GeoArabia, Vol. 6, No. 1, 2001 Gulf PetroLink, Bahrain Arab-D Reservoir, Ghawar Field, Saudi Arbia Geology and Production Significance of Dolomite, Arab-D Reservoir, Ghawar Field, Saudi Arabia David L. Cantrell, Saudi Aramco, Peter K. Swart, University of Miami, Robertson C. Handford, Consultant, Christopher G. Kendall, University of South Carolina, and Hildegard Westphal, University of Hannover ABSTRACT At least five distinct types of dolomite occur in the Arab-D Reservoir in Ghawar field, Saudi Arabia – one of which appears to be responsible for high flow or ‘super-k’. These dolomite types are distinct petrographically, geochemically and stratigraphically: • a finely-crystalline non-fabric-preserving (NFP) variety of dolomite in the lower Arab- D (Zone 3) with low oxygen isotope values and generally poor reservoir quality; • a medium-crystalline NFP dolomite with high oxygen isotope values and very poor reservoir quality in the upper Arab-D (Zone 2); • a medium to coarsely-crystalline NFP dolomite with low oxygen isotopic values and very good reservoir quality (‘super-k’) occurring in Zone 2; and • a finely-crystalline fabric-preserving (FP) dolomite in the uppermost Arab-D (Zone 1) that contains high oxygen isotope values and has generally fair to poor reservoir quality. Previous studies have documented a rare fifth type of dolomite, baroque or ‘saddle’ dolomite, that occurs locally in the reservoir as well. This study also quantified and mapped the abundance and distribution of dolomite across the field, using all available core and log data. Analysis of dolomite distribution map patterns reveals that dolomite occurs in Ghawar as a series of linear trends extending for tens’s of kilometers. These map pattern trends are best-developed in Zone 2B, but are also visible in Zones 2A and 3A as well. Baroque dolomite appears to be limited to a few areas of vertically pervasive dolomite occurring on the same trends of high dolomite content. The linearity of these dolomite trends strongly suggests that some structural element is responsible for controlling their orientation. We interpret these linear patterns to have formed in response to a series of fracturing and/or faulting events that allowed dolomitizing fluids to move up into the reservoir from below, and preferentially dolomitize there. Both a qualitative and a quantitative analysis of performance data (flowmeters) in southern Ghawar (Haradh) indicate that these trends of high dolomite have a profound influence on fluid flow in the reservoir. A qualitative analysis of occurrences of ‘super-k’ in the Arab-D in Haradh suggests that most ‘super-k’ zones (seen as ‘spikes’ or step profiles on the flowmeter) occur in the high dolomite trend in Haradh. A quantitative analysis of flowmeter data and a comparison of this analysis with dolomite map patterns indicate that most reservoir flow occurs where dolomite is abundant, and suggests that there is a direct relationship between patterns of high flow and high dolomite. INTRODUCTION The Arab-D reservoir in Ghawar field (Figure 1) contains a significant amount of dolomite rock (carbonate rocks that are composed of more than 75% of the mineral dolomite). These rocks typically formed during diagenesis of pre-existing limestones and thus their pore systems and reservoir characteristics are fundamentally different from those of limestones. The occurrence of dolomite in 45 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/1/45/4559695/cantrel.pdf by guest on 23 September 2021 Cantrell et al. Bakr Fadhili Abu Sa’fah Samin Dhib Qatif Al-Rayyan Dammam North Dome N V A F U A N G BAHRAIN Fazran Abqaiq Awali 26 ’Ain Dar Shedgum ’Uthmaniyah Ghawar Doha Khurais Dukhan 25 Hawiyah QATAR Qirdi Harmaliyah Farhah Haradh Mazalij Sahba Tinat 24 Wadayhi Abu Shidad Lughfah Waqr N Shama’ah Jawb 0 50 km 23 SAUDI ARABIA Oil field Gas field 48 49 50 51 ARAB-HITH TERMINOLOGY Fm Member Reservoir Manifa GENERALIZED UPPER JURASSIC HITH STRATIGRAPHIC SEQUENCE FORMATION MEMBER LITHOLOGY ARAB-A Arab-A HITH Arab-A ARAB-B Arab-B Arab-B ARAB Arab-C ARAB-C Arab-D Arab-C ARAB JURASSIC JUBAILA 1 ARAB-D 2A HANIFA 2B 3A Wackestones/ Arab-D Anhydrite Grainstones/ Packstones Mudstones 3B JUBAILA 4 Figure 1: Basemap showing the various operational areas of Ghawar field, and a schematic display of the reservoir zonation naming scheme used in Ghawar. 46 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/1/45/4559695/cantrel.pdf by guest on 23 September 2021 Arab-D Reservoir, Ghawar Field, Saudi Arbia the Arab-D has a pronounced impact on reservoir quality, in a number of ways. At times, dolomite is responsible for producing zones of very high flow (‘super-k’) in the reservoir, while at other times these rocks act as permeability barriers or baffles. ‘Super-k’ in the reservoir is typically defined as an interval in which flow of greater than 500 barrels per day per foot of vertical interval occurs. There are important differences between dolomites and limestones in the reservoir, both in terms of their impact on reservoir quality as well as in terms of the reservoir management strategies that are needed for these rocks. Dolomite typically has a higher grain density and acoustic velocity than limestones, which affects the response of most porosity wireline logging tools to dolomite rocks. In addition, dolomite reacts more slowly to the presence of acids than does limestone, resulting in uneven responses to acidization treatments. Surface chemistry differences may also result in dolomite and limestone differing in their wettablity and adsorption characteristics within a reservoir. Finally, limestone and dolomite differ greatly in their strength and ductility. Abundant experimental (Handin et al., 1963; Hugman and Friedman, 1979) and field data (Stearns, 1967) suggest that dolomite is generally stronger and more brittle than limestone at the same burial depth. The preferential development of fractures in dolomite beds often enhances their permeability (without greatly affecting their porosity) and potentially influences the overall quality of the reservoir. This Study This study examined these dolomite rocks and their impact on fluid flow in the reservoir, and included three basic work phases: • geochemical and petrographic characterization of dolomite types present in the Arab-D; • quantitative evaluation of the abundance and distribution of dolomite in the reservoir; and • assessment of the impact of dolomite on reservoir performance and delineation of likely flow units—and especially the high flow or ‘super-k’ units—in the reservoir. In this paper, we will focus on a qualitative and quantitative characterization of dolomite in the Arab D, and integrate this information with reservoir performance data to improve our understanding of these important reservoir rocks. Since dolomite represents both the very best and the very worst reservoir quality rocks in the Arab-D, it is critical that we accurately describe their properties and distribution in the reservoir. This study thus represents a first step in our efforts to geologically characterize these rocks and, ultimately, develop a predictive model for variability in their reservoir quality. GEOCHEMICAL AND PETROGRAPHIC CHARACTERIZATION OF DOLOMITE—or Not All Dolomite is Created Equal! This study identified the occurrence of five different types or styles of dolomite in the Arab-D at Ghawar field. These different dolomite types are distinguished on the basis of their petrographic and geochemical signature, and each has a different impact on reservoir quality. The four main types of dolomite include: Zone 3 non-fabric preserving (NFP) dolomite, Zone 2 non-’super-k’ NFP dolomite, Zone 2 super-k NFP dolomite, and Zone 1 fabric preserving (FP) dolomite. These four dolomite types are basically restricted to the reservoir zone for which they are named. A rare fifth type of dolomite, baroque or ‘saddle’ dolomite, has been previously documented in the Arab-D (Cantrell and Hagerty, 1988); this type of dolomite is volumetrically insignificant in the reservoir, and will not be discussed further in this paper. Zone 3 NFP Dolomite Most examples of dolomite from this zone occur as beds of partly dolomitized mud- to wackestones with considerable amounts of preserved calcite (Figure 2). In these rocks, dolomite typically has obliterated all traces of the original limestone fabric, to create a non-fabric-preserving (NFP) style of dolomitization. Petrographically, this dolomite is finely crystalline, with typical crystal sizes reaching about 100 µm in size. Dolomitization in this part of the reservoir appears to have occurred preferentially 47 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/1/45/4559695/cantrel.pdf by guest on 23 September 2021 Cantrell et al. 01mm 01mm Figure 2: Thin section photomicrograph of a Figure 3: Thin section photomicrograph of non- partially dolomitized mudstone from the lower super-k NFP dolomite from the upper Arab-D Arab-D (Zone 3) in a Ghawar well. Cores of the (Zone 2) in a Ghawar well. Sample is 100% dolomite crystals in this sample are partially dolomite. calcitic. In this and all later thin section photomicrographs, calcite is stained pink to red, and porosity is filled by blue-dyed epoxy. 01mm 01mm Figure 4: Thin section photomicrograph of Figure 5: Thin section photomicrograph of FP super-k NFP dolomite from the upper Arab-D dolomite from the uppermost Arab-D (Zone 1) (Zone 2) in a Ghawar well.This sample located in a Ghawar well. Note the exquisite only 5 ft away from the previous example, in the preservation of original depositional fabric in same well. Sample is 100% dolomite. this sample. Sample is 100% dolomite. in and around non-depositional (hiatal) surfaces and their associated burrows or borings. 18 Geochemically, these rocks display relatively negative (light) oxygen isotope values (-3 to -6‰ δ O) 13 and positive (heavy) carbon isotope values (2.5 to 3.3‰ δ C)(Figure 6).