17 Uppermost Jurassic Arab-A carbonates, Saudi Arabia GeoArabia, v. 10, no. 1, 2010, p. 17-26 Gulf PetroLink, Bahrain Calcimicrobe tubules in uppermost Jurassic Arab-A carbonates of Saudi Arabia Geraint Wyn ap Gwilym Hughes ABSTRACT Carbonates from the Arab-A Member of the Arab Formation, of uppermost Jurassic age, in the subsurface of eastern Saudi Arabia reveal hollow, intertwined, locally bifurcating, microtubules consisting of a bead-like, uniserial series of inaperturate, stacked, concavo-convex crescentic chambers. Previously confined to the Oxfordian of North America, and possibly the Lower Cretaceous of Oman, this is believed to be the first observation of such forms in uppermost Jurassic carbonates. The carbonates consist of a lower, transgressive, ooid-foraminiferal lithofacies and an upper, regressive, stromatolite-ostracod lithofacies of which the microtubules are developed in the latter. Comparison of these structures with fossil and Recent cyanobacterial microborings and Oxfordian calcimicrobial tube- like forms suggests probable cyanobacterial affinity. Despite their very limited stratigraphic extent and open-chambered character, they may contribute to improving mouldic microporosity of the Arab-A carbonate reservoir. INTRODUCTION Qatif field, located in eastern Saudi Arabia, contains stacked Jurassic carbonates that are prolific hydrocarbon-bearing reservoirs. The Arab Formation of Saudi Arabia is of Late Jurassic, Kimmeridgian to Tithonian age (Sharland et al., 2001; Hughes, 2004) and consists of four carbonate units that have been designated member suffixes D, C, B and A in ascending stratigraphic order (Powers, 1968; Wilson, 1985; Moore, 2001, his Figure 6.44; Lindsay et al., 2006). Each of the carbonate units of the Arab Formation is separated from the next by beds of anhydrite, termed the D, C and B anhydrite with respect to the underlying Arab-D, Arab-C and Arab-B carbonate members, except for the anhydrite that overlies the Arab-A carbonate which is called the Hith Formation (Figures 1 and 2). The Arab-A carbonate in the study well consists of a 34 ft thick succession of carbonates that overlies the Arab-B anhydrite and is overlain by the Hith anhydrite. During routine, semi-quantitative, micropalaeontological analysis of thin sections prepared from core plug samples from the Arab-A carbonates a single sample displayed a cavity containing a collection of enigmatic uniserial, chambered, hollow, intertwined, bead-like, vermiform, tubular microfossil moulds. Dense, grey, dendroidal masses of microgranular thrombolite and intra-thrombolite cavities characterise the core from where the sample was cut (Figure 3). Microstromatolitic structures are present within the dendroid framework below the chambered microtubular forms at a level sampled by core-plug 10 (Figure 4h), whereas microgranules are typical of the section above the zone of interest (Figure 4a). The presence of blue-dyed epoxy resin, injected into the core plug during thin section preparation to display visible porosity, enhances the appearance of these forms (Figures 4b to 4g). Their unusual nature, and source of mouldic microporosity, is of hydrocarbon reservoir interest despite their apparent absence from the remaining part of this core. The chambered microtubules have not been named in formal publications, but they show similarity to early Proterozoic Frutaxites sp. in Canada (Hofman and Grotzinger, 1985), and closely resemble un-named, Renalcis-like cyanobacterial tubules of Oxfordian age from North America (Fischer et al., 2007) and Lower Cretaceous forms termed Gakhumella huberi from Spain (B. Granier, written communication). MORPHOLOGY The structures are present as a concentration of hollow, possibly intertwined and not necessarily mutually supporting, gently and randomly curving, vermiform, chambered microtubules (Figures 4b to 4g). Maximum tubule length observed in the thin section is 300μ, with a consistent width of