Research paperChemostratigraphy and biostratigraphy of Devonian, Carboniferous and Permian sediments encountered in eastern Saudi Arabia: An integrated approach to reservoir correlation

- The chemostratigraphic scheme comprises a hierarchical order of zones, subzones, divisions and subdivisions.
- Zonation is largely based on based on variations in elemental ratios reflecting changes in source/provenance.
- Some chemozones are missing in particular sections as a result of erosion/non deposition.
- A fully integrated biostratigraphy/chemostratigraphy approach was utilized to produce the highest level of resolution.

The following study was completed on inorganic geochemical data acquired from 850 core and cuttings samples taken from Devonian, Carboniferous and Permian sediments encountered in six wells in the arabian gulf, eastern saudi arabia. The principal objective of the study was to employ chemostratigraphy to produce a 'robust' high resolution correlation scheme. A secondary aim was to integrate this with existing biostratigraphic data as part of a multidisciplinary approach to reservoir correlation.Data were derived for 50 elements using ICP-OES (Inductively Coupled Plasma - Optical Emission Spectrometry) and ICP-MS (Inductively Coupled Plasma - Mass Spectrometry), but definition of chemostratigraphic boundaries are based on changes in the following key element ratios: Cr/P, Th/Nb, Th/Ta, Zr/Th, Zr/P, Nb/Ti and Nb/P. Variations in these parameters are largely dependent on changes in source/provenance, which affects the relative concentrations of detrital heavy minerals.To avoid geochemical variations relating to changes in grain size/lithology, separate frameworks were constructed for data derived from sandstone and mudrock lithologies. Both comprise a hierarchical order of zones, subzones and divisions, with subdivisions also being identified in the sandstone dataset. The schemes are considered to be 'robust' as chemozones (general term used to refer to any zone, subzone, division or subdivision) are clearly defined using geochemical profiles and binary diagrams and most are correlative between three or more wells. Given the similarities in definitions of zones, subzones and divisions in the two schemes, and the fact that these chemozones are considered to be roughly time equivalent, both were combined to produce a 'final' chemostratigraphy scheme. In a further stage of interpretation, the correlation was refined by integrating the interpretations of this chemostratigraphy study with lithostratigraphic markers (identified by significant trends in GR that exist in two or more wells) and an existing correlation based on biostratigraphy.In the Tawil and Jauf formations there are obvious inconsistencies between the placement of chemostratigraphic and biostratigraphic boundaries, with some subzones occurring within different palynozones in adjacent wells. Such disparities may be explained by the existence of separate fluvio-deltaic/shallow marine sandstones having a similar source/provenance but being deposited in different parts of the study area at different times. This interpretation is supported by the available seismic data showing that the Jauf, and to a lesser extent the Tawil, formations take the form of laterally discontinuous sandstones. By contrast, the chemozones associated with the overlying Jubah Formation, Berwath Formation, Unayzah Group (Juwayl and Nuayyim formations) and Base Khuff Clastics Member show a very close match with the placement of palynozones in each well and were probably deposited at a similar time on both field and subregional scales.