High-Resolution Sequence Stratigraphy of the Kharaib Formation (Lower Cretaceous, U.A.E.)

Abstract

A new sequence stratigraphic framework is proposed for the Lower Cretaceous Kharaib Formation (Barremian and Lowermost Aptian) of the United Arab Emirates. This framework is based on the integration of core and well-log data from Abu Dhabi oil fields with outcrop data from Wadi Rahabah, Ras Al-Khaimah (U.A.E.). The Kharaib Formation is part of the late transgressive sequence set of a second-order supersequence, built by two third-order composite sequences. Fourteen fourth-order parasequence sets build into two third-order composite sequences and show predominantly aggradational and progradational stacking patterns, typical of greenhouse cycles. On the basis of faunal content, texture, sedimentary structures, and lithologic composition, eleven reservoir lithofacies and eight non-reservoir "dense" lithofacies are identified from core. These same lithofacies are also identified in time-equivalent rock exposures studied in Wadi Rahabah. The analyzed lithofacies range from open platform, lower ramp to restricted platform subtidal to intertidal environments. Intensively bioturbated wackestone and packstone, and interbedded organic- and siliciclastic-rich limestone characterize the three so-called dense zones (Lower, Middle, and Upper Dense Zone). Locally, mud-cracks, blackened grains, and rootlets are observed. The two reservoir zones (Lower and Upper Kharaib Reservoir Unit) correspond to the late transgressive and, dominantly, highstand systems tracts characterized by parasequence sets that show shallowing-upward trends from open platform, burrowed skeletal wackestone to skeletal, peloidal packstone and algal, coated-grain grainstone/rudstone, and rudist, algal floatstone/rudstone. Well-developed Thalassinoides firmgrounds (Glossifungites surfaces) indicate temporary cessation in sedimentation and cap several parasequence sets and parasequences. Stylolitic intervals within the reservoir units predominantly correspond to major facies changes related to third-, fourth-, and fifth-order sequence boundaries, parasequence set boundaries, and parasequence boundaries. In outcrop, low-angle clinoforms that cannot be seen in core data are observed within the highstand systems tract of the upper third-order composite sequence (Upper Kharaib Reservoir Unit). Integration of subsurface and outcrop data leads to more insightful and realistic geological models of subsurface stratigraphy. Introduction Large hydrocarbon accumulations have been discovered and produced from platform carbonates of the Upper Thamama Kharaib Formation in Abu Dhabi1,2,3. The Kharaib Formation (Barremian and Lowermost Aptian) contains two reservoir units (Lower and Upper Kharaib Reservoir Unit) separated and encased by three zones of very low porosity and permeability, subsequently referred to as dense zones (Lower, Middle, and Upper Dense Zone). Thickness of the Upper Kharaib Reservoir Unit is between 150 and 170 feet, and thickness of the Lower Kharaib Reservoir Unit is about 80 feet. Highly permeable beds (several Darcy) that mainly consist of coated-grain grainstone and rudist floatstone/rudstone facies are recognized in Field-A (Fig. 1). Generally, reservoir quality (porosity and permeability) decreases from crest to flank across the structure. This is due to compaction and cementation caused by the interaction with formation water. Diagenetic patterns of Field-B, Field-C, and Field-D (Fig. 1) are significantly different to those observed in Field-A. Whereas Field-A displays very good petrophysical properties, only locally affected by minor early- and late-diagenetic events, the reservoir quality of Field-B, Field-C and Field-D is impacted negatively by early-diagenetic porosity-destructive calcite cementation. This early-diagenetic calcite cement prevented significant compaction but occludes much of the primary interparticle porosity of the grainstone facies.

Petrographic and diagenetic studies of thick transition zone of a middle-east carbonate reservoir

“…It was recognized that the permeability depends on the type of porosity and the average pore size. Again Strohmenger et al (2006) discussed the texture of this zone. The reservoir zone is dominated by shallow marine planktonic foraminifera wackestone/packstone with interbedded (dense) bioturbated, burrowed wackestone to mudstone.…”

“…This zone is defined as the interval between the top of one of the reservoir zone and the base of the tight, argillaceous formation and represents the lower part of the Shuaiba Formation (Strohmenger et al, 2006). The field under study ( Figure 1A), which is an onshore oil field in Abu Dhabi, is a double-plunging anticline trending NE-SW formed during the Campanian (Al Bloushi, 2013;Vahrenkamp et al, 2014).…”

“…According to the regional geology, this zone was deposited within the mega sequence AP8 when the Arabian Plate was undergone several tectonic events such as the opening of the Indian Ocean that results in the formation of a new passive margin within the NeoTethys as shown in Figure 1B (Sharland et al, 2013). Strohmenger et al (2006) interpreted the reservoir zone as a third-order sequence, corresponding to the AP3A 18 sequence of the composite transgressive systems tract of a second-order composite sequence. Strohmenger et al (2006) and Al Bloushi (2013) studied the depositional environment and texture of this zone and they stated that this reservoir zone was deposited on an open platform, subtidal, and depositional environment in a carbonate ramp to basin system.…”

“…Strohmenger et al (2006) interpreted the reservoir zone as a third-order sequence, corresponding to the AP3A 18 sequence of the composite transgressive systems tract of a second-order composite sequence. Strohmenger et al (2006) and Al Bloushi (2013) studied the depositional environment and texture of this zone and they stated that this reservoir zone was deposited on an open platform, subtidal, and depositional environment in a carbonate ramp to basin system. The zone has various porosity ranging from <1 to 38% comprising of intragranular, intergranular, microporosity and moldic porosities.…”

“…The reservoir zone is dominated by shallow marine planktonic foraminifera wackestone/packstone with interbedded (dense) bioturbated, burrowed wackestone to mudstone. The sequences start with the subaerial exposure to a transgressive period of mudstone/wackestone, then ends with a regressive interval of packstone/grainstone cycle (Strohmenger et al, 2006;Vahrenkamp et al, 2014;Van Buchem et al, 2013).…”