GeoArabia, Vol. 8, No. 1, 2003 Gulf PetroLink, Bahrain Tilted OOWC, Arab Reservoir, Ghawar field Tilted original oil/water contact in the Arab-D reservoir, Ghawar field, Saudi Arabia Bruno Stenger, Tony Pham, Nabeel Al-Afaleg and Paul Lawrence ABSTRACT A review of the electrical logs, fluid properties, and production history of 195 flank wells drilled in the Arab-D carbonate reservoir of the Ghawar field, Saudi Arabia, showed that the original oil/water contact was regionally tilted. The contact was about 200 ft higher in the southern Haradh sector than in the northern Shedgum and ‘Ain Dar sectors. In Haradh, the fluid contact was also locally tilted down from west to east by as much as 800 ft. In the reservoir, the oil and aquifer densities changed from lighter oil and denser water in the north to lighter water and denser oil in the south. Decreasing methane content caused the increase in oil density and a reduction in the water density was the result of a salinity decrease. The evolution of fluid densities was closely correlated to a decreasing regional-scale geothermal gradient, probably indicating that temperature controlled the distribution of fluid densities. Simple analytical calculations showed that the magnitude of the observed tilt of the original oil/water contact from north to south might be explained by changes in fluid densities. On the western flank of central Haradh, the Arab-D reservoir water was anomalously young and fresh and this created a large salinity gradient between the western and eastern aquifer legs. This anomaly was explained by pressure-dependent vertical leakage along the Wadi Sahba structural trough between the Arab-D reservoir and the shallower Biyadh aquifer. Consequently, the integrity of the Hith Formation seal above the Arab-D reservoir might be locally compromised under particular conditions. A full-field reservoir simulation model, specific geological features, and examples from the technical literature supported a static interpretation of the tilted original oil/water contact in the Arab-D reservoir of Ghawar through the combined effects of changes in oil and water densities. INTRODUCTION In fully buried reservoirs, trapped fluids are generally in a static condition prior to production, and the Original Oil/Water Contact (OOWC) would normally be horizontal. However, naturally occurring tilted OOWCs have been described. The most commonly accepted explanations are ‘frozen-in’ diagenetic trapping combined with late-tectonic tilting (‘forced’ static tilt), or regional hydrodynamic aquifers (dynamic tilt). Wilson (1977) introduced the idea that diagenetic porosity reduction in the aquifer combined with tectonic tilting may create tilted OOWCs. Yeats (1983) and Carlos and Mantilla (2000) interpreted tilted OOWCs as a result of tectonically induced rapid development of structural folding or tilting in reservoirs of low absolute permeability. Willingham and Howald (1965), Pelissier et al. (1980), Wells (1987, 1988), Winterhalder and Hann (1991), Beckner et al. (1996), Gauchet and Corre (1996) and Luebking et al. (2001) relied on aquifer hydrodynamics to account for tilted original contacts. For a geological formation that crops out at a high topographic elevation, rainwater infiltrates into the aquifer. If the aquifer system is also outcropping at a lower elevation, differences in the hydraulic head will induce water flow. Where hydrocarbons are trapped deeper in the sedimentary basin, the flow in the aquifer leg may lead to the dynamic tilting of the OOWC. Dickey (1963), Dickey and Soto (1974) linked aquifer activity with chemical composition at the scale of the sedimentary basin, and showed that highly saline brines are characteristic of static aquifers. Of special importance for this paper was the concept introduced by Bond (1973, 1975) of aquifers in static equilibrium even though changes in hydraulic head were measured. The apparent paradox was explained by variable salinity at the regional scale. To the best of our knowledge, this concept had not been applied to tilted OOWC prior to the work of Stenger (1999). 9 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/8/1/9/4564487/stenger.pdf by guest on 28 September 2021 Stenger et al. 40 50 60 TURKEY Caspian 48°E49° 50° Arabian Gulf Sea BAHRAIN SYRIA Med Sea IRAN N Manama IRAQ 0 300 Abqaiq JORDAN km 30 Fazran 30 KUWAIT 26° A ra G bian ulf Awali BAHRAIN QATAR EGYPT Arabian UAE Shield OMAN SAUDI ARABIA 'Ain Dar 20 20 Shedgum Red SUDAN Sea ERITREA YEMEN Arabian Sea QATAR ETHIOPIA Gulf of Aden 40 50 10 Khurais Uthmaniyah Dukhan Ghawar 25° 25°N Hawiyah Abu Jifan Qirdi Riyadh Farhah Manjurah Harmaliyah Mazalij Mazalij-24 Jafura Haradh Reem N Sahba 05024° Ghazal 24° Wudayhi Tinat Dilam km Shaden Waqr Raghib Lughfah Abu Shidad Tinat South Niban Abu Rakiz Shamah Jawb 47° 48° 49° 50° Figure 1: Ghawar field location map. From north to south the Ghawar field is divided into the following sectors: ‘Ain Dar, Shedgum, Uthmaniyah, Hawiyah, and Haradh. We propose to interpret the tilted OOWC in the Ghawar Arab-D reservoir by the combined effects of changes in oil and water densities in and around the Ghawar field. After discussing the regional setting, the tilted OOWC in Ghawar Arab-D reservoir will be described through field observations. A discussion on the static or dynamic nature of the tilted OOWC will review different mechanisms and interpretations. Finally, conclusions on the origin of the tilted OOWC will be submitted together with a brief review of implications for the on-going development of the southernmost area of Ghawar. As indicated by Aramco (1959) and discussed by Stenger (1999) and Stenger et al. (2001), the tilted OOWC in Ghawar does not lend itself to a straightforward classification. GHAWAR FIELD History The Arab-D carbonate reservoir of the Upper Jurassic Arab Formation in the onshore Ghawar field was discovered in 1948. Following further separate discoveries along the structure’s main axis, five production areas were quickly identified as parts of the giant Ghawar oil field (Figure 1): from north to south they are ‘Ain Dar, Shedgum, Uthmaniyah, Hawiyah and Haradh. At the Arab-D level, the field is a NNE-trending composite anticline 230 km long and about 30 km wide (Figure 2a). The gently dipping crestal region is composed of several sub-parallel axes. The anticline is asymmetric and fairly steep-sided (up to 10º dip). In the southernmost extension of Ghawar (southern part of South Haradh) the west flank is steeper (Figure 3). Farther north, for example in central Haradh and Uthmaniyah, the eastern flank is steeper. Figure 2b is a regional depth map of the top Jurassic. 10 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/8/1/9/4564487/stenger.pdf by guest on 28 September 2021 Tilted OOWC, Arab Reservoir, Ghawar field TOP ARAB-D FORMATION Figure 2a (left): Top Arab-D depth map of Ghawar 49 E 49 30' 26 26 N showing the composite nature of the anticline with several subparallel crestal axis. North Haradh has 'Ain Dar 4,800 been producing since 1996, whereas the development of central Haradh is on-going. Ghawar shows 5,000 changing cross-sectional assymetry along its Shedgum N-S strike e.g. in central Uthmaniyah and south 5,200 Hawiyah, the east flank is more sleeply dipping, 5,400 whereas in northern Hawiyah and south Haradh it is the western flank that dips more steeply 5,600 (Figure 3). 5,800 6,000 6,200 Depth (ft subsea) 6,400 6,600 6,800 7,000 Uthmaniyah 7,200 48°E 50 52 7,400 28°N 28 25 25 Hawiyah 26 26 Ghawar North 0 24 24 Haradh Central 7,000 N 030 South 22 Depth (ft subsea) 22 km 14,000 24 WESeismic Line 24 48 50 52 Figure 3 49 49 30' Figure 2b: Top Jurassic regional depth map. Points north and east of the red line are extrapolated. 11 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/8/1/9/4564487/stenger.pdf by guest on 28 September 2021 Stenger et al. SEISMIC TIME CROSS-SECTION, SOUTH HARADH West East 1.0 1.0 Two-way Time (sec) Two-way 2.0 2.0 West East Aruma Ahmadi Shu'aiba 1.0 1.0 Arab-D Two-way Time (sec) Two-way Jilh 2.0 2.0 05km Figure 3: Seismic cross-section through the southernmost part of the Ghawar field in South Haradh (see Figure 2a). In this part of Ghawar, the west flank is more steeply dipping than the east flank. 12 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/8/1/9/4564487/stenger.pdf by guest on 28 September 2021 Tilted OOWC, Arab Reservoir, Ghawar field According to Wender et al. (1998), the growth history of the Ghawar structure consisted of the following four main phases: Hercynian Orogeny (Carboniferous); Zagros rifting (Early Triassic); Early Alpine Orogeny (Late Cretaceous); and Late Alpine Orogeny (Tertiary). Post-Jurassic tectonic activity was generally mild and limited to the multistage rejuvenation (uplift and erosion) of the Ghawar structure— a part of the greater En Nala Anticline—that is bounded by major N-trending Hercynian basement faults. The ‘Ain Dar and Shedgum areas went on stream in 1951 and development progressed southward by stages. Due to the lack of aquifer support, peripheral gravity water-injection was started in the late 1960s to maintain the reservoir pressure at an adequate level. Since the early 1980s, powered seawater injection has replaced gravity injection in a bid to conserve freshwater resources. During the last fifty years, thousands of vertical wells have been drilled on 1-km spacings. Most of these wells have open-hole completions. In the last five years, the use of horizontal drilling has intensified, especially in the southernmost area where the degraded reservoir quality limits the productivity of vertical wells.