Late Permian to Late Jurassic “Microproblematica” of Saudi Arabia: Possible Palaeobiological Assignments and Roles in the Palaeoenviromental Reconstructions
Total Page：16
File Type：pdf, Size：1020Kb
GeoArabia, 2013, v. 18, no. 1, p. 57-92 Gulf PetroLink, Bahrain Late Permian to Late Jurassic “microproblematica” of Saudi Arabia: Possible palaeobiological assignments and roles in the palaeoenviromental reconstructions Geraint Wyn Hughes ABSTRACT Palaeoenvironmental interpretation of Permian and Jurassic intertidal to very shallow-marine carbonates is difficult where typical shallow- marine microfossils are either absent or sparse. A collection of microfossils originally considered as “microproblematica” because of their uncertain biological affinities are, however, often present. These include species of Aeolisaccus, Gakhumella, Prethocoprolithus, Thaumatoporella, Favreina and Terebella. Observations of their vertical distribution and relationship with carbonate fabrics reveal their environmental preferences, and these contribute to palaeoenvironmental interpretation within a spectrum of very shallow- marine settings that previously precluded refinement. The recognition of high- frequency depositional cycles and definition of cryptic reservoir layering in such shallow to marginal-marine carbonates is now facilitated by the use of these microfossils from the Khuff, Hanifa, Jubaila, Arab and Hith formations. Aeolisaccus dunningtoni is interpreted as either a fossilised cyanobacterial tube or possible foraminifera of Early Permian to Late Jurassic age. It is well represented within mudstones, wackestones and packstones of supratidal flats to very shallow intertidal palaeoenvironments with occasional freshwater influence. The microbialitic Gakhumella cf. huberi is locally present in these Upper Jurassic intertidal to very shallow-marine bioconstructions. Prethocoprolithus centripetalus is a faecal ribbon, considered to be of mollusk origin, within shallow subtidal grainstones and packstones. Thaumatoporella parvovesiculifera is considered a green alga that is typically found encrusting biocomponent fragments. It ranges from the Middle Triassic to Upper Cretaceous and is extensively present in intertidal, possibly hypersaline to shallow-marine, normal salinity lagoon grainstones and mud-lean packstones. Certain types of the distinctively canaliculate, microcoprolitic decapod crustacean faecal pellets, of the genus Favreina, are diagnostic of Late Jurassic intertidal to shallow subtidal conditions found within packstones. Terebella lapilloides is an agglutinated polychaete tube, typical of Upper Jurassic intertidal to shallow-marine packstones. INTRODUCTION Palaeoenvironmental interpretation of carbonates is critical for optimal predictive support for hydrocarbon reservoir exploration, as well as developing intra-reservoir layering for reservoir modelling and improved production strategies. Within very shallow-marine and marginal-marine lithofacies, subtle palaeoenvironmental variations are not easily detectable due to the limited sedimentological variation and a lack of rich microfossil assemblages. The information presented here derives from detailed micropalaeontological analysis of subsurface shallow-marine carbonates from Saudi Arabia (including wells shown in Figure 1) containing less common microfossils, often considered as “microproblematica,” for which palaeoenvironmental interpretation is poorly defined. This study of non-foraminiferal microfossil types will assist determination of palaeoenvironmental variations in the otherwise problematic, very shallow to marginal-marine palaeoenvironmental spectrum of Late Permian to Late Jurassic age Saudi Arabian carbonates. Their distribution, together with other fossil microorganisms, is illustrated with reference to cored subsurface sections to reveal palaeoenvironmental interpretation and depositional cyclicity. Despite their poor biostratigraphic value, and their relative scarcity, integration of these undervalued 57 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/18/1/57/4568926/hughes.pdf by guest on 26 September 2021 Hughes 47°E448° Kuwait City 9° 50° 51° KUWAIT IRAN 4 28°N 28° 3 2 27° 47° 27° 1 34°E 38° 42° 46° 50° 54° 58° 38°N TURKEY Caspian 38° Sea SAUDI ARABIA CYPRUS SYRIA 34° LEBANON IRAN 34° Med IRAQ N Manama Sea 3000 JORDAN 30° km 30° Gulf KUWAIT of Suez BAHRAIN BAHRAIN 26° 26° 26° N SAUDI ARABIA QATAR EGYPT Gulf of 0 100 UAE Oman Arabian Study 22° Shield Area 22° OMAN km SUDAN Red Sea 18° 18° Arabian QATAR Sea ERITREA YEMEN 14° 14° SOCOTRA ETHIOPIA Gulf of Aden 34° 38° 42° 46° 50° 54° 58° 48° 49°50° 51° Banog Figure 1: Eastern Saudi Arabia with locations of wells 1–4 (see Figures 11–15). microfossils with foraminifera, calcareous algae and microbialites enables their use for high- resolution palaeoenvironmental interpretation and for subdivision of reservoir-bearing carbonates. The proposed palaeoenvironmental interpretations for each of the groups is based on their vertical stacking arrangement as seen by the author during semi-quantitative micropalaeontological analysis of thousands of carbonate thin sections, including the wells used to illustrate the biofacies tiering (Figure 1) and especially in the Jurassic sequence of Saudi Arabia. Studies of this nature provide the only insight into the preferred palaeoenvironments of non-foraminiferal microfossils for which this type of information is absent or sparse (Figure 3). Early micropalaeontological analysis of thin sections of Permian, Triassic and Jurassic carbonates, collected by Iraq Petroleum Company geologists, revealed certain microfossils that were difficult to assign to precise taxonomic groups. These were the “microproblematica” that have been identified from four wells during recent micropalaeontological analysis of cored sub- and intra-evaporitic carbonates of the Upper Permian to Triassic Khuff Formation, and the Upper Jurassic Arab and Hith formations in Saudi Arabia (Figure 3). The six problematical groups identified here include: (1) the possible cyanobacterial tube Aeolisaccus dunningtoni Elliott, 1958 (Figure 4); (2) the chambered cyanobacterial and sinuous cylinder Gakhumella cf. huberi Zaninetti (Figure 5); (3) the form here assigned to the ribbon-like gastropod microcoprolite Prethocoprolithus centripetalus Elliott, 1962 (Figure 6); (4) the encrusting alga Thaumatoporella parvovesiculifera (Raineri, 1922; Pia, 1927) (Figure 7); (5) the crustacean microcoprolite genus Favreina Brönnimann 1955 (Figure 8); and (6) the agglutinated polychaete tube Terebella lapilloides Münster (in Goldfuss, 1833) (Figure 9). For the first time, their presence and sequential distribution in thin-sections of core plugs can be related to palaeoenvironmental variations within the carbonate hydrocarbon reservoirs. They can assist recognition of very shallow- to marginal-marine high-frequency depositional cycles and definition of intra-reservoir layers that may not be evident in the sedimentological description. 58 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/18/1/57/4568926/hughes.pdf by guest on 26 September 2021 “Microproblematica” of Saudi Arabia Litho- Stage Age Formation Member Reservoir logy Period Age Ma System (ICS 2004) 145 Sulaiy Tithonian Manifa Hith 150 A B Arab A–C L Arab C D Kimmeridgian Arab D Late J2 E Jubaila J1 155 Hanifa L Ulayyah Hanifa Oxfordian M Hawtah 160 E Non Deposition Hadriya L Tuwaiq T3–T1 Upper Fadhili Callovian M Mountain Hisyan (D7) E Atash 165 L Lower Fadhili D6 JURASSIC Bathonian M Middle E Dhruma D4–D5 Faridah and L D3 Sharar Dibi Lst 170 Bajocian D2 Anhydrite E D1 Dolomite Non Deposition Limestone 175 Late Marrat L. Figure 2: Simplified Middle Toarcian M. Marrat 180 lithostratigraphy of the Early Jurassic Shaqra Group of E. Early Banog Saudi Arabia. The Khuff Formation (Powers, 1968; Powers et al., 1966) is of Late Permian to Early Triassic age and consists of a succession of four carbonate units termed, in the subsurface and in ascending order, as the Khuff D, C, B and A that are separated by anhydrite beds. Micropalaeontological analysis of the Late Permian succession (Hughes, 2005b, 2009c; Vachard et al., 2005) indicates shallow-marine to intertidal conditions during the deposition of the carbonate sediments. The cyanobacterial tube Aeolisaccus dunningtoni is present within the intertidal to shallow subtidal carbonates of the Khuff C. The predominantly carbonate succession of the Jurassic Shaqra Group (Figure 2) of Saudi Arabia is composed of seven depositional sequences of the Marrat, Dhruma, Tuwaiq Mountain, Hanifa, Jubaila, Arab and Hith formations (Hughes, 2005b; Hughes and Naji, 2009; Powers, 1968; Powers et al., 1966). The depositional environments represented by these formations range from deep to shallow-marine that become hypersaline in the Upper Jurassic Arab and Hith formations (Azer and Peebles, 1998). A microbialite cylinder composed of en echelon chambers, assigned to Gakhumella cf. huberi is present in the Late Jurassic carbonates of the Arab Formation (Hughes, 2010). The shallow- marine carbonates of the Arab C, B and A members of the Arab Formation and the carbonates of the Manifa Member of the Hith Formation contain microbialite microgranules in which the agglutinated tube Terebella lapilloides is rare to common. Microcoprolites are well-preserved and include faecal ribbons of Prethocoprolithus centripetalus and the highly internally-structured faecal pellets assigned to the genus Favreina. The encrusting calcareous form Thaumatoporella parvovesiculifera is well-represented, and considered as a separate group of green algae (De Castro, 1990) and, of the forms interpreted here, represents the most open-marine environment.