2005). The black solidline in (b) is the median trend for Middle East Cretaceous carbonatesshown in (a). The dash–dot line in each plot represents themaximum-porosity trend for each lithology from the global data ofEhrenberg & Nadeau (2005).

S. N. Ehrenberg et al.312

the complete lack of discrimination of stratigraphic units,depositional settings or early diagenesis in the dataset. Palaeo-geographical reconstructions (Zeigler 2001), regional correla-tions (Harris et al. 1984) and the many case histories availablefor Arabian Platform Cretaceous reservoirs (Table 1) show thatthe data in Figure 7 must represent widely contrasting mixturesof depositional facies having widely varying degrees of earlydiagenetic modification by dolomitization, karsting and cemen-tation. It is well documented that depositional fabric stronglyaffects initial porosity (Beard & Weyl 1973; Enos & Sawatsky1981), that early diagenesis greatly alters pre-burial porosity of

Fig. 8. Top reservoir depth versus average porosity for Middle EastCretaceous data plotted separately for major stratigraphic groups(Fig. 2).

Fig. 9. Arithmetic-average permeability versus average porosity forMiddle East Cretaceous reservoirs. Symbols indicate: (a) lithology(colour) and country (shape); (b) lithology (colour) and tectonicprovince (shape). Solid lines show median trends for Middle EastCretaceous carbonates (black) and sandstones (red). Dashed linesshow median trends for corresponding lithologies from the globaldataset of Ehrenberg & Nadeau (2005).

Reservoir quality in the Middle East Cretaceous 313

carbonates (Harris & Frost 1984; Wagner 1990) and that bothdepositional fabric and early diagenesis result in widely diver-gent pathways of porosity evolution during subsequent burial(Oswald et al. 1995; Ehrenberg 1997; Ehrenberg et al. 2006a).Several authors have also claimed that porosity of both sand-stones and carbonates commonly is increased during late burialby dissolution and removal of soluble components via bulkflow of porewater (Mazzullo & Harris 1992; Sattler et al. 2004).It is therefore surprising to find overall porosity–depth corre-lations for both carbonates and sandstones when the entireArabian Platform Cretaceous is viewed together.

It is proposed, however, that the correlations in Figure 7demonstrate the overriding control of burial diagenesis on theaverage porosity of larger sedimentary bodies. Although indi-vidual beds commonly show extreme and highly localizedporosity variations, reflecting differences in facies and earlydiagenesis, these high-frequency spatial fluctuations becomesmoothed with increasing scale of observation, such thatdifferences determined by factors uniformly affecting largerrock volume become more apparent. Shallowly buried carbon-ate strata tend to have very high porosity overall, despiteextremely heterogeneous ranges that typically include manyvery low values (Melim et al. 2001; Ehrenberg et al. 2006a; b). Asthe strata are buried, this high initial porosity is destroyedprogressively by chemical compaction and associated cementa-tion, and some lithologies lose porosity more rapidly thanothers, thus preserving or even enhancing petrophysical hetero-geneity. All portions of the affected strata, however, movecloser to and are constrained by the zero-porosity limit, thusultimately reducing both the average porosity and the porosityrange of the aggregate sedimentary body. This model providesa plausible explanation for why Arabian Platform reservoirsshould conform to an overall trend of decreasing aver-age porosity with increasing depth, despite having enormousinternal variation in facies proportions and early diagenesis.

The Zagros province differs from the Arabian Platform inhaving experienced widely variable amounts uplift and erosion(Bordenave & Burwood 1990; Beydoun et al. 1992), such thatmany reservoirs have had maximum burial depths substantiallygreater than the present depths plotted in Figure 5. We suggestthat these discrepancies are the main cause for the lack ofcorrelation between porosity and present burial depth in theZagros data. The comparison between the two tectonic prov-inces in Figure 5b indicates that amounts of overburden re-moval between 0 km and 2.5 km must be postulated to restoreZagros porosity values to the approximate trend shown by theArabian Platform data. Furthermore, the lower porosities ofZagros, compared with Arabian Platform carbonate reservoirs(Fig. 6), are suggested to reflect generally greater maximumburial of the Zargos reservoirs. These hypotheses are difficult totest, however, because the dataset does not include maximumburial values for the individual Zagros reservoirs. Additionalfactors that could also contribute to lower porosity and lack ofporosity–depth correlation in the Zagros reservoirs includepossible hydrothermal activity and lateral deformation associ-ated with the Zagros tectonic activity.

An alternative explanation is that Zagros reservoirs mayhave lower porosities because of more distal depositionalsettings compared with Arabian Platform reservoirs, insofar asthe open ocean was situated toward the NE and the Arabiancontinental interior was relatively toward the SW duringCretaceous time (Alsharhan & Nairn 1997; Sharland et al. 2001).However, the distribution of shallow-water carbonate platformsthroughout Cretaceous time does not show any simple prefer-ence for Arabian Platform rather than Zagros territory (Zeigler2001). Arabian Plate palaeogeography was dominated by several

Fig. 10. Arithmetic-average permeability versus average porosity forMiddle East Cretaceous reservoirs plotted separately for majorstratigraphic groups (Fig. 2).

S. N. Ehrenberg et al.314

intracratonic basins, whose occurrence was determined by localtectonics rather than distance from the continental margin.

Comparison of the world-wide data reported by Ehrenberg& Nadeau (2005) and the Middle East Cretaceous reservoirs ofthe Arabian Platform tectonic province shows that the latterdata: (1) tend to have higher porosity values for a given depth;and (2) have more steeply decreasing trends of P50-porosityversus depth for both carbonates and sandstones (Fig. 7).These differences may reflect the inclusion in the global datasetof many reservoirs from uplifted and eroded terranes analogouswith the Zagros province. Reservoirs whose burial depth hasbeen reduced greatly from previous maximum values will, ingeneral, have lower average porosity for a given depth thanreservoirs presently at their maximum burial depth. Anotherpossible factor is differences in geological age between the twodatasets, but this has not yet been evaluated systematically.

Sandstones and carbonates of the Arabian Platform prov-ince have similar P50 and maximum porosity–depth trends(Fig. 7). This is surprising because it is well known thatcarbonates are more chemically reactive than quartzose sand-stones and should therefore tend to lose porosity faster duringburial, resulting in a steeper porosity–depth gradient. Thisexpected contrast was observed in the global comparison ofsandstone and carbonate petroleum reservoirs presented byEhrenberg & Nadeau (2005), but is not apparent in the presentsubset of the same database.

Evidence from many earlier studies supports a positiverelationship between dolomitization and porosity (Ehrenberget al. 2006a), but the possible influence of dolomite is difficult toevaluate here because only 7 of the 367 carbonate reservoirs inthe present compilation (2%) are listed as having dolomitelithology and six of these seven are from a small area in Syria.However, 94 of the 367 carbonate reservoirs (26%) are listed aspartly dolomitized, the latter occurring predominantly in theZagros province (Fig. 4). Because all data are from petroleum-filled intervals, the present compilation is also unsuitable forevaluating the possible effect of hydrocarbon charge on poros-ity preservation, as has been discussed in various earlier studies(Feazel & Schatzinger 1985; Oswald et al. 1995; Neilson et al.1998; Bjørkum & Nadeau 1998).

Permeability

Lack of correlation between average permeability and porosityin Middle East Cretaceous carbonate reservoirs (Fig. 9) prob-ably reflects the wide variations in dominant pore types knownto exist in these strata (Wagner 1990; Saotome et al. 2000;Russell et al. 2002; Smith et al. 2003) and possibly also varyingcontributions of fracture flow to the reported permeabilityvalues. Lack of permeability–porosity correlation in the sand-stones (Fig. 9) probably also reflects varying pore types.Porosity is expected to be dominantly intergranular in the bettersandstone reservoirs, but this may not, so commonly, be thecase for the better carbonates. Fracture flow should not be asignificant cause for non-correlation between the porosity andpermeability in the sandstones because porous sandstonesrespond to stress mainly by ductile deformation, unless the rockis hardened by quartz cementation (Fisher et al. 2003; Wen-nberg et al. 2008). Nevertheless, the Middle East Cretaceoussandstones plot at distinctly higher permeabilities with respectto most carbonates, consistent with the expected greaterdominance of intergranular porosity in the former.

The presence of a high-porosity and high-permeabilitysubgroup of Arabian Platform data (Fig. 9b) is suggestive of animportant class of these reservoirs where the pore systems aredominated by well-connected matrix flow, involving large

interparticle pores. Conversely, the presence of a low-porosityand high-permeability subgroup of Zagros data (Fig. 9b) issuggestive of an important class of these reservoirs where thepore systems are dominated by fracture flow, as has beensuggested for the global carbonate data (Ehrenberg & Nadeau2005). Reservoirs characterized by high porosity and lowpermeability are present in both tectonic provinces, possiblyreflecting dominance of chalky microporosity and poorlyconnected vugs.

CONCLUSIONS

The main contribution of this paper is the presentation of acompilation of average numerical values for depth, porosity andpermeability for producing Cretaceous reservoirs throughoutmost of the Middle East – a dataset that is far more compre-hensive and quantitative than any that has been shown pre-viously. Some readers may well question the value of examiningsuch data in the absence of detailed information on the internalheterogeneity, geology and identities of the individual reservoirsinvolved. Obviously, the purpose cannot be to explain thepositions of particular data points. The main value of thiscompilation is that it provides a quantitative and objectivepicture of empirical reality, showing the extreme heterogeneityexisting in a major petroleum province. The probability distri-butions describing this picture can be used as fundamentalreferences for both exploration risking and evaluation oftheoretical models for controlling processes.

Despite considerable scatter, porosity–depth correlationwithin the Arabian Platform tectonic province is attributed toburial diagenesis during relatively simple burial histories, forwhich the present depth is close to maximum depth in mostareas. Absence of porosity–depth correlation and markedlylower porosities in the Zagros Fold Belt probably results fromsignificant and widely varying amounts of uplift and erosionfollowing maximum burial.

Median (P50) trends for Arabian Platform carbonate andsandstone porosity decrease with depth are steeper than theworld-wide P50 trends, probably because the global datasetincludes many reservoirs uplifted from previous maximumburial depths, as exemplified by the Zagros data. Middle EastCretaceous carbonate reservoirs show no correlation betweenaverage porosity and permeability, reflecting wide differences indominant pore types between different reservoirs and varyingcontributions of fracture flow, as documented in many previousstudies (Table 1). Average permeabilities tend to be muchhigher for Middle East sandstones than for carbonates. Thepossibilities for testing these hypotheses are limited by lack ofconsistent data on mineralogy, sedimentology and diagenesiscovering the 435 reservoirs of this study, together with thenecessarily anonymous nature of the data presented. In future,such evaluations can nevertheless be made for individualreservoirs by comparison with the ranges of average valuespresented in this study.

Helpful suggestions for earlier drafts of this manuscript wereprovided by A.D. Horbury, F.N. Sadooni, V.P. Wright, S. Burley,W.A. Morgan, R.J. Barnaby, R. Abegg, C. Stabler and J.E. Amthor.

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