Pervasive cements can significantly reduce rock properties and connected volumes Potential to create bypassed oil volumes, particularly in shingled oolitesĭiagenesis can significantly modify the original depositional connectivity in carbonate sedimentsĭolomitization can potentially create good connectivity by modifying fine-grained sediments Numerous hydraulic units and highly layered reservoirs Generally have lower recoveries than sandstone reservoirs difficult to locate wellsĮarly water breakthrough and high water production ratesįractures can create widespread connectivity in an otherwise heterogenous matrix rockĬan form thief zones with rapid water breakthroughĬommon high-frequency cycles on a meter cycle Table 1 Factors influencing connectivity and reservoir development in carbonate reservoirs.Ĭarbonates form highly heterogenous reservoirs Reservoir management is difficult because the accurate targeting of production and injection wells is problematic, and sweep may be inefficient as a result of this. Heterogeneity at all the reservoir scales can make them a challenge to model, and it is not an easy task to make reliable predictions about their production performance. The lower energy drive mechanisms such as solution gas drive are common. They can have lower primary recoveries as connected volumes may be areally limited with no contact to a large aquifer. A combination of depositional geometry and diagenesis creates highly heterogeneous reservoirs (Table 1). They generally have poorer recoveries than siliciclastic sediments (e.g., Sun and Sloan). Many carbonate reservoirs offer a challenge to the production geologistĬarbonate reservoirs can be difficult to develop for a variety of reasons. Various sedimentary bodies such as beaches, barrier islands, shelf sediments, gravity flows, and dune sands are also found in carbonate settings. There are some similarities to siliciclastic environments. There can be a great diversity of grain sizes and shapes in most carbonate sediments compared to sandstones. Consequently, uniform grain sorting is not a major characteristic of carbonates. The coarser material tends not to be widely spread or abraded by waves and currents. Carbonate sediments tend to form and be deposited in situ, with enormous volumes of calcareous material provided by the death, disintegration, or digestion of plant and animal matter. 10 Karstification and paleocave systemsĬarbonate sediments have several features that set them apart by comparison with siliciclastics.6 Typical settings for carbonate reservoirs.2 Many carbonate reservoirs offer a challenge to the production geologist.1 Carbonates are different from sandstones.The Joulters Cay ooid shoal is a single carbonate sand body with a mobile border 25 km (15 mi) long and between 0.5 and 2 km (0.3 and 1.2 mi) wide. The size of individual sediment bodies on the Bahama Banks can be impressive too ( Figure 1). The shallow submerged platform area of the Bahamas extends more than 400 km (248 mi) north–south and covers an area of about 125,000 km 2 (48,263 mi 2). The reason for the very large size of some carbonate reservoirs is not surprising when one considers the sheer scale of even modern-day carbonate settings. Some very large oil fields have carbonate reservoirs, including the largest conventional oil field in the world, the Ghawar field of Saudi Arabia. Many of these are located in the Middle East, Libya, Russia, Kazakhstan, and North America. Depositional Environments and their Flow CharacteristicsĪ significant proportion of the world's oil reserves are found in carbonate reservoirs.
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