Organic Geochemical and Petrographical Characteristics of the Major Lower Cretaceous Petroleum Source Rock (Makhul Formation) in Kuwait - Arabian Gulf
Keywords:Cretaceous, Petroleum geology, Makhul Formation, organic geochemistry, source rocks.
The Lower Cretaceous Makhul Formation is one of the major petroleum source rocks in Kuwait. It is composed of dark grey to black highly bituminous, pyrite-rich limestone with average thickness of 220 m.
The purpose of this study is to evaluate the Makhul source rock for its organic matter richness and its relation to the rock composition and depositional environment. A total of 117 core samples were collected from five wells in Raudhatain, Ritqa, Mutriba, Burgan and Minagish oil fields north and south Kuwait. The rock petrographical studies were carried out using transmitted and polarized microscope, as well as, SEM and XRD analyses on selected samples. Total organic matter TOC and elemental analyses were done on isolated kerogen and followed by identification of kerogen type optically. Solvent extracts were analysed using GC and GC-MS as well as carbon isotope ratio.
The results of this study show that at its earliest time the Makhul Formation was deposited in an oxic shallow marine shelf environment. During its middle part of deposition, the water oxicity level was fluctuating from oxic to anoxic condition as a result of changes in sea water level. At the end of Makhul and the start of upper Minagish Formation, the sea level raised forming oxic open marine ramp depositional condition.
Organic geochemical results show that the average TOC of the Makhul Formation is 2.39% wt. High TOC values of 6.7% wt. were usually associated with the laminated mudstone intervals of the formation. The kerogen is type II and is dominated by marine amorphous sapropelic organic matter with a mixture of zoo- and phytoplankton and rare terrestrial particles.
Solvent extract results indicate non-waxy oils of Mesozoic origin that is associated with marine carbonate rocks. The formation is mature and at its peak oil generation in its deepest part in north Kuwait.
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