Stratigraphic hierarchy and architecture of the upper Thamama (Cretaceous) Lekhwair, Kharaib and Shu’aiba formations at a giant oil field, offshore Abu Dhabi, United Arab Emirates

Abstract

At a giant oil field located in offshore Abu Dhabi, the Lekhwair, Kharaib, and Shu’aiba formations record deposition during a second-order supersequence. This supersequence comprises at least three second-order composite sequences that, in turn, are composed of a stack of third-order sequences. Major third-order sequence boundaries or their correlative conformities are interpreted at the base of the Thamama III, below the base of the Thamama IIIA reservoir unit, the top of Thamama III, the top of Thamama II, and above Thamama IA reservoir units. The vertical distribution of lithofacies in the Lekhwair, Kharaib and Shu’aiba formations is remarkably similar from well to well, and predictable fieldwide. Consistency in lateral facies distribution across the field suggests that deposition occurred on a highly aggradational, flat-topped platform with no appreciable depositional geometry. The Thamama III consists of stacked fourth-order parasequence sets. Porous and permeable packstones/grainstones dominated variously by rudist debris, coated grains and algal lumps comprise the reservoir units that form the highstand portion of these fourth-order cycles. These units in turn are sharply overlain and separated by non-porous and impermeable dense units that consist of stylolitic wackestones/packstones and grainstones with common ostracodes, dasycladacean algae, miliolid foraminifers, and intraclasts. These dense units are interpreted as forming the transgressive portion of the fourth order cycles. Similarly, within the Thamama II, lithofacies associations seen in cores have a predictable vertical succession, and represent a classic shallowing-upwards succession. In ascending order these are: peloidal-skeletal wackestone-mudstone; orbitolinid-peloidal- skeletal wackestone-packstone; algal-intraclasticpeloidal packstone-mud-lean packstone; fining-upward cycles of floatstone-rudstone-boundstone grading up into skeletal-peloidal mud-lean packstone-grainstone; and miliolid foraminifer-dasycladacean algae-sponge spicule wackestone-packstone. The Thamama IA shows an overall deepening-upwards succession of lithofacies associations. In ascending order these are: Lithocodium- Bacinella floatstone-boundstone; skeletal-peloidal packstone; skeletal-peloidal packstone-wackestone; foraminiferal wackestone/mudstone; and planktonic foraminifer-skeletal wackestone-mudstone. No obvious sequence boundary is seen at the top of the Thamama IA, suggesting that offshore, this surface is a correlative conformity.