Geologically based integrated approach for zonation of a Late Jurassic–Early Cretaceous carbonate reservoir; a case from Persian Gulf

In this study, our attempt is to integrate sedimentological and petrophysical data for reservoir evaluation in the sequence stratigraphic framework. Petrographic analysis of the Late Jurassic–Early Cretaceous Fahliyan Formation reservoirs of two oilfields in the northwest of the Persian Gulf led to recognition of twelve microfacies. They can be classified into four facies associations, including open marine, shoal, lagoon and tidal flat, which are deposited in a homoclinal ramp carbonate. Sequence stratigraphy of the studied successions led to the recognition of three third-order depositional sequences based on vertical changes in microfacies and gamma ray analysis. Except for the upper boundary of the third sequence, the other sequence boundaries are type I (SBT.1). Dissolution is the most important diagenetic feature that affected the lower depositional sequence which is caused by the development of subaerial exposure after the deposition of the Fahliyan Formation, whereas cementation is the main diagenetic feature affecting the second- and third depositional sequences, causing their lower reservoir quality. In order to identify the flow units, the flow zone index methods, porosity throat radius (R35) and modified Lorenz based on stratigraphy were applied. The key wells studied in this area have shown good correlation throughout the studied oilfields which may potentially be used for hydrocarbon exploration and field development in the Late Jurassic–Early Cretaceous deposits of the Persian Gulf. This study integrates geological and petrophysical data (rock typing) toward sequence stratigraphic framework.

Lamottia (Favositida, Tabulata) from the Decorah Formation (Kirkfieldian, Ordovician) of Iowa

The early tabulate coral Lamottia heroensis has been identified from the Ion Member of the Decorah Formation (Upper Ordovician) in northeast Iowa. This extends the stratigraphic range of this species upward from Lower Chazyan to Kirkfieldian, and extends the geographic range from the Vermont-New York border area to include the north-central Midcontinent. Thin section and SEM studies strongly support the contention that the longitudinal pattern of alternating light and dark bands observed in corallite walls reflects a primary structural grain rather than a secondary diagenetic feature.

Early diagenetic bedded cherts of the Tuscan sequence (Miocene) near Città di Castello, Italy

Summary. A distinctive level of bedded, internally laminated, deep-sea cherts in the area west of Città di Castello, central Apennins, has been studied. These cherts form part of the Lower Miocene Monte Santa Maria Tiberina Formation, and belong to the Tuscan sequence, consisting of pelagic and turbidic sediments. The sediment is enriched in diatoms and Radiolaria. The diatoms provided silica as well as barium, now present as submicroscopical baryte concretions. Synsedimentary volcanism could have provided silica as well, through calcitization of volcanic fragments within the sediment. Since the laminations display early diagenetic microdeformations,. it can be concluded that the cherts themselves are an early diagenetic feature.

The age of metasomatic anhydrite in Mississippian reservoir carbonates, southeastern Saskatchewan

Replacement anhydrite porphyroblasts are abundant in Mississippian carbonates from south-eastern Saskatchewan and have caused deterioration in oil field reservoir quality. Previously they were interpreted to have formed at the time overlying Mississippian bedded anhydrites were deposited. Porphyroblasts are here reported to be absent from a late diagenetic altered zone, composed of dolomitized and anhydrite-plugged beds, that occurs immediately beneath the Mississippian unconformity. This absence from strata that normally contain them indicates that anhydrite porphyroblast growth occurred after the formation of the sub-unconformity altered zone. This zone is believed to have been rendered impermeable, or otherwise made unsuitable for anhydrite replacement, before introduction of fluids that elsewhere precipitated the anhydrite porophyroblasts. The altered zone is dated as post-Watrous redbeds (post-Triassic); consequently the growth of replacement anhydrite post-dates this period. The occurrence of included bitumen in many porphyroblasts indicates that some replacement occurred after or during the introduction of oil into the Mississippian reservoirs (Late Cretaceous?).An early diagenetic origin for pre-replacement-anhydrite joints, stylolites and dolomite need not be invoked now that replacement anhydrite has been shown to be a late diagenetic feature.