Diagenesis of the Aptian Shuaiba Formation at Ghaba North Field, Oman
Published:January 01, 2000
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Abdulrahman A. Al-Awar, John D. Humphrey, 2000. "Diagenesis of the Aptian Shuaiba Formation at Ghaba North Field, Oman", Middle East Models of Jurassic/Cretaceous Carbonate Systems, Abdulrahman S. Alsharhan, Robert W. Scott
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The Aptian Shuaiba Formation is subdivided into two major reservoir zones in Ghaba North Field, Oman. The lower Shuaiba zone was deposited in a lagoonal setting with restricted to partially restricted marine circulation conditions. The upper Shuaiba zone was deposited under open circulation conditions in a shallow marine setting. Facies succession in the Shuaiba indicates an overall major shallowing-upward cycle, culminating in a regional unconformity at the top of the formation.
The Shuaiba reservoir is heterogeneous, and diagenesis has been responsible for both heterogeneity and enhancement of reservoir properties. Production is from secondary macroporosity, matrix microporosity, and fracture porosity. Reservoir storage capacity is dependent mainly upon matrix microporosity, which occurs as intercrystalline porosity within microrhombic calcite matrix. Reservoir quality is enhanced vertically in the proximity of two recognized subaerial exposure surfaces. Meteoric-water diagenesis at local exposure horizons and the regional unconformity led to matrix stabilization and development of both secondary macroporosity and microporosity within micro-rhombic calcite matrix. This interpretation is supported by the decrease in secondary macroporosity and matrix permeability downward from the exposure surfaces. Stable-isotope data, in conjunction with stratigraphic and sedimentologic observations, support the diagenetic influence of meteoric water in creation of favorable reservoir properties. The lower Shuaiba zone shows relatively invariant stable-isotope compositions and is interpreted to have been diagenetically stabilized in a confined aquifer under conditions of low water/rock ratio. The upper Shuaiba zone shows carbon-isotope depletions at the local subaerial exposure surface and at the regional unconformity. Attenuation of carbon-isotope shifts likely resulted from poor development of vegetation and soil in an arid setting. The potential for improvement of reservoir properties by local subaerial exposure should be considered in reservoir development plans.
The Shuaiba Formation is a prolific hydrocarbon reservoir in the Arabian Gulf, in general, and in northern and central Oman in particular. The formation is Aptian in age and is the uppermost formation of the Lower Cretaceous Thamama Group. It was deposited during an extensive marine inundation of the eastern Arabian Shield, and was marked by a series of thick rudist grainstone buildups (Alsharhan, 1985). Murris (1980), and Alsharhan (1995) suggested that during the Aptian, in the southern part of the Arabian Gulf and United Arab Emirates in particular, the intrashelf basin of Abu Dhabi had developed and fringing shelf-margin rudist buildups grew where conditions were favorable, creating the prolific Shuaiba reservoirs.
Hughes Clarke (1988) interpreted the Shuaiba Formation in Oman as a full cycle of carbonate sedimentation that can be subdivided into two parts. The lower part is described as an algal (Lithocodium) skeletal wackestone to boundstone. This passes up into argillaceous carbonates, which then grade up into cleaner foraminiferal, and in some places rudist-rich, wackestones to packstones. The Shuaiba in Oman and the United Arab Emirates overlies the Kharaib Formation, which consists of shoal deposits on a carbonate shelf.
The Shuaiba is terminated in the southern part of the Arabian Gulf by a shale seal called the Nahr Umr Formation, the lower formation in the Wasia Group of Middle Cretaceous age. The Nahr Umr Formation (Albian) records a shift to a mixed siliciclastic-carbonate system. Pratt and Smewing (1993) believe that it was deposited during drowning of the shelf in the Albian. Detrital sediment constituting the Nahr Umr Formation was generated by uplift on the west side of the platform. Large-scale evolution of the margin in the central Oman Mountains imposed major sedimentological changes on the platform in generating the siliciclastics of the Nahr Umr Formation (Pratt and Smewing, 1993). A regional unconformity, representing erosion or nondeposition of the middle Aptian section, marks the contact between the Shuaiba and the overlying Nahr Umr Formations in this area (Frost et al., 1983; Harris et al., 1984).
The present study focuses on Ghaba North Field of central Oman, discovered in 1972 (Fig. 1). The field lies within the Ghaba Salt Basin (Petroleum Development Oman report, 1992), where great thicknesses of Precambrian to early Paleozoic elastics, salts, and marine carbonates accumulated (Alsharhan and Kendall, 1986). Abundant structures formed by salt tectonism in this basin have developed prolific oil traps in the area.
The structural setting of the Ghaba North Field (Fig. 2) indicates an anticlinal structure trending NE-SW with a fractured and faulted crest at the Shuaiba Formation level (Al-Bastaki, 1993; Petroleum Development Oman report, 1992). The structure likely formed by deep-seated movement of the Ara Salt. Stresses caused by the salt movement, together with wrench faulting associated with the Maradi Fault Zone, created a NE-SW striking fault system consisting of normal faults (Al-Bastaki, 1993; Uitentuis, 1981).
Methods of the Study
In order to understand the controls on porosity evolution and the development of reservoir quality, this study focuses on the sedimentological history and diagenesis of the Shuaiba Formation. Much of the work for the present study came from detailed analysis of Ghaba North core No. 23 (GN-23). This core, taken in 1990, is 50 m long and 19.5 cm in diameter and is still in good condition. This large-diameter core affords an excellent opportunity for megascopic study. Moreover, three other cores from this field are used in this study; GN-1, GN-2, and GN-13 (see Fig. 2 for locations), all of which are 10.2 cm in diameter.
Eighty-five core samples were taken from core GN-23 at an approximate interval of 0.5 m. All representative facies were sampled in GN-23, with the emphasis being on recognition of sediment types, bounding features, and diagenetic phases. On the basis of the detailed sampling in GN-23, representative samples were also taken from the other three cores for comparison. Thirteen, four, and eight representative samples were taken from the cores GN-1, GN-2, and GN-13, respectively.
Thin sections were prepared from the core samples, some were stained, and all were examined using standard transmitted-light petrography. Broken surfaces of five samples from GN-23 were chosen for a scanning electron microscope (SEM) investigation. Three of these samples come from intervals with high porosities and two were from intervals with relatively low porosities. These high-porosity and low-porosity intervals alternate with each other from the bottom of the core to the top.
Powdered samples for stable carbon and oxygen isotopic analyses of matrix and cements were collected from slabbed core plugs with a dental drill. Samples of the matrix were repeatedly cleansed of hydrocarbons in a toluene bath. All samples were prepared for isotopic analysis by off-line extraction of CO2 in 100% orthophosphoric acid at 90°C. Stable carbon and oxygen isotopes were analyzed on a VG Micromass 602E mass spectrometer according to standard procedures outlined in McCrea (1950). Isotopic ratios are expressed in the standard delta notation and are given relative to PDB standard in per mil (‰) notation. Precision on blind duplicates for 10% of the samples yields a mean half range of 0.20‰ for δ18O and 0.07‰ for δ13C.
Biofacies and Lithofacies
Three biofacies were identified at Ghaba North Field. Specific types of allochems characterize each biofacies, but the boundaries between these biofacies are transitional and may display faunal mixing.
Foram Biofacies: This biofacies is dominated by Orbitolina and small benthicforams, including miliolids, uniserial and biserial forams, peneropolids, and agglutinated and spiral forams. The faunal assemblage is indicative of a restricted to partially restricted shallow marine setting (e.g., Wilson, 1975). This biofacies lacks any rudists or rudist fragments, which are indicative of higher-energy conditions.
Algae-Foram-Rudist Fragment Biofacies: This biofacies is characterized by abundant green algae (Lithocodium), Orbitolina, small forams, and small-sized rudist fragments. In addition, a few scattered whole rudists are present. Skeletal debris of echinoderms and mollusks is common as well. The algae are present as either nodules or layers. This faunal assemblage is indicative of a restricted to partially restricted, shallow marine setting (Wilson, 1975). However, in this case, the presence of rudist fragments indicates that water-circulation conditions were slightly better than in the previous biofacies. This indicates that the depositional site on the shelf/ramp was closer to some rudist bioherms, in inter-biohermal sites, because of the presence of transported rudist fragments in wacke-packstone microfacies.
Rudist-Orbitolina Biofacies: Rudists and rudist fragments dominate this facies and in places are present in growth position. The benthic foram Orbitolina is abundant in this biofacies; however, other taxa of small benthic forams are rare or absent. The abundance of calcareous algae is low and, if present, occur as nodules only. This biofacies is indicative of a less restricted, much shallower setting. It represents deposition near, or within, rudist bioherms on the shelf.
Two major lithofacies groups were identified in Ghaba North Field, with each group composed of a variety of subfacies.
Restricted Lagoon and/or Below Fair-Weather Wave Base Lithofacies: This lithofacies comprises the foram biofacies and the algae-foram-rudist fragment biofacies. This lithofacies was deposited on the landward side of the shelf/ramp. Therefore, it is indicative of relatively shallow waters but with partial restriction of circulation in a lagoonal setting. Alternatively, it might indicate deposition under slightly deeper water depths, below fair-weather wave base. It is represented by the lower Shuaiba zone, which occurs below 619.45 m in GN-23.
Relatively Open Shelf/Ramp and/or Above Fair-Weather Wave Base Lithofacies: This lithofacies is represented by the rudist-Orbitolina biofacies. It was deposited above fair-weather wave base, in shallower water and more open-circulation conditions than the restricted lagoonal lithofacies. This lithofacies is represented by the upper Shuaiba. In general, rudist boundstones, rudstones, and minor rudist floatstones dominate this zone. In addition, a grainier bioturbated packstone microfacies is present. The lithofacies formed by vertical stacking of rudist biohermal/biostromal facies, intercalated with inter-biohermal floatstone facies. Several high-frequency cycles of bioherm deposition occur in the upper Shuaiba zone.
Lithofacies succession at Ghaba North Field is indicative of a shallowing-upward trend. The restricted-lagoon lithofacies predominate in the lower zone, whereas the relatively open shelf/ramp lithofacies represents the upper Shuaiba zone. Stacked, high-frequency cycles constituting the upper zone culminated in grainstone deposition and inferred subaerial exposure. This overall shallowing-upward trend ended with a major drop in sea level and subaerial exposure, represented by the regional unconformity between the Shuaiba and Nahr Umr formations (Harris et al., 1984; Alsharhan, 1995).
Reservoir properties considered include total measured porosity, macroporosity, microporosity, and horizontal permeability. Total measured porosity and horizontal permeability were obtained from an unpublished report (Petroleum Development Oman, 1990). Grain volume, for plugs taken from the core, was determined using a helium porosimeter. Subsequently, the total measured porosity and grain density of each plug were calculated. Macroporosity represents the porosity observed and estimated for all intervals in the Shuaiba Formation from the core, plugs, and thin sections in this study. Microporosity percentages are calculated from the difference between the total measured porosity and the estimated macroporosity for each interval.
Total Measured Porosity.—
The Shuaiba lower and upper zones have similar characteristics regarding variation in total measured porosity (Fig. 3A). Both of these zones start with high total measured porosity at the base, which then decreases gradually through the middle sections of the zones. A gradual increase in the porosity then occurs as the top of each zone is approached. However, the lower zone has a lower average total porosity (27%) than the upper zone, which has an average of greater than 30%. Thus, the Shuaiba Formation, in general, shows a trend of increasing total measured porosity toward the top.
The lower Shuaiba zone has relatively low average macroporosity (5%). This percentage holds relatively constant throughout the lower zone except for few intervals that show even lower values (Fig. 3B). Macroporosity increases gradually above the contact between the lower and upper zones. The upper zone is characterized by a higher average macroporosity (> 8%), and it reaches a maximum of greater than 15% at the top of the zone (top of the Shuaiba Formation).
Microporosity estimates represent the difference between total measured porosity and estimated macroporosity. The plot of microporosity values versus depth (Fig. 3C) is similar to the plot of the total measured porosity variation with depth (Fig. 3A). Trends are not obvious; however, average microporosity values are slightly lower in the lower Shuaiba zone.
Horizontal permeability is shown in Figure 3D. In general, the Shuaiba Formation is characterized by relatively low permeability values. The permeability is lower at the bottom and becomes slightly higher toward the top of the formation. In order to look at permeability variations in the microporous matrix, data in Figure 3D were replotted using a cutoff of 35 md, effectively removing the interconnected macroporosity (Fig. 3E). The lower zone shows that matrix horizontal permeability values are generally less than 10 md, averaging 6 md, and increasing to about 15 md at the top of the zone. The upper zone exhibits higher and more variable permeability values, with an average estimated to be about 15 md. The highest permeabilities, over 200 md, occur at or near the top of the upper Shuaiba zone (Fig. 3D).
Diagenesis of the Shuaiba Formation
Macroscopic cements are volumetrically minor at Ghaba North Field. Original interparticle macropore spaces were sparse because of the predominance of wackestone fabric in the Shuaiba sediments of the area. The few cements observed are calcific in composition but with variable morphologies. Cementation patterns vary with depth and facies of the Shuaiba Formation.
Two major types of calcite cements are observed. The first occurs as a euhedral to subhedral, fine-crystalline calcite, and the second occurs as a coarse-crystalline pore-filling calcite. The fine-crystalline calcite cements occur in intraparticle pores of algae, Orbitolina, and other forams, as well as within microfractures, borings, shelter porosity, and primary interparticle pores. These cements partially to completely fill the pores. In partially filled pores, fine-crystalline calcite cement lines the pores and the rest is filled by hydrocarbons. This fine-crystalline calcite cement has a patchy appearance in the core and plugs. This is due mainly to localization of the cements within algae intraparticle pores, and therefore it corresponds to the distribution of algae. Microfractures filled with fine-crystalline calcite are later than these pore-filling cements, because they crosscut allochems that contain the earlier cement.
The coarse-crystalline calcite cements occur mainly in secondary moldic porosity or primary macroporosity, such as in rudist internal pore spaces. These cements are generally equant-blocky and subhedral to euhedral, and show a trend of increasing crystal size toward the centers of the pores. The presence of broken rudist fragments, of spalled micrite envelopes, and of brecciated fragments of matrix floating in these cements indicates that these cements occur later in the paragenesis. Higher abundance of these cements occurs in the upper Shuaiba zone and corresponds to rudist-rich intervals.
Rudists and rudist fragments in the Shuaiba may either be preserved as replaced or recrystallized skeletal material, or partially to completely leached. Almost all of the rudists and rudist fragments in the lower zone are preserved with no signs of leaching. Here, some rudists retain their wall microstructures, while others lack such preservation because of pervasive recrystallization.
Data and observations from cores, plugs, and thin sections of the upper zone provide unequivocal evidence for dissolution and creation of moldic pores. This moldic porosity is present either as whole molds, now filled by hydrocarbons, or as partially cemented molds. The latter contain coarse-crystalline calcite cements lining the pores (Fig. 4). Development of this moldic porosity resulted in an increase in both macroporosity and horizontal permeability in the upper Shuaiba zone. Importantly, dissolution and leaching features are observed to be intense and abundant below distinct stratigraphic boundaries and decrease with depth away from them. These boundaries are the regional unconformity, which occurs at the top of the Shuaiba and separates it from the overlying Nahr Umr Shales, and a subaerial exposure surface identified at the top of a rudist rudstone facies at a depth of 609.45 m in GN-23.
Micro-Rhombic Calcite Matrix.—
Matrix microporosity accounts for most of the hydrocarbon storage capacity in the reservoir at Ghaba North. SEM investigation of matrix samples shows that this microporosity occurs because of abundant intercrystalline pores associated with micro-rhombic calcite (Fig. 5). Micro-rhombic calcite crystals are generally equant and may show partial rounding of crystal terminations, reflecting limited dissolution. Micro-rhombic calcite occurs in two fabrics, a blocky-crystal framework (Fig. 5) and a crystal mosaic (Fig. 6). Individual micropores average about 5 to 10 μm in diameter; however, some micropores approach 62 μm in diameter. The latter are secondary in nature and include micromolds and microvugs.
Eighty-four samples of micro-rhombic calcite matrix, comprising all the plugs from GN-23, were analyzed for stable carbon and oxygen isotopes. In addition, three samples of late (post-leaching) coarse-crystalline calcite cements were analyzed. The δ13C of the micro-rhombic calcite matrix ranges from +3.11 to +5.95‰ and δ18O ranges from-7.31 to -4.10‰ (all relative to PDB) (Fig. 7). The three cement samples represent the most depleted carbon and oxygen isotope values.
As discussed previously, the lower Shuaiba zone is dominated by the foram and the algae-foram-rudist fragment biofacies, and is represented by the restricted lagoonal lithofacies. The upper zone is dominated by the rudist-Orbitolina biofacies, and is represented by the relatively open shelf/ramp lithofacies. Analysis of the variability trends of total measured porosity, macroporosity, microporosity, and horizontal permeability in Figures 3A–E, indicates that the upper zone, in general, has better reservoir characteristics. The upper zone has higher averages of total measured porosity, macroporosity, microporosity, and horizontal permeability. It is important to note that the upper zone contains fewer algae, more rudists, and more Orbitolina and other allochems than the lower zone. This difference is attributed to the depositional conditions that dominated during deposition of each zone. Obviously, sedimentology, allochemical constituents, and biofacies forming the lithofacies of the Shuaiba Formation, which are dependent on the depositional conditions, play an important role in determining the quality of the reservoir. The Shuaiba, in general, has very high total measured porosity (Petroleum Development Oman, 1990). However, examination of cores, plugs, and thin sections shows that macroporosity is very low in the lower zone and becomes higher in the upper zone. Cores and samples examined in this study were saturated with hydrocarbons throughout the Shuaiba interval, with heavier hydrocarbon staining toward the top.
The Shuaiba Formation has been described as a chalky limestone (Moshier, 1989a, 1989b; Al-Bastaki, 1993; Gallagher et al, 1990) because of its micritic texture. Chalky-textured limestone petroleum reservoirs in the Mesozoic and Tertiary deposits of the Middle East are very common (Wilson, 1975). The chalky texture is due to abundant and disseminated microporosity existing within the micritic matrix of typical carbonate platform sediments (Moshier, 1989a). The Shuaiba in Ghaba North Field has high porosities, which are attributed to the microporosity of the micro-rhombic calcite matrix; however, there is an observed variability in this microporosity with depth in both zones of the Shuaiba (Fig. 3C). Development of secondary moldic porosity in the upper zone, due to the leaching of rudists and rudist fragments, enhanced the horizontal permeability of the matrix, thus augmenting the reservoir quality and capacity to hold hydrocarbons. Therefore, in addition to the role played by the depositional environment in determining reservoir properties, there is a diagenetic role that further enhances these properties.
Controls on Microporosity.—
Similarity between the vertical variability of microporosity and of total measured porosity is indicative of the control that microporosity has on hydrocarbon storage capacity in this reservoir. Microporosity in the lower and upper Shuaiba zones is highly variable (Fig. 3C), although the lower zone has a slightly lower overall average porosity in comparison with the upper zone.
Further analysis of microporosity variability in the Shuaiba Formation was done by selecting five samples for SEM investigation. These samples correspond to alternating intervals with high and low excursions in the microporosity plot (Fig. 3C, Table 1). SEM observations of these samples indicates that microporosity occurs as intercrystalline porosity within the micro-rhombic calcite matrix. Sample No. 19, which corresponds to an extensively cemented rudstone interval, has relatively low microporosity because of the presence of a neomorphic calcite microspar that heals matrix microporosity. Sample No. 77 shows very high macroporosity but relatively low microporosity. This is due to the presence of two types of fabrics in the matrix: a blocky-crystal framework fabric and a crystal mosaic fabric (Figs. 5 and 6, respectively). The former has high microporosity values, whereas the latter has low microporosity values.
|Sample .||Depth (m) .||Total Porosity % .||Macroporosity % .||Microporosity % .||Horizontal Permeability md .|
|Sample .||Depth (m) .||Total Porosity % .||Macroporosity % .||Microporosity % .||Horizontal Permeability md .|
Controls on Macroporosity.—
The fact that the upper zone exhibits a higher average macroporosity than the lower zone (Fig. 3B) is consistent with the higher rudist content in the upper zone. Nearly all macroporosity occurs as a secondary moldic porosity that developed because of leaching of rudists and rudist fragments by undersaturated diagenetic fluids. The upper zone also shows a trend toward increasing macroporosity toward the top of the upper zone. This reflects the presence of abundant caprinid and caprotinid rudists rather than monopleurid rudists. Caprinid and caprotinid rudist shells had thick aragonitic layers and thin calcific layers (Peter Skelton, 1997, personal communication); thus, the rate of stabilization of these rudists to low-Mg calcite (LMC) is low during diagenesis (Al-Bastaki, 1993; Borgomano, 1991). Therefore, these rudists were leached more easily than rudists that stabilize to LMC at a faster rate (e.g., monopleurids).
The upper zone is capped by a regional unconformity, separating the Shuaiba Formation from the Nahr Umr Formation shales (Harris et al., 1984), which acted as a subaerial exposure surface from which potential diagenetic fluids invaded the Shuaiba. Thus, the lower zone has lower macroporosity values because it either contained lesser amounts of rudists for leaching and/or it was far from the reach of undersaturated meteoric fluids that caused leaching of rudists in the upper zone.
Controls on Horizontal Permeability.—
Note the similarity between the plots for macroporosity (Fig. 3B) and for matrix permeability (Fig. 3E). Higher permeability values in the upper zone and the increase in these values toward the top of the zone correspond to the increase in macroporosity and effective porosity, defined as the interconnected pores. This results from leaching of rudists and rudist fragments, where leaching creates moldic porosity and enhances the connectivity of the pore system (Al-Bastaki, 1993; Borgomano, 1991). The upward increase in rudist abundance also means an increase in the amount of coarser-grained sediments at the expense of finer-grained materials, also leading to enhancement of matrix permeability. Anomalously high values of horizontal permeability are observed near the top of the formation in Figure 3D, and result from moldic macroporosity and the presence of open fractures.
Diagenesis at Ghaba North Field
Rudist Leaching and Macroporosity Development.—
Rudists are a major skeletal constituent of the Shuaiba Formation and are represented by different taxa and morphologies and with different original mineralogies. Rudists and rudist fragments which have original shells that have thicker high-Mg calcite (HMC) layers (such as monopleurids) are more easily and rapidly stabilized to LMC than rudists originally with thicker aragonitic shell mineralogy (caprinids and caprotinids) (Al-Bastaki, 1993; Borgomano, 1991; P.W. Skelton, personal communication, 1997). The latter type are more commonly subjected to dissolution than to stabilization. Saturation with respect to LMC is attained much faster in sediments containing mixed stabilized and unstabilized constituents, because of higher solubilities of metastable mineralogies relative to LMC. This mechanism is fabric-selective and is termed "mineral-controlled alteration", which acts in low-activity phreatic realms (low water/rock ratio) (James and Choquette, 1984). However, if the phreatic realm was very active or all the constituents of the sediment were already stabilized, then alteration of the sediment occurs depending on the nature of the fluid (saturated or undersaturated). This process is non-fabric selective and is termed "water-controlled alteration". Leaching here might occur if large volumes of undersaturated fluids flowed through the sediment (high water/rock ratio) (James and Choquette, 1984). This process then proceeds in the sediment depending on saturation state, flow rates, porosity, permeability, and grain size. Rudists and rudist fragments, in intervals below the 619.45 m boundary separating the lower and upper Shuaiba zones in GN-23, are preserved and extensively cemented. Above 619.45 m rudists are only rarely preserved and the majority show development of moldic porosity. The degree of moldic porosity creation increases with proximity to the local subaerial exposure surface at 609.45 m and the regional unconformity at the top of the Shuaiba Formation. Different styles of rudist modification in the upper and lower Shuaiba zones suggest that the formation was affected by more than one diagenetic event in the Ghaba North area.
Development of Micro-Rhombic Calcite Matrix and Microporosity.—
Development of micro-rhombic calcite matrix in the Shuaiba is a very important event because intercrystalline microporosity of this matrix accounts for most of the hydrocarbon storage capacity in the reservoir. This micro-rhombic calcite has previously been identified in the Shuaiba formation in the United Arab Emirates by Moshier (1989b) and Budd (1989).
Most lime mud accumulating in modern shallow-platform settings is formed from post-mortem disintegration of calcareous green algae, precipitation from seawater, and bioerosion and abrasion of skeletal debris (e.g., Bathurst, 1975). Calcite micro-rhombs and associated microporosity in ancient rocks, on the other hand, most likely are the products of stabilization of this type of original metastable lime mud. However, the diagenetic environment in which stabilization occurred has been debated by Moshier (1989b) and Budd (1989). On the basis of geochemical data, Moshier (1989b) suggested that microporosity of the Shuaiba and other Lower Cretaceous formations in the United Arab Emirates formed by stabilization in a closed system of marine pore waters. On the other hand, Budd (1989) argued for the development of micro-rhombic calcite by meteoric fluids through a two-step process involving subaerial exposure and shallow burial diagenesis.
Matrix in the Shuaiba Formation at Ghaba North Field has both types of fabrics, blocky-crystal framework and crystal mosaic fabrics, identified and described by Moshier (1989a, 1989b). In general, blocky-crystal framework texture has higher microporosity and permeability, whereas crystal mosaic texture has lower values of both. Dissolution along crystal boundaries creates secondary intercrystalline microporosity and enhances permeability by enlargement of interconnected porethroats (Moshier, 1989a). Moshier suggested that a framework fabric developed by stabilization of wet, unconsolidated lime muds. Additional cementation and compaction of crystals of the framework resulted information of the mosaic texture with less microporosity. In addition, he suggested that framework texture forms in systems with low water/rock ratio, whereas crystal mosaic texture forms in (open) systems with high water/rock ratio. His argument was that open systems introduce excess carbonate for thorough cementation.
Analysis of Stable Carbon and Oxygen Isotopes.—
A cross-plot of δ13C and δ18O of both the matrix and late coarse-crystalline calcite cements (Fig. 7) shows ranges of +3.11 to +5.95 ‰ δ15C and -8.25 to -4.10‰ δ18O (PDB). The more enriched values for carbon and oxygen are generally taken to be closer to the original marine composition, while the more depleted values could be interpreted in different ways. For instance, Moshier (1989b) interpreted the δ13C range of +3.4 to +4.0‰ and the δ18O range of -4.0 to -5.0‰ of micro-rhombic calcite matrix of the Shuaiba in the Sajaa Field in the United Arab Emirates to be indicative of stabilization of matrix in a closed system with marine porewaters. Because a range of -2 to -2.5 ‰ PDB was reported for δ18O of Aptian marine calcite (Scholle and Arthur, 1980; Moldovanyi and Lohmann, 1984), he suggested that his more depleted δ18O values are the result of dissolution and reprecipitation through a process of replacement of original matrix materials during diagenesis. Thus, Moshier (1989b) suggested that the Shuaiba matrix was depleted by 1.5 to 2.0‰ relative to its precursor marine sediment.
On the other hand, Budd (1989) interpreted the δ13C range of micro-rhombic calcite matrix of +3.1 to 4.5‰ PDB to be primary and unaltered by diagenesis. However, the δ18O range of -4.5 to -6.0‰ PDB was interpreted as a reflection of alteration by meteoric waters. Inasmuch as isotopic values similar to those in both the Budd (1989) and the Moshier (1989b) studies occur at Ghaba North Field, one could interpret these data as either indicative of stabilization in a system with marine pore-water composition or in meteoric water.
Evidence exists for subaerial exposure and communication with meteoric waters in Ghaba North Field. Two surfaces recognized in GN-23 have potential importance for this argument: a subaerial exposure surface recognized at 609.45 m and the regional unconformity at the top of the Shuaiba. Further evidence comes from observation of extensive leaching beneath these surfaces and from calcite cement fabrics. Other workers have recognized subaerial exposure both within and at the top of the Shuaiba Formation (Wagner, 1990; Vahrenkamp, 1996).
Isotopic data from micro-rhombic calcite (Figs. 7, 8B) can be separated into two clusters between those samples with δ18O values more depleted or more enriched than a δ18O value of -6 ‰. Although this is a mostly arbitrary demarcation, virtually all samples above 609.45 m in GN-23 are more enriched than -6‰ δ18O (Fig. 8B). These data likely suggest that the calcite rhombohedra developed in more than one event. If this is the case, it explains the deviation of data from the pattern suggested by Allan and Matthews (1977, 1982) for the variation expected in carbon and oxygen isotopic compositions of limestones during fresh-water diagenesis. Typically, a narrower range of δ18O values and a wider range of δ13C values results from fresh-water diagenesis, in part because water has a large reservoir for oxygen relative to the rock (Lohmann, 1988). Thus, most of the oxygen incorporated in a diagenetic product is sourced from water, and the oxygen-isotope composition should be relatively invariant for a single diagenetic event. On the other hand, carbon reflects a combination of sources, such as soil-gas CO2, precursor CaCO3, and atmospheric CO2. The carbon-isotope composition reflects water-rock interaction and proximity to an exposure surface and therefore varies with depth (Allan and Matthews, 1982; Lohmann, 1988). However, if more than one diagenetic event affected the sediments, then this pattern would not hold, and compositions of diagenetic carbonates would be dependent upon the isotopic composition of the different waters involved.
Variations of δ13C and δ18O with depth in the Shuaiba Formation (Fig. 8A and 8B) should be used together with petrographic and stratigraphic observations. The two plots can be divided into three zones (Zones I-III) with different isotopic characteristics. The first zone starts from the bottom upward to a depth of 619.45 m. The second zone begins at 619.45 m and ends at 609.45 m. The third zone starts at 609.45 m and ends at the top of the Shuaiba at 596 m. Each one of these diagenetic zones is discussed below.
Zone I is distinguished from the rest of the Shuaiba by its nearly invariant δ18O and δ13C values. The 619.45 m upper boundary was also recognized as the stratigraphic boundary, a marine hardground, between the lower Shuaiba zone (interpreted as restricted-lagoon lithofacies) and the upper Shuaiba zone (interpreted as relatively open-shelf lithofacies). Because -2 to -2.5 ‰ PDB was suggested for the δ18O value of calcific sediments deposited from Aptian seawater (Scholle and Arthur, 1980; Moldovanyi and Lohmann, 1984), then an average δ18O value of -6.7‰ PDB of the first zone is depleted at least 4% PDB relative to marine values. If matrix stabilization had occurred in marine pore waters, then δ18O of the matrix should be around -4‰ PDB, which indicates depletion by 1.5 to 2.0‰ relative to its marine precursor (Moshier, 1989b). We suggest that stabilization in meteoric waters was responsible for the additional 2.7‰ depletion beyond this value.
The average δ13C of micro-rhombic matrix samples in Zone I is approximately +4‰ PDB, similar to values reported by other workers studying Shuaiba reservoirs (Moshier, 1989b; Budd, 1989; Wagner, 1990; Vahrenkamp, 1996). Carbon incorporated in the diagenetic stabilization process that produced the micro-rhombic calcite was dominated by carbon sourced from the rock. This suggests diagenesis under conditions of low water/rock ratio and stabilization far away from a source of depleted carbon, such as a subaerial exposure horizon. A hardground present at 619.45 m likely acted as a permeability barrier separating Zone I sediments from overlying subaerial conditions. Confined flow of meteoric water and stabilization below the hardground could have produced the relatively invariant isotopic compositions of the lower zone. In such a scenario, flow rates are low, water/rock ratio is low, oxygen-isotope values are depleted relative to marine conditions, and the carbon-isotope composition is buffered by the composition of the precursor limestone. A low water/rock ratio also would have the effect of producing slow stabilization of rudist material rather than leaching. Although timing of meteoric diagenesis of Zone I is unclear, it probably occurred prior to subaerial exposure within and atop the upper Shuaiba zone.
Zone II extends from 619.45 m to 609.45 m, where an erosional surface is recognized in core. The δ18O of Zone II averages about -6.4‰ PDB. Apart from the δ18O value at the base of this zone, other values are almost constant and then become enriched toward the top of the zone. Carbon-isotope values in this interval become progressively depleted toward the top. Carbon-isotope depletion and oxygen-isotope enrichment upward toward an erosional surface is a common pattern of isotope variability imparted by meteoric diagenesis at a subaerial exposure surface (Allan and Matthews, 1978, 1982). In an arid climate with only limited plant and soil-zone activity, carbon-isotope depletion is attenuated at subaerial exposure surfaces (Budd, 1989). This may account for the limited depletion in carbon-isotope composition at the exposure surface. Depletion may also be attenuated because most arid vegetation would be expected to follow the C4 photosynthetic pathway, and soil-gas CO2 in arid regions may be enriched in δ13C by as much as 8‰ relative to soil gas in humid regions (Rightmire and Hanshaw, 1973). It is also likely that subsequent marine submergence of this surface would have led to erosion of whatever thin soil zone may have been present.
Stratigraphic evidence, represented by the presence of the erosional surface, and petrographie evidence, represented by extensive leaching of rudists and rudist fragments below the surface, corroborate these isotopic data. We envision localized subaerial exposure and development of a small freshwater lens in a rudist bioherm. This may have occurred at a number of places on the Shuaiba shelf where local seafloor topography was exposed during a minor relative sea-level fall.
Zone III extends from 609.45 m upward to the regional unconformity separating the Shuaiba from the overlying Nahr Umr Formation. Vertical δ18O and δ13C variations again show a pattern indicative of subaerial exposure and meteoric diagenesis. The presence of extensive leaching and moldic secondary porosity, especially below the unconformity, is further support for subaerial exposure at the end of Shuaiba time. The occurrence of a boundary at 600.4 m in this zone is obvious from δ13C and δ18O variation plots with depth (Fig. 8A, B). Sharp enrichment in δ13C and δ18O directly below 600.4 m and the depletion trend of these above it toward the top is a pattern indicative of a vadosephreatic boundary (Allan and Matthews, 1982). The Shuaiba above 600.4 m has very high macroporosity and abundant unconsolidated rubbly intervals, indicative of leaching and a lack of extensive cementation, characteristics of subaerial diagenesis.
Late Coarse-Crystalline Calcite Cements.—
Coarse-crystalline calcite cements postdate compaction but predate hydrocarbon migration, and therefore precipitated at intermediate burial depths. These cements occupy secondary moldic pores that we interpret to have formed during early subaerial exposure and meteoric diagenesis. Samples of these cements have an average δ18O of -8‰ PDB and an average 813C of +3.3‰ (Fig. 7). These cements have the most depleted oxygen values in this study, and are interpreted to have precipitated under slightly higher-temperature, moderate burial conditions. It is likely that these cements were precipitated after deposition of the Nahr-Umr Formation. Migration of hydrocarbons into the Shuaiba Formation prevented these cements from completely occluding moldic porosity. Hydrocarbons would not have been trapped in the Shuaiba at Ghaba North Field unless Nahr Umr Shales had sealed the Shuaiba and the structure had already been developed. Thus, coarse-crystalline calcite cements were precipitated after the formation of the anticlinal trap at Ghaba North, which occurred during uplift associated with salt tectonics.
Summary and Conclusions
The Shuaiba Formation (Aptian) in Ghaba North Field consists of two stratigraphically and sedimentologically distinct zones. The lower zone represents deposition below fair-weather wave base under restricted to partially restricted water circulation in a lagoonal setting. The upper zone represents deposition in a shallow relatively open shelf/ramp setting with more open (less restricted) circulation conditions, and above fair-weather wave base. Overall, this stratigraphic stacking represents a shallowing-upward trend.
Favorable reservoir characteristics are best developed in the upper Shuaiba zone. Development of secondary moldic porosity enhances reservoir properties; however, matrix microporosity is responsible for most of the hydrocarbon-holding capacity of the reservoir. The upper Shuaiba zone has higher macroporosity and horizontal permeability, and thus better reservoir quality. Microporosity is present as intercrystalline pores within microrhombic calcite matrix.
Three diagenetic zones are identified by stable-isotope data and supported by stratigraphic and petrographie evidence in the GN-23 core. These zones are indicative of different diagenetic events and the complexity of the Shuaiba diagenetic history. The lower zone was stabilized in a confined meteoric aquifer. The second zone experienced local subaerial exposure and meteoric diagenesis. The upper zone was affected by exposure at a regional unconformity marking the end of Shuaiba time. From isotopic evidence, presence of subaerial exposure surfaces, and the action of leaching, it can be concluded that meteoric waters were the main diagenetic fluids responsible for diagenesis of the Shuaiba Formation at Ghaba North Field. Matrix stabilization, development of micro-rhombic calcite, and the formation of intercrystalline microporosity occurred in the presence of meteoric water. Development of secondary moldic porosity in rudists and rudist fragments was controlled by the presence of abundant rudists, original mineralogy of the rudists, and proximity of rudist-rich facies to subaerial exposure surfaces.
Stabilization of the Shuaiba matrix, development of moldic macroporosity associated with the exposure surfaces, and establishment of the anticlinal trap in Ghaba North occurred relatively rapidly. The presence of partially cemented molds by coarse-crystalline calcite cement indicates that hydrocarbons migrated into the reservoir soon after precipitation of this late cement. Hydrocarbon migration thus prevented occlusion of porosity by cementation or compaction under heavy overburden pressures at deep burial depths.
This study of the Shuaiba Formation is fundamental for a development plan for the Ghaba North Field. This is a heterogeneous reservoir, with the upper Shuaiba zone providing the best reservoir properties. Most hydrocarbon storage is within matrix microporosity; however, reservoir quality is enhanced in the presence of secondary moldic porosity. The presence of two subaerial exposure surfaces is significant for further exploration and exploitation. The subaerial exposure surface at 609.45 m in GN-23, although it likely records a localized exposure event, suggests that others may be present. Local growth of rudist bioherms or buildups above sea level, or a minor fall in relative sea level, could have occurred at other localities on the shelf. Such localized exposure, with subsequent meteoric diagenesis, would provide enhancement of the Shuaiba reservoir, similar to that developed in GN-23. Development of secondary moldic porosity should be denoted by an amplitude decrease in the top Shuaiba seismic reflectors, thus reducing the ordinary velocity contrast between the higher-velocity limestone of the Shuaiba and the overlying lower-velocity Nahr Umr shale (Frost et al., 1983). Variation of reservoir properties should be considered in any development or flooding program of the Shuaiba Formation in Ghaba North Field.
We wish to acknowledge Petroleum Development Oman for providing the data and support of the first author’s Master’s thesis project at Colorado School of Mines. J. Curtis and J. Warme provided constructive criticism during various stages of this work. Editors of the special publication, A. S. Alsharhan and R. W. Scott, are thanked for their encouragement and patience. Insightful reviews by P. M. Harris and V. C. Vahrenkamp significantly improved this manuscript.
Figures & Tables
Middle East Models of Jurassic/Cretaceous Carbonate Systems
This volume will interest tectonic modelers, stratigraphers, sedimentologists, and explorationists. It is the product of the international conference of “Jurassic/Cretaceous Carbonate Platform-Basin Systems, Middle East Models” that was convened in December 1997 jointly by SEPM (Society for Sedimentary Geology) and the United Arab Emirates University in Al Ain, United Arab Emirates. The twenty-three papers present new data and interpretations arranged in three sections: 1) sequence stratigraphy, cyclostratigraphy, chronostratigraphy, and tectonic influences, 2) depositional and diagenetic models of carbonate platforms, and 3) hydrocarbon habitat and exploration/development case studies. New tectonic models of the Arabian Basin, new stratigraphic and sequence stratigraphic reference sections, new geochemical and source rock data, and new reservoir data are presented. New geologic models make this set of papers relevant to geoscientists working outside of Arabia also.