Identification, distribution characteristics, and effects on production of interlayers in carbonate reservoirs: A case study from the Cretaceous Mishrif Formation in Halfaya Oilfield, Iraq

Carbonate reservoirs play an essential role in the oil and gas exploration, especially in the Middle East. Halfaya oilfield is a super-giant oilfield, in which Mishrif Formation is the main oil-bearing formation. Mishrif Formation has a strong heterogeneity with an alternative appearance of interlayers and reservoirs. The appearance of interlayers has an obvious effect on oil production. Therefore, it is necessary to identify and characterize interlayers in Mishrif Formation in Halfaya oilfield. The interlayers in the study area can be divided into three types with different lithology according to the core and well logging data. In this study, a zonal identification method is established to accurately identify these three types of interlayers. In this method, the reservoir and interlayer are divided based on their permeability. The identification criteria of the three types of interlayers are developed based on analyzing the well logging data. After identifying the interlayers at wells based on the newly developed identification criteria, the distribution of interlayers far away from the well is characterized by a zonal interwell comparison method. Then, the planar distributions of the interlayer thickness in each member are generated. The identification results show that barriers develop in members MA1, MB1-1, MC2-1 and MC3-1. For the intercalations, there are 8 members developing large-scale continuous intercalations, which are members MA2, MB1-2A, MB1-2B, MC1-3, MC1-4, MC2-2, MC2-3 and MC3-2. Comparing the distributions of the interlayers and those of the sedimentary microfacies, the development of interlayers is controlled by sedimentary microfacies. It can be concluded that the interlayers prefer to appear in shallow open sea and subtidal flats. To quantitively characterize the effects of intercalations on oil production, intercalation density and intercalation frequency are compared with half-year oil production decline rate. The results show that the half-year decline rate increases with the increase of intercalation density and frequency. The intercalation density can better reflect the effect of intercalations on oil production than the intercalation frequency.