Applications of chemostratigraphy in Middle Jurassic unconventional reservoirs in eastern Saudi Arabia

ABSTRACT The following study was undertaken on inorganic geochemical data acquired for 1,032 core and cuttings samples taken from Middle Jurassic and Upper Jurassic sediments encountered in five wells in eastern Saudi Arabia. The study sections extend from the upper part of the Middle Jurassic Dhruma Formation to the base of the Upper Jurassic Arab Formation, though the principal focus was on the intervening Hanifa and Tuwaiq Mountain formations where potential unconventional hydrocarbon reservoirs have been encountered. The principal objective of the study was to produce a chemostratigraphic framework for these wells. A secondary aim was to utilize the geochemical data to recognize organic-rich zones and seals. The study sections mainly comprise limestones, argillaceous limestones and calcareous mudrocks, with the latter lithologies being more abundant in the Tuwaiq Mountain and Hanifa formations. Although ICP-OES (Inductively Coupled Plasma–Optical Emission Spectrometry) and ICP-MS (Inductively Coupled Plasma–Mass Spectrometry) were used to acquire data for 50 elements, the chemostratigraphic framework is based on changes in the following ‘key’ elements and ratios: Nb/Cr, Zr/Nb, Zr/Cr, Cr/Th, Cr/Ta, La/Lu, Zr/Yb, Nb/Ti and Ti/U. Variations in these parameters are dependent on changes in source/provenance, reflecting increases or decreases in the abundances of particular detrital heavy minerals. The framework comprises a hierarchical order of 5 zones, 10 subzones and 8 divisions. The zones are labeled C0, C1, C2, C3 and C4 in ascending stratigraphic order, with C2, C3 and C4 being divided into subzones. Further divisions are also recognized in some subzones. Zones C1, C2, C3 and C4 are correlative between all five wells, with the absence of C0 in three of the wells, probably explained by the existence of this zone below the present sampling intervals of these wells. Unlike most zones, some of the subzones/divisions are not identified in all wells. This is mostly explained by local variations in provenance being responsible for the presence of some subzones/divisions in certain wells but not others. In other instances the absence of particular subzones and divisions may have resulted from localized erosion/non-deposition. Lithostratigraphic boundaries are relatively easy to place in wells 1, 2 and 3 where there are similar changes in e-log response in all three study sections. However, this does not hold true for wells 4 and 5 where e-log trends are more or less obvious. In these wells it has been necessary to utilize chemostratigraphy in combination with much more subtle increases/decreases in e-log trends to identify these boundaries. In addition to enabling the placement of lithostratigraphic boundaries in wells 4 and 5, it is possible to use chemostratigraphy to produce a correlation scheme of much higher resolution than was possible by employing lithostratigraphy. For example, The Dhruma Formation was previously defined as a single lithostratigraphic unit but a fourfold chemostratigraphic zonation of this formation is now proposed for wells 1 and 2 (i.e. zones C0, C1, subzone C2-1, division C2-2a which occur in ascending order). By comparing profiles plotted for U, Mo and TOC it is possible to identify units in which there is a relatively high proportion of organic matter. Sediments producing high values of these parameters may be considered potential unconventional reservoirs, with the most organic-rich zones present in wells 1 and 2. Potential seals have been identified above the ‘reservoir’ sections of these two wells. These produce elevated values of Al and K, inferring an abundance of clay minerals which may prevent or reduce the propagation of induced fractures during future ‘fracking’ operations conducted in the vicinity of these wells.