INTEGRATION OF HIGH-RESOLUTION SEQUENCE STRATIGRAPHY AND ICHNOLOGY OF JURASSIC (OXFORDIAN) HANIFA FORMATION, CENTRAL SAUDI ARABIA: OUTCROP STUDY FOR RESERVOIR QUALITY PREDICTION

Outcrops provide vertical and lateral continuity of exposure that can help to improve our understanding of reservoir stratigraphy, and offer the opportunity for detailed description of sedimentological and stratigraphic features equivalent to subsurface reservoirs. The purposes of this study are to: 1) document field observations from strata in the Hanifa Formation (complex stratigraphy and heterogeneous lithofacies) and construct a high-resolution sequence stratigraphic framework as a guide for correlation in the subsurface; and 2) explore how bioturbation modifies sedimentary fabric and texture of carbonate lithofacies within a sequence stratigraphic framework.

Field investigation of the Hanifa Formation at four outcrop localities in central Saudi Arabia reveals considerable variation in thickness and lithofacies. Six lithofacies associations in the Hanifa Formation are interpreted to represent deposition in a ramp setting. Five general types of cycles are defined, and stack to form 12 high frequency sequences (HFS) bounded by unconformities. Lithofacies, ichnofacies, and significant surfaces, integrated into a high-resolution sequence stratigraphic framework, reveal discontinuous facies and cycles, but laterally extensive HFS that can be correlated across the area. Flooding surfaces and sequence boundaries of each HFS can be characterized by lithofacies and ichnofacies trends that provide a predictive tool for lithofacies change.

Although bioturbation is ubiquitous throughout the Hanifa Formation, specific patterns are evident within cycles and HFS. In general, each facies and facies association has a specific range of bioturbation intensity (ichnofabric index, ii). The presence of distinct ichnologic assemblages defines surfaces that bound cycles and HFS. For example, laterally extensive (>5 km) firmgrounds of the Glossifungites ichnofacies have an ii of 4–6 and represent HFS boundaries. Glossifungites ichnofacies in mud-dominated lithofacies are important because the coarse sediment infill of the 3D burrow networks provide permeability pathways in an otherwise less permeable medium. This study improves our understanding of how traces are distributed within a sequence stratigraphic framework, and how they can affect the original fabric and textures of carbonate strata.