Contribution of Biofacies Study to Recognize High-Resolution Depositional Cyclicity: A Case Study from Middle and Upper Jurassic Carbonates, Saudi Arabia
Al-Dhubaib, Abdullah J.*1
(1) ETSD, Saudi Aramco, Dhahran, Saudi Arabia.
Thin-section based micropaleontological studies from subsurface and outcrop localities of Middle and Late Jurassic carbonates in Saudi Arabia have led to recognition of environmentally significant biofacies. These are based on vertical tierings of various paleoenvironmentally sensitive biocomponents which represent response to hydraulic energy levels and light penetration. An ascending order of tiers from relatively deep to very shallow marine bio-assemblages has been determined for each formation. This is used to identify short-term paleobathymetric variations on a hierarchical level to fourth and fifth-order sequences.
Studied carbonates from Upper Dhruma Formation to Upper Hanifa Formation demonstrate three third-order shallowing-upward sequences. These sequences start with open marine biofacies in the lower part, which are considered part of a transgressive systems tract, ending with shallow marine biofacies attributed to highstand systems tract. The first sequence, which spans Upper Dhruma to Lower Tuwaiq Mountain formations, represents the first well-developed carbonate rimmed platform in the Saudi Arabian Mesozoic. This includes the Atash Member of Upper Dhruma Formation when small patch reef mounds form a shelf/bank margin, tending to back-step landward. This biofacies dominated the successions during the transgressive systems tract of each sequence. Stromatoporoid/coral bank biofacies of the highstand are dominant at the end of both first and second sequences in the Lower and Upper Tuwaiq Mountain Formation respectively. The second sequence begins in the Middle Tuwaiq Mountain Formation and commences with open marine biofacies predominated by debris-flow deposits, representing the inner and outer slope that received excess bank margin-derived sediments. The last sequence starts in the Lower Hanifa Formation with its predominantly open marine biofacies and terminates in the Upper Hanifa Formation where very shallow marine biofacies become dominant. The identified biofacies can also be applied to determine subtle variations in the depositional environment such that small scale intra-reservoir layers.
The variety of biofacies biocomponents in different units has provided a positive approach to define high-resolution depositional cyclicity. This will help to further subdivide the current reservoirs facies and it can be utilized to locate potential sites of intrashelf basin margin biofacies where potential stratigraphic traps may form.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain