The Integration of Wireline Logs and Sedimentological Data to Predict Sequence Stratigraphic Framework in Carbonate Rocks: An Example from Rumaila Formation (Cenomanian), West Qurna Oil Field, Southern Iraq
Stratigraphy and Geological Correlation
Abstract: A combination of wireline log data and sedimentological characterization provides insight into the sequence stratigraphic framework of the carbonate rocks. The Cenomanian Rumaila Formation is considered one of the Cretaceous representative carbonate units in the Middle East. In Iraq, limited information regarding the regional distribution of the Rumaila Formation within a sequence stratigraphic framework prompted this work. This study integrates geophysical well logs (i.e. gamma ray, sonic and density logs) and a detailed microfacies study of the core and cutting samples to suggest the sequence stratigraphic framework of the studied sections. The detailed carbonate microfacies study of over 100 samples from the four available wells (WQA, WQB, WQC, and WQD) within the West Qurna Oil Field suggests deposition from the outer to the middle ramp with the influence of the shoal environment. The Rumaila Formation could be correlated to the early part of the highstand face of the Wasia Group with a Cenomanian to Early Turonian age. There is no clear evidence of unconformity or a time gap between the Ahmadi and Rumaila formations in the studied area. The sequence stratigraphic study of 80 m of the Rumaila Formation revealed the identification of three 4th-ordered stratigraphic sequences (RLS1, RLS2 and RLS3). Moreover, this study identified a potential regional correlative maximum flooding surface that correlates with the regional established one (i.e. K-130). Two marine flooding surfaces were identified within the studied sections and could be further correlated regionally. Because of the intercalation between the Rumaila and Mishrif formations in southern Iraq, identifying these regional marine intrusion episodes (i.e. K-130 and K-135) provides credible evidence to clearly differentiate them. In the transgressive system tracts (TST), the deepening upward trend is invariably accompanied by an increase in gamma-ray and density log values, a decrease in acoustic and neutron log values, and vice versa in the high stand system tracts (HST). Thus, wireline logs provide insight into the sequence stratigraphic framework in carbonate rocks, especially when sedimentological data is missing.
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