POST-SEDIMENTARY TRANSFORMATIONS OF THE UPPER JURASSIC CARBONATE ROCKS OF THE PRE-CARPATHIAN FOREDEEP OUTER ZONE IN THE CONTEXT OF THEIR OIL AND GAS CONTENT

Abstract

The manifestations of diagenetic and katagenetic processes in the Upper Jurassic carbonate rocks of the Outer Zone of the Pre-Carpathian Foredeep have been investigated and their influ-ence on the formation of the reservoir properties of rocks has been found out. During the diagenesis, the rocks were recrystallized, granulated and cemented. From the min-eralogical point of view, the changes consisted in the transformation of primary aragonite and magnesium-calcite skeletal remains of organisms into calcite, as well as the processes of dolo-mitization, ferruginization and silicification. At the stage of diagenesis, Fe-containing minerals – glauconite and pyrite – have been formed. The dolomitization of Jurassic organogenic limestones of the Pre-Carpathian Foredeep Outer Zone has been caused by the mixing of fresh meteoric wa-ters with buried marine pore waters in the underground phreatic zone adjacent to the ancient coastline. Silicification is a consequence of the metasomatic substitution of the carbonate sub-stance by silicate, which has been caused by a decrease of pH occurred after dolomitization. Katagenetic transformations of the rocks are manifested in the dissolution of the remains of organisms, grains of carbonate cement and late cementation of pores and microfractures. Changes of carbonate rocks are associated with the introduction of certain substances in the sediment (sulphatization, celestitization) or their removal (dedolomitization, decementation). Neogenic minerals of the katagenesis stage are represented by anhydrite and celestite. The processes of recrystallization of carbonate rocks at different stages of katagenesis con-tributed to the selective leaching of limestones and dolomites and led to the formation of secon-dary cavities and caverns, different in shape and size. The predominance of cavities of a certain type determines the type of reservoir, among which are distinguished pores, pore-caverns and joint-caverns. The processes of diagenesis and catagenesis are associated with the dissolution of carbonate material, which differently affects the reservoir properties of sediments. Authigenous mineral formation, with the exception of dolomitization, impairs the reservoir properties of the Upper Jurassic carbonate rocks, and recrystallization improves. Burial of carbonate rocks at depth contributes to the preservation and even improvement of their reservoir properties both due to dissolution and textural heterogeneity (jointing develops at the boundary of the elements of textural heterogeneity). Observations show that the most favour-able in this respect are organogenic and detrital varieties of carbonate rocks. The presence of clay minerals in the composition of their cement increases the textural heterogeneity and, as a conse-quence, the ability to formation of joints.