188 / 2022-02-28 17:07:59

Lithofacies, architecture and evolution of carbonate tidal delta system of Mid-Cretaceous Mishrif Formation in Southeast Iraq: implications for reservoir heterogeneity

architecture,carbonate tidal delta,reservoir heterogeneity,Mishrif Formation,incision

Carbonate tidal delta system used to be put much less attention on unlike the counterparts in siliciclastic systems, and its unique architecture influences the reservoir heterogeneity and corresponding development strategy significantly. Based on detailed observation of cores from 5 wells and integrated analysis on well-loggings and 3D seismic, a flood tidal delta system was recognized and characterized in MB2 member of Mishrif Formation in H oilfield in Southeast Iraq. Four lithofacies associations were recognized in the the MB2 member of Mishrif Formation: (1) back-margin massive packstone to floatstone; (2) high energy channel-like bioclastic grainstone; (3) swamp carbonaceous mudstone interbedded with bioclastic wackestone; and (4) incision brown mud- to wackestone with chert nodular. The stacking pattern of the lithofacies association suggests that the flood tidal delta system experienced the evolution of back margin wave-dominated shoal, channelized shoals and swamp, and eventually incisions, representing a response of sea level falling. Swamp carbonaceous mudstones usually sit on the top of wave-dominated shoals separating from the flooding tidal channels. Incised valleys filled with tight mud- to wackestone, with the depth of up to 30 m, cut and divided the channelized shoals, swamp and wave-dominated shoals into pieces. The internal architecture and stacking pattern of lithofacies associations enhanced the reservoir heterogeneity significantly. Vertically, the swamp carbonaceous mudstone consisting of regional barrier separating the reservoir; laterally, the valley-shaped mudstone and wackestone-filled incision compartmentalized the reservoir that is composed of shoal and channel-filling packstone and grainstone. Furthermore, high resolution 3D seismic data shows that the prograding clinoform occurred within the shoals and channels, which will probably influence the fluid flow like those in siliciclastic sandstone. Additionally, the tidal delta system experienced significant modification of meteoric water dissolution during the formation of incision and exposure of the platform, and consequently plenty of dissolved vugs formed along the sequence boundary, consisting of the high permeable streaks that influence the fluid flow significantly, especially during the water injection. The depositional architecture and diagenetic modification resulted in the complicated reservoir architecture within the carbonate tidal delta system, which should be favorable to the optimization of the development strategy of the oilfield and other counterparts with similar background.