A Revised Sequence Stratigraphic Interpretation of Platform Top Carbonates of the Lower Cretaceous Shuaiba Formation, U.A.E., Based on New Outcrop and Subsurface Observations

Abstract

The Lower Cretaceous carbonates of the Shuaiba Formation forms one of the most prolific carbonate reservoirs in the region. These carbonates were deposited on a shallow epeiric carbonate platform during the Aptian. In addition, intra-shelf basins formed on this vast platform where deeper and more restricted carbonates were deposited. As a result of a long term relative sea-level fall these carbonates started to prograde into the intra-shelfal Bab Basin. The platform ultimately diminished during exposure of the Shuaiba platform and subsequent flooding and the deposition of deeper water carbonates and clastics of the Nahr Umr Formation. The observations described here concentrate on the sequence stratigraphy of the shallow carbonate platform top carbonates in outcrop and the subsurface. The internal succession of the Shuaiba Formation can be briefly described in core and outcrop as follows: The Shuaiba Formation has been deposited on top of the deeper-water carbonates of the Hawar member, which terminated the Kharaib platform. Based on core evidence from several Abu Dhabi oilfields, the Lower Shuaiba Fm. is composed of two depositional cycles composed of Orbitolina wackestones shallowing upward into Lithocodium/Bacinella floatstones (Aptian 1). The succession is overlain by deeper water Orbitolina and foraminiferal wackestones/packstones and in parts contains planktonic foraminifera. Overlying this succession are shallowing-upward sequences with Lithocodium/Bacinella floatstones passing to Lithocodium/Bacinella rudstones or bindstones with stromatoporoids and corals (Aptian 2). The Aptian 2 section is bound by a prominent breccia that can be mapped in the subsurface around the Bab basin. The breccia shows angular clasts, vugs, and in places late stage calcite and dolomite cement rimming (or filling) the pore space. In parts, breccia clasts have been incrusted by stromatoporoids and Lithocodium/Bacinella after renewed flooding. This breccia has all the characteristics of a solution collapse breccia and is the result of a major drop in relative sea-level on the platform. In outcrop (400 km away from the subsurface analogue) an intraclast breccia is observed at the same stratigraphic position and can be traced kilometers along the mountain front in some wadis in the northern emirate of Ras Al Khaimah. This unit displays evidence of exposure and reworking and in areas intense pervasive later cementation. Overlying the Aptian 2 sequence boundary, the Aptian 3 succession has dominant Lithocodium/Bacinella floatstones and locally intercalated rudist floatstones suggesting a renewed deepening. Above, the section passes to massive rudist floatstones and rudstones containing mainly Caprinid, Myophorid and Caprotinid rudists and downlapping in outcrop onto the Lithocodium/Bacinella section (Aptian 3). At the base of the massive rudist section another breccia is exposed incorporating clasts of the underlying and overlying material. In contrast to the first breccia, this breccia displays evidence of mechanical effects involving fluidization, pressure containment/release, and development of pipe-shaped features connected to the parent bed. Internal clasts display angularity with variably weak to strong lithification. The top of the rudist section is represented by a sequence boundary characterized by significant stylolitization and sharp facies change. The upper part of the section is characterized by an alternation of skeletal, peloidal, Orbitolina, algal and rudist floatstones with intercalated mudstones (Aptian 4 and Aptian 5). Close to the contact to the Nahr Umr borings and pyrite nodules are prominent suggesting multiple stacked exposure surfaces leading to the ultimate unconformity on the top of the Shuaiba Platform. Establishing a sequence stratigraphic framework on shallow carbonate platforms heavily relies on identifying sequence boundaries (SBs), which are the best preserved surfaces recognized in these platform settings. Deposits as a result of maximum flooding surfaces are often not distinct since bathymetry was never deep enough on shallow-water platforms to deposit deep water carbonate mudstones and shales. The two most recognizable sequence boundaries for the Shuaiba Fm., indicating widespread regional exposure, are: the solution collapse breccia on top of the Aptian 2 algal section; and at the top of the Shuaiba formation at the contact with the Albian-aged Nahr Umr Formation. Both can be correlated basin-scale over 100s of kilometers between the subsurface and outcrop. More subtle SBs are defined by abrupt facies shifts at the contact of shallowing upward trends and overlain by deeper water deposits. These are positioned in the lower Shuaiba at the top of the two - Orbitolina-algal floatstone cycles, and in the Upper Shuaiba on top of the rudist section. Maximum flooding surfaces are more subtle and have been placed where we have indication of either significant deeper-water deposition in the lower Shuaiba (e.g. Orbitolina and foraminiferal wackestones/packstones with planktonics) or at the contact of slightly deeper facies (algal) to shallower facies (rudist). This interpretation follows common sequence stratigraphic rules and helps to better understand the laterally extensive water break-through zones in producing fields. Consequently, some of the previous interpretations on the Shuaiba shallow platform top carbonates require reconsideration.