A New Bioevent Stratigraphy of Late Jurassic Arab-D Carbonates of Saudi Arabia

Abstract

A series of fine-scale biofacies cycles are recognised by the application of semiquantitative micropalaeontological analysis of cores in eight wells from the reservoir carbonates from the Arab-D and upper Jubaila formations in Saudi Arabia. They are of Kimmeridgian - Tithonian age, and form the primary producing reservoir of the world's largest single continuous reservoir located in the Ghawar field of Saudi Arabia. Three new composite biozones are identified that include a basal, deep marine Biozone D3 with fining-upwards cycles which equate, in part, with the Jubaila Formation. Biozone D2 was deposited in shallower conditions of the lower Arab-D carbonate, and Biozone D1 deposited in a very shallow marine environment of the upper Arab-D carbonate; both zones contain multiple coarsening-upwards cycles. Numerous additional bioevents, of potential subzonal value, are detected within these broad zones, and are considered to be related to discrete shoaling-upwards responses to transgressive episodes. The proposed scheme differs significantly from that erected for the Arab-D Formation in offshore Abu Dhabi. Carbonates of the Arab-D member are concluded to be composed of a series of high frequency palaeobathymetric changes which were superimposed upon an extensive carbonate platform. Gradual shallowing of the platform resulted possibly from a reduction in subsidence rate, a fall in sea level, rise in carbonate productivity rate or a combination of these. The latest episode of very shallow marine carbonate sedimentation terminated with the deposition of the Arab-D evaporites.