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Regional Chemostratigraphic Correlation of the Silurian Qusaiba Formation, Central Saudi Arabia

Abstract

Inorganic whole rock geochemical data have been used to define stratigraphic correlations in the petroleum industry for over a decade. The stratigraphic technique of chemostratigraphy relies upon recognizing changes in element concentrations through time and using those to model changes in geological features, such as paleoclimate and provenance. Absolute chronostratigraphic timelines can be determined by Re-Os isotope stratigraphy which dates the age of deposition of organic matter. This method is potentially capable of providing timelines that can be related to the absolute timescale. Once absolute chronostratigraphic markers are defined, elemental chemostratigraphy can provide a high resolution “lithostratigraphic” correlation scheme, which provides the basis for any stable reference stratigraphic framework, which is of paramount importance for any basin exploration. The zonation and correlation of the Qusaiba reveals a distribution of chemostratigraphic subsequences and packages which on a regional scale suggest that the Qusaiba in the north may be younger than in the south and so could be diachronous. Re-Os isotopic analyses give an Aeronian age of c.428Ma+/- 2.3Ma (=Llandovery – Wenlock boundary) for the ‘hot’ shale in well North-1, with published data showing the southern ‘hot’ shales may be of Rhuddanian to Aeronian age (436 to 443+/-1.7Ma) Moreover, a maximum flooding surface has been potentially identified in the geochemistry midway through the Qusaiba which is considered to correlate with the MFS S10 identified by Sharland et al. 2001. Furthermore, a good match exists between the biostratigraphic correlations of Miller and Melvin, 2005 with the chemostratigraphic zonation and subsequent correlation of the study wells for which there is biostratigraphic data for. When flattened on the MFS S10, the palynostratigraphic horizons and the chemostratigraphic packages downlap onto this surface. These progradational features trend towards the northwest of the study area, with the progradational nature of the chemostratigraphic sequences appearing to comply with paleoocurrent data from the Wajid outcrop belt from sediments of Silurian-Carboniferous age, which show that the delta complex present in the area prograded from the south to the north as described by Stump & Van der Eem, 1995.