Chemostratigraphy of the pre-Khuff Sedimentary Rocks Encountered in the Subsurface of Oman

Abstract

The subsurface sedimentary record of Oman extends back to the Middle Neoproterozoic Era. The sedimentary fill in the pre-Khuff (Middle Permian) Palaeozoic and Neoproterozoic successions is dominated by siliciclastic rocks, many of which are either exploration targets or hosts to considerable accumulations of proven hydrocarbons. Key to the successful exploration and development of the siliciclastic and carbonate reservoirs of Oman is having a robust regional stratigraphic model of the subsurface and whilst a wealth of detailed biostratigraphic, lithostratigraphic, sequence stratigraphic and sedimentological research has been conducted, uncertainties still remain; particularly in the Cambrian and older sedimentary rocks. In recent years, there has been interest among operators in Oman in determining whether elemental chemostratigraphy, when integrated with other stratigraphic, sedimentological and provenance datasets, can resolve stratigraphic uncertainties encountered in the Neoproterozoic and Palaeozoic sedimentary rocks. Presented here is a summary of the regional elemental chemostratigraphic framework that is being developed for the pre-Khuff sedimentary rocks in the subsurface of both northern and southern Oman. The framework is based on a compilation of approximately 1000 samples measured by ICP OES & MS instruments from 15 key wells. The results are formatted to show the stratigraphic variations in the key elements and element ratios by time rather than true subsurface thickness and tied into the Oman Subsurface Stratigraphy Chart of Forbes et al. (2010) for reference. The regional framework is not yet complete and there are some gaps in the stratigraphy that remain to be filled. Therefore the results presented here are regarded as a work in progress. The elements that are identified as being key to the regional stratigraphic framework are Si, Al, Ca, Mg, K, Rb, Cs, Ti, Nb, Zr, Th the rare earth elements (REEs), Mo and U. The elements are interpreted to model temporal variations in lithology, K-minerals (feldspar, mica, illite), heavy minerals (zircon, rutile, monazite), and authigenic minerals (REE-phosphates, molybdenite and uraninite), which themselves are interpreted to be controlled by variations in sediment provenance and depositional environment. From the data, four chemostratigraphic megasequences and seventeen chemostratigraphic sequences (each sequence is broadly equivalent to a formation) are currently defined at a regional level. Many of the broader geochemical signatures are correlative over hundreds of kilometres, while some are also basin-specific. The results of this work demonstrate the efficacy of elemental chemostratigraphy as a tool to resolve stratigraphic problems, both at a regional and reservoir level, in the Palaeozoic and Neoproterozoic sedimentary successions of Oman. The chemostratigraphic framework is closely tied to the established lithostratigraphy. However, with the recent substantial increase in geographic and stratigraphic control, elemental chemostratigraphy is now being employed to test alternative correlations of key clastic and carbonate-prone formations between the northern and southern areas in conjunction with other stratigraphic and provenance datasets.

AAPG Datapages/Search and Discovery Article #90360 © 2019 AAPG Middle East Region Geoscience Technology Workshop, 3rd Edition of the Siliciclastic Reservoirs of the Middle East, Muscat, Oman, October 28-30, 2019