The Eocene Rus Anhydrite: Important Arabian Seismic Reflector and Recorder of Cenozoic History

D. Mark Steinhauff¹, Christian J. Heine¹, and Arthur E. Gregory^2
1Saudi Aramco, Dhahran 31311, Saudi Arabia
²Saudi Aramco, Dhahran 31311, Senegal

The Eocene Rus anhydrite is an important seismic reflector on the Arabian Plate where it is present above the Mesozoic and lower Cenozoic carbonate platforms. It provides reservoir top seal locally (e.g., Neutral Zone). Understanding Rus deposition and diagenesis has broad implications for understanding the Cenozoic: for seismic planning, acquisition, processing, and interpretation; and in understanding exploration and production limitations.
The Rus typically comprises meter-thick anhydrite parasequences interbedded with thinner limestone, dolomite, and marly beds with gross thickness in the shallow-basin centers of more than 700 ft. Sub-meter scale, anhydrite bed sets can be traced on wireline logs for 100's of kilometers. Typically, the Rus has distinctive physical properties, including strong impedance contrasts with over- and-underlying strata. Less common are seismic zones lacking coherent reflectors and distinctive physical properties. Well penetrations show that many incoherent reflectors typically include rubble. These zones present challenges in applying static corrections and interfere in our ability to correlate deeper reflectors.
Previous work has shown that the Rus anhydrite thins and is not present above many structural highs including much of the Qatar Arch, the Ghawar field, the Dammam Dome, and other large anticlines. Features attributed to karst are noted in many areas. Lack of anhydrite can be explained by (1) non deposition because the structures were moving; (2) deposition followed by erosion, or (3) extensive subsurface dissolution of carbonate and sulfate-evaporites under Middle Pleistocene wet climatic conditions. Even though the aforementioned structures probably have unique depositional and tectonic histories; some combination of the three proposed processes likely affected them all.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece