AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
The Palaeoenvironmental and Microstratigraphic Significance of Microcoprolites in Saudi Arabian Upper Jurassic Carbonates
(1) Saudi Aramco, Dhahran, Saudi Arabia.
Reservoir carbonates of the Arab Formation B and A units and Hith Formation contain beds in which microcoprolites are well preserved. Species of the tubule-bearing, rod-like Favreina are attributed to F. salevensis and F. fontana and represent derivation from a decapod crustacean source. Contorted ribbon-like microcoproliths of Prethecoprolithus centripetalus represent derivation from a mollusc source. Their association with cyanobacterial microgranules, often in stromatolitic layers, and monospecific unornamented cyprid ostracods, Terebella lapilloides and absence of foraminifera suggests a bacterial grazing mode of life within a stressed, marine environment that may have experienced elevated salinity and temperature. The ascending succession from: (a) microfaunally-barren anhydrite lithofacies; (b) microfossil-barren, granular cyanobacterial microbialite - Decastronema / Aeolisaccus biofacies; (c) ostracod biofacies; (d) Favreina-Prethocoprolithus - ostracod - cerithid gastropod biofacies; (e) Trocholina - Redmondoides - Palaeopfenderina - Mangashtia - Clypeina-Salpingoporella - Thaumatoporella biofacies; to (f) concentric ooid biofacies. These facies are considered to represent a parasequence within a shallow marine palaeoenvironment. The alternation of such stressed carbonate units and evaporitic sediments is considered to represent episodic flooding of a playa-like evaporitic basin, in which the foraminiferal biofacies probably represents the maximum flooding event and best circulation of marine water. Microcoproliths provide intra-reservoir stratigraphic events to complement micropalaeontologically sparse carbonates and would be expected to provide valuable micro-biocomponents to assist coiled-tube biosteering of late Jurassic carbonate reservoirs.