Biofacies, Palaeo-environments and Stratigraphy of the Ratawi, Minagish and Makhul Formations of Kuwait

Three major micropalaeontological biozones / biofacies assemblages are described from the latest Jurassic (Tithonian) to Early Cretaceous (Berriasian to Early Valanginian) lower part of the Thamama Group in Kuwait. The thin sections studied and analysed are predominantly from core samples but supplemented with a small number of ditch cuttings where core was not available. The sample interval was irregular as the thin sections were originally chosen for reservoir parameter studies that included petrography, microfacies, porosity and permeability. Three major bio-assemblages (defining local biozones) can be identified and approximate the Makhul Formation – radiolarite assemblage of the restricted platform muddy limestones; the Minagish Formation and Ratawi Limestone Member – calcareous algae / foraminiferal assemblage of shallow shelf, clean carbonates including oolite / grainstone shoals; and the Ratawi Shale Member – foraminiferal assemblage of the mixed clastic and carbonate sedimentary environment. Within these three major bio-assemblages a number of subsidiary biofacies subdivisions, temporal and spatial, can also be identified. This allows the recognition of vertical stacking patterns and depositional cyclicity, as observed in the core description and petrographic microfacies studies, of this important hydrocarbon bearing interval.