Micropalaeontology, Palaeoenvironments and Sequence Stratigraphy of the Sulaiy Formation of Eastern Saudi Arabia

摘要

The Sulaiy Formation, which is the oldest unit in the Lower Cretaceous succession, is conformably overlain by the Yamama Formation and it is a challenge to identify the precise age of the two formations using foraminifera and other microfossil assemblages. In the eastern side of Saudi Arabia, the Sulaiy Formation and the base of Yamama Formation are poorly studied. The main objectives of this study is to enhance the understanding of the Sulaiy Formation sequence stratigraphical correlation, regional lateral variations and palaeoenvironmental investigation.Lithological and semi-quantitative micropalaeontological analysis of 1277 thin sections taken from core samples from nine cored wells providing a geographically representative distribution from the Saudi Arabian Gulf. These cores intersected the base of the Yamama Formation and the Sulaiy Formation in the total thickness of cored wells of 843.23 meters (2766.5 feet). On the evidence provided by the foraminifera, the Sulaiy Formation is considered to represent the Berriasian to the lowermost Valanginian. The investigation of the micropalaeontology has provided considerable insights into the biocomponents of Sulaiy and the base of Yamama formations in order to identify their biofacies. These microfossils include rotalid foraminifera, miliolid foraminifera, agglutinated foraminifera, calcareous algae, calcispheres, stromatoporoids, sponge spicules, problematica (e.g. Lithocodium aggregatum), molluscs, corals, echinoderms and ostracods. Systematics of planktic and benthic foraminifera is accomplished using the foraminiferal classification by Loeblich and Tappan (1988) as the main source. The assemblage contains foraminifera that recorded for the first time in the Sulaiy Formation. Other microfossils were identified and recorded to help in the identification of the sedimentary environments. The investigation of the micropalaeontology and the lithofacies analysis have provided evidence the identification of the various lithofacies. About twenty four microfacies were identified on the basis of their bio−component and non-skeletal grains. The lithofacies and the bio−component results have provided the evidence of the sedimentary palaeoenvironmental model namely the Arabian Rimmed Carbonate Platform. This palaeoenvironmental depositional model is characterised by two different platform regimes. They are the Platform Interior and the Platform Exterior each of which have unique sedimentary lithofacies zones that produce different types of lithofacies. Each lithofacies is characterised by special depositional conditions and palaeobathymetry that interact with sea level changes and the accommodation space. The important palaeoenvironments are intertidal, restricted lagoon (subtidal), open marine, deeper open marine, inner shoal, shoal and platform margin. Generating, and testing, a depositional model as a part of formulating a sequence stratigraphical interpretation of a region is a key to understanding its geological development and – ultimately – reservoir potential. The micropalaeontology and sedimentology of the Sulaiy Formation in the subsurface have indicated a succession of clearly defined shallowing−upwards depositional cycles. These typically commence with a deep marine biofacies with wackestones and packstones, capped with a mudstone-wackestone maximum flooding zone and an upper unit of packstone to grainstones containing shallow marine biofacies. The upper part of the Sulaiy Formation is highstand-dominated with common grainstones that host the Lower Ratawi reservoir which is capped by karst that defines the sequence boundary. This karst is identified by its abundant moldic porosity that enhanced the the reservoir quality by increasing its porosities into greater values. Integration of the sedimentology and micropalaeontology has yielded a succession of shoaling−upwards depositional cycles, considered to be 4th order sequences, that are superimposed on a large scale 3rd order system tract shallowing−upwards, highstand-associated sequence of the Sulaiy Formation. The Lower Ratawi Reservoir is located within the latest high-stand portion of a third-order Sulaiy Formation sequence. The reservoir consists of a succession of several sequences, each of which is sub-divided into a lower transgressive systems tract separated from the upper highstand systems tract by a maximum flooding surface (MFS/Z). The last of these depositional cycles terminates in beds of porous and permeable ooid, or ooidal-peloidal, grainstone. The reservoir is sealed by the finer-grained sediments of the Yamama Formation.