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AAPG Geoscience Technology Workshop

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The Diagenetic History of the Jurassic Section in the Southeastern Levant Margin, a Potential Reservoir

Abstract

The recent gas discoveries in Tertiary strata of the Levant Basin of the Eastern Mediterranean stimulated new interest in potential deeper hydrocarbons reservoirs sourced in Mesozoic rocks. Seismic and well data from the Levant margins indicate an assortment of potential structural and stratigraphic oil-traps offshore Israel, while others suggest that Mesozoic oil-prone source rocks have reached thermal maturity in the offshore. Several offshore and onshore boreholes showed oil marks and oil accumulation in the Jurassic section, especially in the Barnea, Sderot and Zohar formations, which indicate the presence of source rocks and hydrocarbon generation. The current study focuses on the diagenetic evolution of potential reservoir rocks within the Jurassic sedimentary section along the Levant margin in Israel in order to track evidence for the accumulation of hydrocarbons within these rocks. One of the most useful tools for the reconstruction of the diagenetic history and burial path of sedimentary rocks is a paragenetic sequence study. Guided by lithostratigraphic description and electric logs, sampling efforts targeted rock cuttings and drill cores in order to choose the best intervals for a diagenetic study of potential reservoir rocks. Forty three samples were collected from four offshore and six onshore boreholes, which have penetrated the Jurassic strata. The samples have been petrographically studied using optical polarizing microscope, Scanning Electron Microscope (SEM) and specialized cathodoluminescence microscope (CL) to determine facies-types, sedimentation environments, cement phases and their chemical composition. Oxygen and carbon isotope ratios of carbonate cement minerals have been measured to determine the isotope composition and temperatures of formation water, which were further interpreted in terms of depositional environment, hydrocarbons connection and possible burial paths of the host rocks. Lithofacies of the studied strata indicate shallow marine sedimentation environment throughout Jurassic times. The majority of the rocks sampled are limestones, with sandy intervals in places. Massive pore-filling carbonate cement is present in most of the samples, and seems complete. Though dissolution phases occurred, the formed cavities had been cemented repeatedly with calcite, dolomite and silica, not allowing any major porosity to develop. Four authigenic carbonate minerals were observed: calcite, siderite, ankerite and dolomite. Calcite is the most common cement mineral, and appears in different phases, mainly of eogenetic formation to late fracture filling cementation. Siderite is of eogenetic stage, while dolomitization is mainly mesogenetic. Jurassic rocks had experienced most of the cementation during early burial stages, in Middle Late Jurassic to Early Cretaceous times, and prior to the Middle Cretaceous oil related cementation found at the Heletz-Kokhav oil field. Most pore-filling calcite cementation have been precipitated in mostly marine waters and in low temperatures, up to about 40°C. Deep burial siliceous cementation occurred in the offshore, precipitating quartz and oligoclase. Deep burial saddle dolomitization occurred especially in Zohar Formation offshore, possibly by hydrothermal activity; whereas deep burial dolomitization onshore occurred in Early Jurassic rocks at the time of maximum burial temperatures, in post Cretaceous times. Hydrocarbons, equivalent to those drilled in offshore Jurassic strata, are suggested to occur within larger dissolution cavities and in macro-fractures systems, which were not filled with cement, and less typically within porous rocks.