Sedimentary and reservoir architectures of MB1-2 sub-member of Middle Cretaceous Mishrif Formation of Halfaya Oilfield in Iraq

Abstract

Based on comprehensive analysis of core, cast thin section, logging and seismic data, the sedimentary and reservoir architectures of the MB1-2 sub-member of Mishrif Formation in Halfaya Oilfield, Iraq, are studied. The MB1-2 sub-member of Mishrif Formation has three types of microfacies, lagoon, bioclastic shoal, and tidal channel, and facies architecture controlled by sequence stratigraphy. In the 4th-order sequence, the lagoon facies aggradated vertically, and the bioclastic shoals in lenticular shape embed in the background of lagoon, the end of the sequence is incised by the “meandering river” shape tide channel, which represents the depositional discontinuity. Three types of reservoirs including tidal channel grainstone to packstone reservoirs, bioclastic shoal grainstone to packstone reservoirs and dissolved lagoon wackestone reservoirs are developed. The reservoir architectures within tidal channel and bioclastic shoal are strickly controlled by grainy facies, whereas the dissolved lagoon reservoirs controlled by both facies and dissolution are related to the sequence boundary. The reservoir sections occur mainly in the 4th sequence highstand systems tract (HST) and are separated by barriers formed in the transgressive systems tract (TST). Complicated facies architecture and dissolution modification resulted in strong heterogeneity within the reservoir, which showed the characteristics of “attic type” architecture. The results of this study can guide the development of similar reservoirs in the Middle East.