Carbonate-platform scale correlation of stacked high-frequency sequences in the Arab-D reservoir, Saudi Arabia
Introduction
Predicting the existence of a facies, and harder still its extent and geometry, in the subsurface posits a defying task to the stratigrapher. This task becomes even more daunting in the realm of carbonate sequence stratigraphy owing to the in situ nature of carbonate sedimentation. The matter becomes further complicated when stratigraphic analysis is conducted in a regional, shelf-to-basin, scale since complete, shelf-to-basin, platformal transects are less common in outcrops (Goldhammer et al., 1990, Mitchum and Van Wagoner, 1991, Handford and Loucks, 1994, Tinker, 1998). Yet, regional stratigraphic analysis is key in comprehending lateral and vertical facies changes and predicting intra-reservoir geometric relationships, which together with diagenesis and fracturing shape the porosity–permeability schemes of carbonate reservoirs. Thus, platform-scaled correlation, if this could be conducted at the high-frequency-sequence scale, can contribute in resolving reservoirs' architectural heterogeneities and pin down fluid-flow patterns.
This paper presents the results of a detailed sequence stratigraphic analysis of one of the world's most prolific reservoirs, the Arab-D reservoir, in Saudi Arabia. The Arab-D is one of four carbonate reservoirs of the Upper Jurassic Arab Formation. Each of these carbonate reservoirs possesses remarkable porosity and permeability, and each is capped by a nonpermeable anhydrite unit (Fig. 1). The Arab Formation reservoirs are sandwiched between the organic-rich mudstones of the Hanifa and Tuwaiq Mountain formations below and the tight anhydrites of the Arab and Hith formations above (Fig. 1). Giant expansive structural traps, such as the anticlines of Ghawar and Khurais fields (Fig. 2), proficiently harvest the Arab Formation oils from the extensive source rocks that lay across the Arabian Peninsula. In 2009, Saudi Aramco successfully completed the largest oil expansion project in the earth's history, known as the Khurais Mega Project bringing to production nationally significant rates from Khurais and adjacent satellite fields mainly from the 100-m-thick Arab-D reservoir (Al-Ghamdi et al., 2008, Al-Mulhim et al., 2010, Mouawad, 2010).
This paper details the stratal patterns of the high-frequency sequences that compose the reservoir, discusses the sequences' characteristics and identification criteria and correlates them on a platform-wide, shelf-to-basin, scale. The study also addresses the long-standing controversy surrounding the progradational direction of the reservoir, which has been suggested to be prograding in virtually all directions — north, south, east and west (Mitchell et al., 1988, Meyer and Price, 1993, Al‐Saad and Sadooni, 2001, Handford et al., 2002, Lindsay et al., 2006, Stephens et al., 2009). To address this controversy, the study uses an extensive, never previously used data set of 32 cored wells described meticulously at a 10-cm scale and 500 thin sections. Working at such high-resolution enables the detection of subtle lithofacies variations and deciphers what they disclose in terms of key geologic factors, their relative significance, and how they interplayed to control the paleoenvironments of deposition. The study also manifests the benefits of expanding the scope of geologic observations to a broader, trans-political-boarders sense, as it makes use of reported observation on the regional thinning and pinches out trends of the Arab and Hith formation carbonates and anhydrites in the United Arab Emirates, as discussed in Section 5.
Examining the cores from Khurais Field offered an excellent opportunity to formulate a regional perspective of the reservoir's configuration and bridge, for the first time, the correlational gap between the Arab-D outcrop in Wadi Nisah, south of the Saudi capital, Riyadh, and the subsurface beneath Ghawar Field (Fig. 2). Two cross-sections are presented, a W–E one that extends from the Arab-D outcrop to Ghawar Field across a distance of 413 km; and a N–S cross-section that extends from the Safaniya area down to the southern tip of Khurais Field along a 397 km line of section (Fig. 2).
The concluded correlation model differs from the one presently in use and relies on the authors' previously proposed depositional environments' model and sequence stratigraphic framework of the reservoir (Al-Awwad and Collins, in press, Al-Awwad and Collins, in review). This model is able to predict lateral and vertical heterogeneities and architectural arrangements of the reservoir facies regionally. This could potentially guide future modeling of three-dimensional-spatial distribution of porosity, permeability, diagenetic, structural and other petrophysical parameters within the reservoir and depict how they dictate the architecture of fluid-flow and -baffle units.
Section snippets
Depositional model and sequence stratigraphic framework
The Arab-D lithofacies recognized in the Khurais Field are detailed in Al-Awwad and Collin's (in press) paper and summarized in Table 1; these lithofacies start at the bottom of the succession with monotonously interbedded couplets of lime mud and intraclastic lithofacies representing offshore basinal turbidites, followed by pelletal lithofacies that represent lower shoreface sands and silts, and then by stromatoporoid lithofacies which formed a shelf fringing reef. The reef protected a lagoon
Regional and structural settings
The Arabian plate formed by the accretion of the Midyan, Hijaz, and Asir terranes (Fig. 2c) to northeastern Africa before 715 Myr (Stoeser and Camp, 1985). After that, the Afif Terrane was accreted between 680 and 640 Myr, forming the Nabitah Suture. The Rayn Terrane, which is comprised of eastern and central Arabia fused with the Afif Terrane in what is known as the Amar collision, which took place between 640 and 620 Myr and yielded the Amar Suture, an obducted N-trending thrust slices of
Methodology
One dimensional sequence stratigraphic analysis was initially conducted on 32 cored wells in Khurais and presented in Al-Awwad and Collins's (in review) paper (Fig. 3a). Lithofacies components, repetitive motifs of lithofacies stacking patterns, recognition of retrogradational, aggradational and progradational modulations, vertical thickening versus thinning trends, upward fining versus coarsening and/or shallowing versus deepening trends (Van Wagoner et al., 1990) coupled with gamma log
Regional correlation
Bridging the correlational gap between the Arab-D outcrop and the subsurface in Ghawar Field has only been tentatively attempted before by Leinfelder et al. (2005). Another regional correlation was attempted by Handford et al. (2002), but it did not account for the area west of Ghawar, which, in light of this study, has a significant role in understanding the regional configuration of the reservoir and interlacing the outcrop to the subsurface (Al‐Saad and Sadooni, 2001).
It has been established
Composite sequence boundaries
The different sequence stratigraphic interpretations concluded by different authors who worked on the Arab Formation, as presented in Fig. 1, are imputed to the inherent difficulties of the succession represented by the inaccuracies of the biostratigraphic–radiometric timing that marks the entire Phanerozoic of the Arabian platform (Al-Husseini and Matthews, 2005, Al-Husseini and Matthews, 2008). This combines with the Arab Formation's lack of biostratigraphic age control (Hughes, 2004) and
Conclusions
Predicting lateral and vertical heterogeneities and architectural arrangements of carbonates in the subsurface are a challenging task. High-frequency-sequence scaled stratigraphic analysis and correlation is crucial in tackling this task and is presented here through the analysis of one of the world's most significant reservoirs, the Arab-D reservoir.
The preserved upward shallowing trend of the Arab-D reservoir across the Khurais Field is manifested by an eastward progradational trend. This is
Acknowledgments
We thank Aus Al-Tawil of Saudi Aramco for his unrelenting support and commitment to this study. We would like to thank Saudi Aramco for granting permission to publish this study. Our appreciation is also extended to Luis Pomar, University de les Illes Balears, Spain for his beneficial discussions and keen insights.
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