The following chemostratigraphy study was conducted on Paleozoic sediments encountered in 14 wells in eastern Saudi Arabia. A total of 1,500 samples were analysed by ICP-OES (Inductively Coupled Plasma – Optical Emission Spectrometry) and ICP-MS (Inductively Coupled Plasma – Mass Spectrometry), with data acquired for 48 elements, ranging from Na to U in the periodic table. The aim was to utilise chemostratigraphy, in conjunction with existing biostratigraphic, lithostratigraphic and sedimentological data, to define the Hercynian Unconformity in each well and to recognise stratigraphic boundaries occurring above and below it. This was necessary as the unconformity eroded to different stratigraphic levels in each well, with Devonian, Silurian and Ordovician sediments found immediately below it in adjacent locations. In the absence of chemostratigraphic, biostratigraphic and sedimentological data, it is often very difficult to define this boundary and others using lithostratigraphy alone as many stratigraphic intervals yield similar GR log trends. For example, a low ‘blocky’ GR response is typical of both the Carboniferous Ghazal Member and the Ordovician Sarah Formation. Similarly, both the Silurian Sharawra Member and the Silurian-Devonian Tawil Formation produce a ‘ratty’ GR trend. Each stratigraphic member and formation was found to have distinctive chemostratigraphic, biostratigraphic, sedimentological and/or wireline log signatures.

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