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

Abstract

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.

The eogenetic karst of the Cambrian Longwangmiao Formation in the Yangtze Platform: Implications for the eustatic and stratigraphic correlations of Toyonian (Cambrian)

“…In terms of the pore type, the moldic pore and interparticle pore resutling from the selective dissolution of grains, gypsum, breccia, and early‐stage cement mainly exist in the palaeokarst zone of the Longwangmiao Formation. Previous palaeokarst features are similar to the eogenetic karst in the Halfaya Oilfield in Upper Cretaceous (Zhong et al, ), Maokou Formation of the middle Permian of the Sichuan Basin (Tan et al, ; Xiao et al, ), and Mississippian in New Mexico (Meyers, ). Besides that, the dissolution of the early‐stage fibrous cement and the selective‐dissolution in oolite indicate that the dissolution occurred in the penecontemporaneous or the early diagenetic stage (Longman, ).…”

“…As regard to the palaeokarst features in the study area, there is an obvious difference between the high‐permeability layer (grainstone or packstone) and low‐permeability layer (mudstone or wackestone) indicating that lithological variations control the developing extent of the palaeokarst, which accords with the attributes of the eogenetic karst (Moore et al, ; Tan, Xiao, & Chen, ; Xiao et al, ). The three dissolution‐filling zones, closely resembling the “Friable Halo” rim led by the diffuse flow in the modern eogenetic karst (Moore et al, ; Zhong et al, ), can be identified in the grainstone (Figures 9a and 14b). In contrast, the mudstone displays the clear conduit boundary without a half‐dissociated zone between the mixed filling and matrix (Figures 9 and 14b).…”

Optimization of Gas Lifting Design in Mishrif Formation of Halfaya Oil Field

“…It has a proven reserve of 4.1 billion barrels and a production potential of 200,000-535,000 barrels per day. Generally, Mishrif is one of the main Cretaceous reservoirs in the Mesopotamian Basin and Middle East [29,30]. The Mishrif Formation, which is the primary producing zone in the Halfaya Oilfield, is approximately 400 meters thick and is characterized by the development of grain shoal reservoirs [28,31].…”