Effect of Petroleum Emplacement on Reservoir Quality in the Thamama Reservoirs of Abu Dhabi.
Controls on reservoir quality in the Kharaib Formation, Thamatina Group, Abu Dhabi, have been investigated during a regional study of several fields.
Lithofacies and subsequent diagenesis are critical in determining reservoir quality. Grainstones and packstones with residual intergranular, vuggy and mouldic porosity can retain permeabilities of 10's – 100's mD. Where extensive burial cementation has occurred only microporosity remains. Although the amount of microporosity may be significant, permeabilities are in the order of a few millidarcies only.
A critical factor in reducing cementation in grainstone and packstone units has been found to be the relative timing of petroleum emplacement. Where petroleum emplacement has occurred relatively early in the reservoirs burial history and prior to significant burial cementation by calcite, reservoir quality is preserved. Where petroleum emplacement has occurred after significant burial cementation, reservoir quality is destroyed.
The regional relationships documented here for the first time in carbonate reservoirs are similar to those observed in clastic reservoirs. This has significant implications for the prediction of porosity in carbonate reservoirs.
The Kharaib Formation of the Thamama Group is one of the most important reservoir horizons in Abu Dhabi. It is therefore essential to understand the controls on its reservoir quality and particularly the complex and often episodic diagenetic changes which affect a sediment once it has been deposited.
A major event in the geological history of any reservoir must be the emplacement of petroleum. Does petroleum inhibit or stop diagenesis?
In recent years, a significant amount of research has been conducted on the relationships observed in sandstone reservoirs between petroleum generation, filling and cementation by quartz and other minerals. There is direct evidence that cementation in a sandstone reservoir and petroleum filling can occur at about the same time. If cementation precedes petroleum emplacement, porosity and permeability may be destroyed. Conversely, where a petroleum charge has occurred prior to, or at the same time as cementation, porosity and permeability are maintained. A key piece of evidence leading to these conclusions was the observation of primary petroleum inclusions in diagenetic minerals in reservoir sequences with enhanced porosity and permeability. It should be noted that this view is not shared by all workers on sandstones, but its opponents have not demonstrated that cementation overlaps petroleum emplacement in the cases they cite.
A particularly important conclusion from this model is that porosity development may not depend solely on stratigraphic and sedimentological factors - structural configuration, insofar as it relates to trap filling by petroleum, can also play a part. The extent to which structure determines porosity distributions depends critically on the relative timing of petroleum input and cementation. Carbonates generally suffer more complex diagenesis than sandstones, so that petroleum emplacement may not be the only process which can lead to porosity trends which crosscut sedimentological trends - freshwater diagenesis such as karstification, or burial diagenesis as a result of decarboxylation reactions in kerogen can also do this.