New Sedimentological Data within the Minagish Formation: Insights from Undeveloped Area, Onshore Kuwait

Abstract

The Lower Cretaceous Minagish Formation forms one of the most prolific oil reservoirs in onshore Kuwait, with key reservoir units located in thick oolitic grainstones forming the lower half of the Middle Minagish Member. Within the central and eastern part of Kuwait, the upper part of the Middle Minagish consists of inner ramp skeletal packstones, that are variably cemented, for which the reservoir potential is poor. Following exploration campaigns carried out by KOC in the unexplored south-western area of Kuwait, unusual oil staining has been observed in the uppermost part of the Middle Minagish Member highlighting a potential new hydrocarbon play. As part of a multiwell study investigating the Middle and lowermost Upper Minagish Members cored in south-western Kuwait, this work focuses on understanding the occurrence of hydrocarbon stained deposits within the uppermost Middle Minagish and assesses their extent. The study aims to characterise the sedimentological make-up and analyse the field-scale depositional organisation to assist in the prediction of reservoir architecture. A facies analysis has been carried out and has resulted in the interpretation of the depositional environments. The key surfaces characterised in core along with openhole log data have helped in the interpretation of a sequence stratigraphic framework across both the reservoir and non-reservoir units. The facies analysis and vertical facies evolution across the cored Minagish succession suggest deposition in an intertidal to marginal/proximal mid-ramp setting, with the development of oolitic geobodies both in a marginal shoal corridor and sand bars formed in a more landward position on the inner ramp. The more proximal sand bars are typically recorded in the south-west Kuwait area and are not present further towards the east (e.g. Minagish and Umm Gudair Fields; Davies et al., 2000; Davies et al., 2001). The vertical depositional organisation indicates that the Middle Minagish is part of a large-scale regressive trend, characterised by the upward evolution from marginal inner ramp/proximal mid-ramp oolitic grainstones to skeletal packstones terminating with deposition of internal oolitic/carbonate sand bar to locally intertidal deposits. Hydrocarbon staining is observed where internal oolitic/carbonate sand bar/intertidal deposits are capped by key surfaces, locally associated with dissolved cavities suggesting exposure. These surfaces are interpreted as a sequence boundary above which a distinct change in deposition from clean carbonates to clay/organic matter-prone deposits (Upper Minagish) occur. The well correlation highlights a condensation of the sand bar/intertidal deposits in the south-western part of the study area, which is a result of a progressive decrease in accommodation at the end of the regression that is amplified in the landward position of the system. The work has been carried out prior to the establishment of the reservoir architecture and has helped in the prediction of rock type distribution.