Identification of facies-controlled eogenetic karstification in the Upper Cretaceous of the Halfaya oilfield and its impact on reservoir capacity

Penecontemporaneous dissolution has been considered as the dominant origin of reservoir formation, whereas epigenetic dissolution has also played certain roles in the improvement of reservoir capacity in the Upper Cretaceous Mishrif Formation in the Middle East. Here, we report a possible new reservoir origin based on a case study in the Halfaya oilfield of Iraq, that is, facies-controlled eogenetic karstification. The most representative evidence is that early selective dissolution is not only present in relative high-energy categories of rocks but also in low-energy ones in the cored intervals. Meanwhile, the occurrence frequency of karstification varies among different environments and associated lithologies. In grainstones and packstones, spongy-like dissolution pores and irregular karst channels are widely developed, with fillings and massive plastic breccias. In contrast, in wackestones, lots of high-angle karst channels are developed based on biological burrows, and the karst is more frequently observed in the shoal environment than in the low-energy environments. These characteristics are indication of eogenetic karstification. The model of this karstification can be attributed to a multiple superimposition of short-term exposure during the penecontemporaneous stage and the medium-term exposure after the shallow burial stage. The karst has impact on reservoir capacity, according to which 3 areas are divided in the increasing order, namely, the tight bedrock area, spongy-like dissolution pores area, and karst channels and vugs-filled area, among which the karst channels and vugs-filled area is the most favourable for oil accumulation. This understanding might be general to the Mishrif reservoirs in the entire Middle East.