Characteristics and Genesis of Low-Energy Fine-Grained Carbonate Reservoirs: A Case Study from Middle Cretaceous Mishrif Formation in H Oilfield, Iraq

The Mishrif formation in H oilfield of Iraq is primarily characterized by lagoon deposits in the MB1–2 interval, consisting mainly of packstone and a small amount of wackestone. The reservoir exhibits a high degree of heterogeneity, and the effectiveness of water flooding is unclear, requiring further investigation. Based on extensive core and thin section observations, combined with core analysis, well logging, and seismic data, it is recognized that the MB1–2 interval was deposited in a semi-restricted platform environment, controlled by fourth-order cycles, with tidal channel deposits as the typical characteristic. The reservoir is predominantly developed in the upper part of the cycles, within packstone layers, characterized by foraminiferal pore cavities, bioturbation mold pores, and matrix micropores. Non-fabric-selective dissolution pores, such as dissolution pores and microfractures, are also present. The cemented hardground section forms barriers within the reservoir. Logging characteristics show relatively high natural gamma values, low density values, high neutron porosity, and high resistivity, along with relatively low gamma and high-density thin layers representing interbeds. The development of the reservoir indicates that it formed during the tidal channel development stage in the late stage of sequence evolution, experiencing atmospheric water dissolution and modification after the exposure of the lagoon environment. Strong localized dissolution and modification have resulted in the formation of effective reservoir zones. By combining natural gamma inversion with seismic impedance inversion, the spatial distribution of this type of reservoir was predicted, revealing that it mainly develops in the upper part of the cycles and exhibits a patchy distribution on the plane. The research findings presented in this paper hold significant implications for enhancing the development efficiency of H oilfield in the Middle East and understanding the heterogeneity of bioclastic limestone reservoirs in other oilfields within the Mishrif formation.