ABSTRACT

Najmah Formation is a Middle Jurassic tight, fractured formation, spread across many fields in Kuwait. The existing conventional name is Najmah Shale but it could be better called Najmah Mudstone as the unit comprises mainly carbonates with abundant kerogen and bituminous matter.

The entire formation is interesting for its prospectivity and it is cored in different locations in Kuwait in several wells, since its reservoirs are often vertically and laterally heterogeneous due to depositional variability and diagenetic alteration through space and time.

The bituminous intervals, often encountered drilling the Najmah, can induce damaging effects on oil recovery such as reduction of total reservoir porosity, creation of permeability barriers, diminution of water driving efficiency, modification of fluid circulation and rock wettability. Drilling challenges associated with bitumen presence may also arise.

While fundamental questions still remain unclear and controversial, such as how bitumen behaves at in-situ conditions (high stress and high temperature), what shape is bitumen formation, what mechanisms drive bitumen into wellbore, the current consensus is to avoid bitumen intervals as much as possible.

This paper describes how geochemical data allowed for: the characterization of the organics present in the main reservoir, the control of the presence of bitumen and the estimation of its amount in key cored well samples, and the establishment of a methodology to tentatively extend the previous results to non-cored wells.

Both organic and inorganic geochemical investigations were employed, namely Total Organic Carbon (TOC), Pyrolysis analysis, Thermal Desorption Gas Chromatography (TD-GC), X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD).

Pyrolysis analysis can distinguish between free oil (the S1 peak) and heavy organic matter resulting from thermal cracking (the S2 peak), but it cannot differentiate the kerogen from the bitumen, being both included in the S2 peak.

However kerogen is, by definition, insoluble in all organic solvents. This chemophysical property helped in the correct identification and isolation of this range of organic compounds: a long cycle treatment in a Soxhlet extractor using a high-polarity solvent and a low-polarity one guaranteed an efficient extraction of all the soluble components, including the soluble part of bitumen.

The comparative Pyrolysis, i.e. the comparison of the original analysis with the second, post-treatment, analysis revealed the amount of the removed organic matter.

An estimation of bitumen solubility, based on previous analysis on pure bitumen samples, allowed the assessment of total bitumen amount.

An analogous procedure was set up for cuttings samples and its feasibility proved to depend on the type of drilling fluid used while drilling.

This study represents the first integrated characterization of a formation working both as source rock and reservoir rock with the inclusion of comparative Pyrolysis in order to determine the presence of bitumen.

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