Depositional Cyclicity and Palaeoenvironments of the Middle Dhruma Formation, Saudi Arabia

Robert F. Lindsay*, Nassir S. Alnaji, and Geraint W. Hughes
Saudi Aramco
*[email protected]

The Dhruma Formation forms a major component of the Shaqra Group of Saudi Arabia and in outcrop consists of at least 1,300 ft of Middle Jurassic, Bajocian to Bathonian, carbonates. Sequence stratigraphic analysis of stacking patterns of individual cycles, cycle sets and high-frequency sequences (HFS) and microbiofacies provided important insights to the stratigraphic location of three hydrocarbon reservoirs in the subsurface. A new road cut at Khashm al Mazru’i exposes over 300 ft of continuous carbonates, part of which is assigned to informal zones D5 to D6 of the Bathonian age Middle Dhruma. The upper part of zone D6 includes grainstones that host the Lower Fadhili reservoir in the subsurface of eastern Saudi Arabia. The western flank of the outcrop belt is characterized by impoverished foraminifera typical of a proximal shallow lagoon. In the eastern subsurface, Lower Fadhili reservoir carbonates yield higher-diversity foraminiferal assemblages together with encrusting and branched stromatoporoids typical of a more distal lagoon and bank depositional setting. The Middle Dhruma consists of a succession of high-frequency sequences, with transgressive systems tracts composed of mud-rich to marl-rich cycles and packstones/grainstones in highstand systems tracts. A typical 40–65 ft thick HFS contains 12-18 individual carbonate cycles that form 2–5 cycle sets. Thin karst features, filled with terra rosa, cap some HFS.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain