Integrated Core-based Sequence Stratigraphy, Chemostratigraphy and Diagenesis of the Lower Cretaceous (Barremian?Aptian), Biyadh and Shu'aiba Formations, a Giant Oil Field, Saudi Arabia
Alghamdi, Nasser Mohammad S.
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This study provides the most updated stratigraphic, depositional and diagenetic histories of the Early Cretaceous Biyadh and Shu'aiba formations. Carbon isotope data were integrated with core descriptions and well logs to define the age model beyond the resolution of biostratigraphy; they were also used as a geochronology tool for correlating between wells and across platform to basin transitions. The Biyadh Formation consists of one 3rd-order sequence of four high-frequency sequences (S1-S4). S1 and S2 are the TST, composed of deep chalky limestone facies. S3 and S4 are the HST composed of skeletal rudist grainstone deposited in shallow water environments. The Biyadh sequence is capped by a regional subaerial boundary (SB1) corresponding to a global sea level fall. The Shu'aiba Formation consists of one 2nd-order sequence of four 3rd-order sequences and ten HFS's (S1-S10). S1 records the initial TST, followed by the regional MFS (K70) of S2. S3 is the late TST and is dominated by Lithocodium aggregatum/coral facies. S4 to S6 formed the early HST and are dominated by rudist buildups that transitioned basinward into fore-bank, slope and basinal settings with pronounced clinoform geometries. S7 and S8 are composed of shallow lagoonal milliolid packstone, representing the late HST. S9 and S10 are new identified Upper Aptian prograding sequences that formed during forced regression on the northern-block within platform edge and slope settings. The correlation between the Arabian Plate stratigraphic record and the standard Tethys isotope record and eustatic sea-level suggests a direct influence of the 3rd-order sea-level fluctuations on the Biyadh and Shu'aiba formations. This correlation also suggests that the Aptian were mainly controlled by glacio-eustatic fluctuations associated with eccentricity cycles. The Barremian records low magnitude sea-level changes and thinner sequences likely reflecting obliquity cycles. The Shu'aiba Formation records major subaerial hiatus reflecting glacial intervals, interrupted by flooding units reflecting global warming intervals. Trace elements, microprobe analysis, Cathodoluminescence (CL) and stable isotope data were used to define the diagenetic history of the Shu'aiba Formation within the sequence stratigraphic framework. The Shu'aiba Formation was mainly affected by meteoric diagenesis associated with the major unconformity at top of Shu'aiba.