Chemostratigraphy of the Silurian Qusaiba Member, Eastern Saudi Arabia

• Produced a chemostratigraphic scheme of 4 zones, 7 subzones and 4 divisions.
• Definition of chemozones based mainly on changes in provenance.
• Most chemozones are correlative between the five study wells.
• Chemozones are absent in some wells as a result of erosion/non-deposition.
• Mid Qusaiba reservoir sandstones are clearly identified using geochemical data.

Given the unavailability of high resolution biostratigraphic data and difficulties in using lithostratigraphy for stratigraphic correlation, it was decided to employ chemostratigraphy to propose a scheme for the Silurian Qusaiba Member encountered in five wells in Eastern Saudi Arabia.Chemostratigraphy may be defined as a reservoir correlation technique involving the utilization of inorganic geochemical data. Although Inductively Coupled Plasma – Optical Emission Spectrometry (ICP-OES) and Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) were used to acquire data for 50 elements, the scheme is based on changes in the following ‘key’ element ratios: Zr/Th, Cr/Ti, Th/Nb, Zr/P, Y/Yb, Zr/Yb and Y/P. Variations in these parameters are largely dependent on changes in source/provenance, reflecting increases or decreases in the abundances of particular detrital heavy minerals. The scheme comprises a hierarchical order of four zones, seven subzones and four divisions. The zones are labelled C1, C2, C3 and C4 is ascending stratigraphic order, with two, three and two subzones identified in C2, C3 and C4 respectively. In addition to this, chemostratigraphic divisions are noted in two of the subzones.The chemostratigraphic scheme is considered robust as chemozones (general term used to describe any zone, subzone or division) are clearly defined in each well using geochemical profiles and binary diagrams plotted for key element ratios. Furthermore, high levels of statistical confidence are associated with the chemozones and most are correlative between three or more wells. The nonexistence of chemozones in particular wells is mainly explained by the sampling strategy employed. For example, the absence of subzone C3-2 (occurring towards the center of zone C3) in wells 4 and 5 is most likely to be explained by the uppermost part of the Qusaiba Member not being sampled. In other instances, particular chemozones may be missing as a result of erosion/non-deposition on a local scale.One positive aspect of the study is that it was possible to identify the mid-Qusaiba reservoir sandstones which occur within chemozone C3-3 (at the top of zone C3) in each well. Consequently, chemostratigraphy may be utilized to recognize these sandstones in future wells, particularly in subhorizonal ones where their identification may be more difficult using wireline log signatures alone.