Hydrocarbon Habitat of the Early Permian Unayzah Formation in Abu Dhabi.
- Ahmed Taher (Abu Dhabi Natl. Oil Co.) | Abdulla Abdul Qader Al Shateri (Abu Dhabi Natl. Oil Co.) | Khalil Al Mehsin (Abu Dhabi Natl. Oil Co.) | Johan Witte (ADNOC) | Mohamed R. Al-Zaabi (ADNOC E&P)
- Document ID
- Society of Petroleum Engineers
- Abu Dhabi International Petroleum Conference and Exhibition, 11-14 November , Abu Dhabi, UAE
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 5.1.5 Geologic Modeling, 5.1.1 Exploration, Development, Structural Geology, 2.4.3 Sand/Solids Control, 1.1 Well Planning, 1.14 Casing and Cementing, 1.6 Drilling Operations, 1.2.3 Rock properties, 5.1.2 Faults and Fracture Characterisation, 5.6.1 Open hole/cased hole log analysis, 5.3.4 Integration of geomechanics in models
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The pre-Khuff principal hydrocarbon reservoir, Unayzah Formation, consists mainly of distal braid plain sandstones characterized by aeolian and sabkha facies with minor fluvial units. It extends between the pre-Khuff and the Hercynian unconformities. In Abu Dhabi, the Unayzah-A is further subdivided into three members, Members 1 and 2 are comprised of sandstone reservoirs and Member 3 consists of siltstone and shale sediments.
Facies controls on reservoir quality are weak. The main controls on porosity reduction of the reservoir are mechanical compaction and silica cementation. Quartz cementation tends to be the most severe in the cleanest, coarsest sandstones and near certain fractures. The presence of clay mineral grain coatings, although reducing the permeability, but locally protects the rock from secondary quartz overgrowth and preserve the porosity to great depths of burial. Without the grain coating, porosity will decrease with depth until the reservoir rock is completely tight.
Unayzah reservoir seals are provided by the Basal Khuff Clastics, tight Basal Khuff Carbonate and Middle Khuff Anhydrite. The Basal Khuff Carbonate seal does not appear to be regionally extensive but localized and potentially prospect specific. However, there are insufficient data to accurately define the seal for the Unayzah hydrocarbon accumulations.
Due to lack of deep penetrations in Abu Dhabi, basin modeling for Silurian hot shale source rock is challenging. Therefore, much of the unknown source and tectonic information were derived from the surrounding countries. This comes from understanding the regional tectonics and depositional trends of the southeastern Arabian plate, which helped to extrapolate the source trends into the Abu Dhabi area. The basin model shows that oil from Silurian source rock was generated early in the basin history and was widespread by the Late Triassic (220 Ma). Significant gas generation occurred during Lower Cretaceous (140 Ma) and dominated the hydrocarbon system by Middle Cretaceous (110 Ma). During the Early Tertiary (50 Ma), the source rock was highly mature for gas generation and at present-day, the charge is still active in the north offshore of Abu Dhabi.
The pre-Khuff charge history showed that the southern offshore and onshore structures are underfilled. The filling of these structures ranges between 50% and 80%, but in some onshore structures the filling is less than 50%. The middle and northern offshore structures are expected to be filled to spill point.
The Unayzah Formation in Abu Dhabi forms a potential target for future gas exploration. Many structures remain to be drilled especially in offshore Abu Dhabi and some of these prospects, may contain significant volumes of gas.
The facies variation, depositional environment, reservoir properties, and hydrocarbon potential of the Unayzah Formation were evaluated using data from key wells that are distributed over all Abu Dhabi (Figure-1). The data used in Unayzah evaluation included logs, drilling reports, selected cores and regional seismic lines. The available basin modeling results were incorporated into this evaluation. The Paleozoic basin modeling not only describes the maturation history of the Qusaibah source rock, but also predicts the filling percentage of the Abu Dhabi prominent fields.
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