The characteristics and origins of thief zones in the Cretaceous limestone reservoirs of Central and southern Mesopotamian Basin

Abstract

Thief zone is a common reservoir unit with super permeability developed in giant carbonate reservoirs and readily leads to the breakthrough of fluids bring the challenge to water flooding development or tertiary oil recovery. To provide insight into the characteristics and origins of thief zones in Cretaceous carbonate reservoirs in Central and Southern Mesopotamian Basin, core analyses, well log, and production performance data analyses were performed in three formations from two giant oilfields. Thief zones are indicated and located by higher water injection and liquid production rates and classified based on their sedimentary environment, that is, rudist shoal thief zone (RS-TZ), tidal channel thief zone (TC-TZ), and bioturbated-associated thief zone (BS-TZ). Both RS-TZ and TC-TZ are characterized by similar or higher porosity, much higher permeability, and much larger pore-throat radii than overlying and underlying adjacent non-thief zones. The RS-TZ comprises microfacies of rudist rudstone and rudist grainstone with interparticle pores, interparticle dissolved pores and vugs. It occurs in the upper-middle section of the uppermost rudist shoal in the shoal complex, shows the sheet-shaped horizontal distribution in the late highstand system tract of the field. The TC-TZ comprises microfacies of bioclastic grainstone with interparticle pores and few interparticle dissolved pores, and presents banded horizontal distribution. The BS-TZ went through strong bioturbation within hardground substrate leading to the great difference of microfacies, pore types and pore structures in burrows and matrix, and this type has lower porosity but much higher permeability than adjacent reservoirs. The key to developing the thief zones is to form the great permeability difference between the thief zone and its adjacent reservoirs. The origins of the three types of thief zones can not only be attributed to the primary porosity having well-connected pore structures resulted from high-energy sedimentary facies or high-energy sediments filling bioturbation-associated burrows, and freshwater dissolution in the eogenesis stage also enlarged the pores and pore-throat sizes and further improved pore connectivity, which enhanced the porosity and significantly improved permeability. Due to the difference in rock textures, pore structures and sedimentary successions, the three types of thief zones have different well log response characteristics which could be used for thief zone identification in non-coring wells and other oilfields that occurred the similar thief zones.