Critical Casing Points: Identification of Najmah Top and Sublayers with Wellsite Inorganic Geochemistry in Kuwait Carbonate Reservoir
Authors: Dashti Jalal , Al-Awadi Mashari , Moshref Moustafa , Shoeibi Ahmad , Pozzi Alessandro , Estarabadi Javad 
Published: October 2018

The Middle Jurassic strata of the NE Arabian Plate compose part of the largest world-class petroleum system, with more than 250 billion barrels of proven hydrocarbons. The Najmah Formation, one of those productive strata located in Kuwait, represents a transgressive deposition within a deep basinal settings and anoxic environments; with its black shales interbedded with bituminous limestones the Najmah Formation works as both reservoir and source rock. Due to its organic richness and maturity, the middle Jurassic formation can be considered the best potential conventional/unconventional play in the Kuwaiti Province.

Evaporates of Gotnia, a HPHT formation overlaying the Najmah reservoir, are dealt with high mud weight (19-21 ppg), to counter the high pressured patches. The identification of Najmah stratigraphic top is crucial for setting the casing point, then reducing the mud weight for the final drilling phase. Missing this critical casing point may lead to several rig NPT and related operational cost increments, such as cement jobs and, in extreme cases, may lead to missing and abandoning the well. When the standard investigation methods, as the optical microscopy, or Gamma Ray failed in identifying the Najmah top, due to the similarity between its limestones and those of Gotnia Formation, the ED X-ray fluorescence (XRF) and X-ray diffraction (XRD) established distinctive formations geochemical ‘fingerprints’, as well as their sedimentary patterns, providing absolute certainties about the casing point position despite a misleading stratigraphy.

The technique of Chemostratigraphy, applied in this study on five exploratory wells, can increase the value of such geochemical fingerprints, providing not only applications as critical casing points ID but also a means to unify stratigraphic schemes, i.e. develop stable reference stratigraphic frameworks: changes in rock geochemistry reflect changes in the relative sea level, thus sediment supply/accommodation, oxygenation and diagenetic conditions.

Once inside the Najmah Formation, the elemental and mineralogical patterns point out different formation sublayers, corroborating many sedimentological and stratigraphic evidences obtained from outcrops and cores analyses. Some redox-sensitive trace metals are delivered to the sediment in presence of organic matter (Ni, Mo, V and U) under anoxic-euxinic conditions and tend to exhibit covariation with TOC, highlighting the best pay zones in the Najmah Kerogen sublayers. Some other metals such as Mn, Fe and Zn, in carbonate sequences, can evaluate the amount of carbonate cement (sparite) among the microcrystalline matrix (micrite); such metals, correlated with mud gas concentration, reveal the most porous sections within calcareous sublayers.

Having access to more detailed rock properties allow for one time decisions making, such as the identification of casing/coring points and the characterization of a reservoir in all its sublayers. Chemostratigraphy has led the operational team to minimize NPT and related costs, the completion team to the right well profiles and the production team to a better overview of the reservoir.