Stratigraphic Analysis of the Nahr Umr Formation in the Luhais Oil Field, South Iraq

Abstract

The Nahr Umr Formation, which is Albian-Early Turonian in age, was investigated in the Nahr Umr Formation succession at two wells of the Luhais oil field in southern Iraq. The study defined sedimentary facies, types, and distribution of porosity and their relationship to sedimentary facies, as well as different types of diagenetic processes. It also included the distribution of facies in the Nahr Umr Formation in relation to suggested depositional settings. Thin sections of the cores from the two chosen boreholes (Lu 39 and Lu 43) in the Luhais Oil Field were used to study the petrography and lithofacies of the Nahr Umr Formation. The five fundamental facies identified in this study are poorly and well-sorted, quartz arenite sandstone, in additional to clayey sandstone poorly sorted, sandy shale, and shale, it represented many sub-environments present in the delta plain depositional environment, the distributary channel depositional environment, and the delta front depositional environment. The Nahr Umr succession in the examined wells (Lu -39 and Lu-43) in the Luhais oil field, southern Iraq, has three asymmetrical cycles of the third order, which are identified by the variable frequency as well as the symmetry of the relative sea level. They show successive events involving sea-level rise and stillstand. In the study area, the succession is formed by surface erosion, which represents the boundary of the sequence type 1 (SB1) distinguished in all wells of the study. As a result of the significant drop in the relative sea level during the stage of the marine retreat (regressive), which indicates coarsening-shallowing upwards and with continuous decreasing, a sequence boundary of type 2 (SB2) is formed at the end of this tract. The boundary of SB2 represents the decrease in sea level, indicating the beginning of the second stratigraphic sequence, which shows the deposition of sand in the river channels as it reflects progressive accretion patterns towards the basin represented by (MFST. Then the sediments begin to thin, and the shale layer appears at the Nahr Umr Formation top, which reflects the regional marine progress and marks the sedimentation of the limestone formation that separates it from the Nahr Umr Formation on the regional flood surface.