| High permeable streaks (HPS) are commonly developed in the Cretaceous bioclastic limestone reservoirs in
Middle East. With the progress of water injection, the existence of HPS leads to rapid water channeling, which brings
serious challenges to oilfield development. Therefore, the study on the cause and distribution of HPS is of great
significance. Based on the data of cores, cast thin sections, physical property and logging, this paper analyzes the
characteristics of sedimentation, lithology, physical property and electrical property of the HPS in Mishrif Formation of W
Oilfield in Iraq, and clarifies their types, genesis and distribution pattern. The result shows that the HPS of Mishrif
Formation can be divided into three types according to their genesis: sedimentation-dominated type, diagenesisdominated
type, and tectonism-dominated type. The sedimentation-dominated HPS are mainly developed in tidal
channels of MB1 member and in inner ramp shoals of MB2L member. The former is dominated by bioclastic grainstones
and rudstones with average porosity of 23.5% and average permeability of 173×10-3 μm2, the logging curves are boxshaped
and bell-shaped; the latter is mainly composed of bioclastic packstones with average porosity of 26.7% and
average permeability of 103×10-3 μm2, the logging curves are funnel-shaped. The diagenesis-dominated HPS are mainly
distributed in the platform margin of MB2U member which are mainly composed of bioclastic rudstones, grainstones and
packstones,with an average porosity of 27.0% and an average permeability of 216×10-3 μm2, and the logging curves are
box-shaped + funnel-shaped. The tectonism-dominated HPS are mainly distributed in the mounds and shoals of the
limited platform in MA member, which are mainly composed of bioclastic rudstones, grainstones and packstones, with
average porosity of 23.6% and average permeability of 108×10-3 μm2. The logging curves are characterized by low natural
gamma-ray, low density, and high interval transit time.
The genesis, development model, distribution pattern of these three types of HPS are different. The sedimentationdominated
HPS are formed by the superposition of high-energy sedimentary bodies and the dissolution of fresh water in
the syn-sedimentary and pene-sedimentary period with network and flake pattern laterally. The diagenesis-dominated
HPS are formed by two periods of atmospheric fresh water dissolution during the syn-sedimentary and pene-sedimentary
period and supergene period with mat pattern laterally. The tectonism-dominated HPS are formed by multi-stage
dissolution caused by atmospheric fresh water during syn-sedimentary and pene-sedimentary, meteoric fresh water and
formation acidic fluids flowing along the fault into sediments during burial period, and the distribution along the faults is
patchy. The research results have important theoretical value for the development of Cretaceous bioclastic limestone
reservoir in Middle East, and have practical guiding significance for the development of similar oilfields. |
