The Neocomian Fahliyan Formation is one of the important oil reservoirs in the Abadan Plain Basin, SW of Iran. To evaluate the pore pressure regime of the Fahliyan reservoir, 164 in situ pressure data points (MDT, XPT, and RFT) were analyzed from seven wells belonging to six oilfields. The pressure versus depth plot revealed that the Fahliyan reservoir is highly overpressured in all fields. The formation was characterized as a multilayered stacked reservoir with different pore pressure decreasing downward in a step-wise manner. Also, there is a major pressure step in the middle part of the reservoir, suggesting the presence of a regional efficient seal dividing the reservoir into two stacked compartments, where the upper compartment is more overpressured than the lower one. The stepped pressure pattern of the Fahliyan Formation is a regional phenomenon controlled by a factor governed regionally, the depositional condition, and facies lateral changes during the deposition of shallowing upward sequence of the Fahliyan reservoir. In addition, direct relationship is observed between the reservoir pressure and burial depth. This matter could amplify the initially generated overpressure state more possible due to dewatering of sediments and by-pass product of oil migration from Garau source rock to the Fahliyan reservoir.
Kangan Formation of Lower Triassic is one of the most important hydrocarbon reservoirs in Fars and Persian Gulf areas. The Kangan Formation unconformably is underlain by Permo-Triassic while its upper boundary with Aghar Shale Member of Dashtak Formation is conformable. Containing mainly carbonate rocks, Kangan Formation is interbedded especially in the Upper part with shale and claystone which can be considered as equivalents of Sudair Shale sourced from south west. In some parts of the Fars area, the nearly clean carbonates of the Kangan Formation ranging from tidal flat to shoal in terms of depositional environment, abruptly change to argillaceous limestone and shale of outer ramp (Figure1). In this case, a clay content map provides a powerful mean to delineate the area rich in clay; where is called "Gray zone"(GZ) hereafter. With a length of 350 km., a width of 100 km and an Azimuth of 150°, the GZ extends from Khalfani field located on the coast of Persian Gulf to Dashtak field in interior Fars area, forming ~30° angle against main Zagros fault. Such argillaceous facies, named "vermicular limestone", has been reported from Dena and Oshtorankuh both located in high Zagros outcrops. Generation of such a narrow, long trough could be considered as a result of the activation of a number of main faults during the Neothetis opening. In terms of dimensions and the depositional settings, the GZ. likely provided a location for anoxic conditions, hence organic matter enrichment, due to limited oceanic water circulation. Thin section study revealed the presence of such enriched lamina, however, analyzing three geochemical samples from the well SBN ranks them as poor source rocks (TOC: 0.42-0.6).
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