A distinctive signature of the Lower Triassic to Middle Jurassic succession of the Arabian Plate is, within the exception of the uppermost Khuff Formation and equivalents, the scarcity of grainstone intervals. . Typically, the limited number of grainstone intervals present are composed of ooids, peloids and oncoids. Microbial laminites can be common. These geological characteristics reflect the proximity of two paleoecological crises: the End Permian and the End Triassic mass extinctions, which led to significant faunal reductions close to these events. However, fauna, notably microfossils and ammonoids, is present in much of the succession in suitable sediment and permits the biostratigraphic calibration of 3rd order sequence-based correlation.
The interval described in this book starts at the base of the Lower Triassic succession at an age of ~ 250 million years with the lowermost lithostratigraphic unit, the upper Khuff Formation and its equivalents. The interval ends at the top of the Middle Jurassic Callovian stage, with an ago of ~ 163 million years, lithostratigraphically represented by the Tuwaiq Mountain Limestone Formation and its equivalents. Thus, the interval discussed spans over 80 million years, recorded by a rock succession ranging from a few hundred to up to 2000 meters in thickness.
Lithologically, the Lower Triassic to Middle Jurassic sequence is dominated by carbonate deposits, which are interbedded with a significant amount of siliciclastic beds. Fine-grained carbonates and mostly low-energy clastics create a unique signature for this geological interval. This is in stark contrast to the thick stacks of grainstone in the Khuff Formation below and the Arab Formation above. Evaporites are a notable feature of Middle and Upper Triassic deposition. Their distribution is often restricted to intraplate sags. The Lower Triassic to Middle Jurassic sequence represents the drift stage of the Neotethys Ocean, with subtle differential tectonics. Re-activated basement structures form several intraplate sags that can contain organic-rich sediments and can be fringed by narrow grainstone belts. The notable exception to this general tectonic quiescence is during the latest Triassic to earliest Jurassic, which was marked by regional uplift, a widespread hiatus and sheet-like sandstones fringing uplifted areas. This uplift may have created subcrop traps in some areas.
The volumes of hydrocarbons stored in this interval represent just a small fraction of the total reserve present across the Arabian Plate. However, the interval is underexplored and warrants further attention, especially if charge risk and reservoir quality risks can be better understood.
This EAGE special volume invites the reader to explore this poorly-known stratigraphic interval with fascinating insights from the microscopic to the basin-wide scale.