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

摘要

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.