The Jurassic Period in Saudi Arabia is represented by the deposition of the Shaqra Group (Figure 1 after Fischer et al., 2001; Powers et al., 1966; Powers, 1968; Manivit et al., 1990). This group includes the prolific Arab reservoirs. In the Arabian Orbital Stratigraphy (AROS) framework, Al-Husseini and Matthews (2005) correlated model second-order sequence boundary SB2 11 (predicted at c. 159.7 Ma in the early Oxfordian Stage; i.e younger than Oxfordian/Callovian = Late/Middle Jurassic = 161.2 ± 4.0 Ma; Gradstein et al., 2004) to the boundary between the Hanifa Formation and underlying Tuwaiq Mountain Limestone (denoted Hanifa/Tuwaiq) in Saudi Arabia and Oman. In the subsurface of eastern Saudi Arabia (C.D. Redmond, inPowers, 1968) and Oman (Rousseau et al., 2005; 2006), the signature of the maximum sea-level drop associated with SB2 11 is manifested as a regional unconformity that involved erosion of the Tuwaiq Mountain followed by Hanifa onlap.

Above the Hanifa/Tuwaiq Boundary, orbital second-order depositional sequence DS2 11 is interpreted to consist of the Hanifa, Jubaila, Arab and Hith formations (Figure 1), and to have been deposited in a period of about 14.58 million years (my) from c. 159.7 to 145.1 Ma (Al-Husseini and Matthews, 2005). This note focuses on the stratigraphic architecture of the Hanifa Formation. Unlike the erosional Hanifa/Tuwaiq Boundary, the Jubaila/Hanifa Boundary is apparently conformable (Powers, 1968) and is here interpreted as a third-order sequence boundary (SB3).

Vaslet et al. (1983) divided the Hanifa Formation in the Wadi ar Rayn quadrangle, central Saudi Arabia, into two formal members: Hawtah Member (52.5 m thick) and overlying Ulayyah Member (53 m thick). The basal Ulayyah consists of a 4.5 m-thick bioclastic coral limestone (rich in echinoderm debris) with a carbonate-pebble conglomeratic base filling channels cut in the Hawtah Member. In the outcrops of Ar Riyad quadrangle the Hawtah (57 m) and Ulayyah (68 m) members (Vaslet et al., 1991; Mattner and Al-Husseini, 2002) each consists of 4 units (from base-up):

Hawtah Member

Unit 1: (15 m) Yellow fossiliferous (brachiopods, bivalves) clayey limestone and beige bioclastic limestone.

Unit 2: (22 m) Yellowish to gray clayey limestone interspersed with ocher bioclastic and pelletoidal calcarenite containing rare silicified stromatoporoids and with three massive meter-thick beds of beige to ocher bioclastic cherty limestone.

Unit 3: (10 m) Cream pelletoidal clayey limestone and ocher bioclastic and intraclastic calcarenite. This assemblage has yielded large nautili (1 m in diameter) and is capped by a hardground.

Unit 4: (10 m) Yellowish clayey limestone, bioclastic limestone containing rare ammonites, and gray bioturbated limestone in the upper part.

Ulayyah Member

Unit 1: (15 m) Yellow to gray clayey limestone with silicified corals and stromatoporoids; ocher bioclastic and pelletoidal calcarenite showing hummocky stratification.

Unit 2: (15 m) Cream reef limestone containing silicified corals and stromatoporoids, forming massive biostromal beds 50 cm thick; yellow to gray bioclastic limestone interrupted by several hard-grounds.

Unit 3: (15.5 m) Cream bioclastic massive reef limestone, containing silicified corals and stromatoporoids, either in growing position or broken; ocher oncolitic intraclastic calcarenite.

Unit 4: (22.5 m) Ocher pelletoidal, bioclastic, and lithoclastic (reworking of mudstone pebbles) calcarenite forming massive beds 50 cm thick, interspersed with cream clayey limestone (mudstone). This assemblage is capped by a massive meter-thick bed of ocher pelletoidal cross-bedded calcarenite.

Age of Hanifa Formation: The age of the Hanifa Formation is Oxfordian-early Kimmeridgian (Vaslet et al., 1991; Figure 1 after Fischer et al., 2001). At its base, the Hawtah Member is early Oxfordian (?Cordatum zone) based on brachiopods (Boullier, inManivit et al. 1990, and inFischer et al., 2001). The middle and upper parts of the member are middle Oxfordian (Plicatilis zone) according to the Euaspidoceras ammonite fauna (Enay et al., 1987, inFischer et al., 2001), nautiloids (Tintant, 1987, inFischer et al., 2001) and nannoflora (Manivit, 1987, inFischer et al., 2001). The Ulayyah Member is late Oxfordian in its basal part, based on the occurrence of foraminifera Alveosepta jaccardi (Andreieff, inManivit et al., 1990, and inFischer et al., 2001), and brachiopods (Boullier, inManivit et al., 1990, and inFischer et al., 2001). The upper part of the Ulayyah Member yielded echinid faunas (Clavel, inManivit et al., 1990, and inFischer et al., 2001) that suggest an early Kimmeridgian age (?Hypselocyclum zone).

Third, Fourth and Higher-order Depositional Sequences

Mattner and Al-Husseini (2002) interpreted the two members of the Hanifa Formation as two third-order sequences (Figure 2a). In turn, these authors interpreted the lower Hawtah Member in terms of five cycles. Each of the Hawtah cycles is about 10–20 m thick and consists of clayey limestone capped by a bioclastic limestone some one meter thick (Vaslet et al., 1991). In some cycles (e.g. Hawtah cycle 4 = unit 3 of Vaslet et al., 1991) the top of the capping limestone is a hard ground, indicating probable subaerial exposure and a fourth-order sequence boundary.

In the AROS framework, the Hawtah Member (denoted DS3 11.1) appears to consist of five fourth-order orbital cycles (DS4 11.1.1–11.1.5) that were deposted in a period of 2.025 my (5 × 0.405 my) between c. 159.7 and 157.7 Ma. This age span implies an Oxfordian age (161.2–155.7 ± 4.0 Ma, Gradstein et al., 2004) for the Hawtah Member, as consistent with its biostratigraphic age (Fischer et al., 2001).

The Ulayyah Member appears to consist of seven cycles that are similar in thickness and lithology to those of the Hawtah Member (Figure 2a). The Ulayyah Member (DS3 11.2) apparently consists of seven fourth-order orbital cycles (DS4 11.2.1–11.2.7) that were deposited in a period of c. 2.835 my (7 × 0.405 my) between c. 157.7 and 154.8 Ma (i.e. late Oxfordian and early Kimmeridgian; Kimmeridgian: 155.7–150.8 ± 4.0 Ma, Gradstein et al., 2004). An Oxfordian and early Kimmeridgian age for the Ulayyah Member is consistent with its biostratigraphic age (Fischer et al., 2001).

In the Hanifa outcrops (Figure 2a, b), the basal Hawtah fourth-order cycle can be further divided into four “bedding bundles” that are each some 2–4 m thick (Figure 2c, Mattner and Al-Husseini, 2002). According to the orbital calibration, these bundles would each have a period of c. 95–125 ky. The 30-cm cycles may be related to precession and tilt and have periods of 20–40 ky.

Moujahed Al-Husseini founded Gulf PetroLink in 1993. Gulf PetroLink is a consultancy aimed at transferring technology to the Middle East petroleum industry. Moujahed received his BSc in Engineering Science from King Fahd University of Petroleum and Minerals (1971), MSc in Operations Research from Stanford University (1972), PhD in Earth Sciences from Brown University (1975) and Program for Management Development from Harvard University (1987). Moujahed joined Saudi Aramco in 1976 and was the Exploration Manager from 1989 to 1992. In 1996, Gulf PetroLink launched the journal of Middle East Petroleum Geosciences, GeoArabia, for which Moujahed is Editor-in-Chief. Moujahed also represented the GEO Conference Secretariat, Gulf ΡetroLink-GeoArabia from 1999-2004. He has published about 30 papers covering seismology, exploration and the regional geology of the Middle East, and is a member of the AAPG, AGU, SEG, EAGE and the Geological Society of London.

Robley K. Matthews is Professor of Geological Sciences at Brown University, Rhode Island, USA, and is general partner of RKM & Associates. Since the start of his career in the mid 1960s, he has had experience in carbonate sedimentation and diagenesis and their application to petroleum exploration and reservoir charcterization. Rob’s current interests center around the use of computer-based dynamic models in stratigraphic simulation.

Joerg Mattner is a Geoscience Consultant and the Executive Editor for the journal GeoArabia. In 2001 he founded GeoTech in Bahrain, an independent Geoscience Consultancy for Middle East geology and hydrocarbon reservoirs, joerg received his PhD in 1990 from Clausthal University, Germany. During his studies and subsequent teaching assignment, he worked on geological projects in Europe, South America and Northern Canada. In 1990, he joined the Petrophysical Evaluation Group of Western Atlas in London. Subsequently joerg moved to Syria, and established a log analysis center. In 1994 he became Chief Geologist for the Middle East and opened Western Atlas’ regional Geoscience Center in Bahrain. Through mergers he joined Baker Hughes, and in 2000 took up an assignment as Director of Marketing and Business Development with Western Geophysical. joerg’s special interests are structural geology and fractured reservoir characterization. He was member of the GEO 2000, 2002 and 2004 Conference Technical Committees.