Lithogenetic preconditions of reservoirs and reservoir rocks formation in the Middle Devonian sediments of the Skhidnosaratske deposit (Dobrudja Foredeep)

Abstract

The lithological features of the Middle Devonian sediments of the Skhidnosaratska structure of the Dobrudja Foredeep in the aspect of their influence on the formation of reservoir rocks and hydrocarbon reservoirs have been considered. Nine lithocycles of regressive nature (e-1 – e-5; g-1 – g-4), characterized by a two-membered structure have been identified in the section. Each lithocycle represents a separate productive horizon. The lower parts of the latter are composed of porous and fractured reservoir rocks (limestones, dolomites, siltstones, sandstones), and the upper parts are composed of fluid-resistant packs (marls, anhydrites).Features of cyclic sedimentation conditions caused spatial-age variability of trap localization and morphology, nature of distribution of reservoir rocks of various types, and post-sedimentation processes caused spatial heterogeneity of petrophysical properties of rocks, which entailed different prospects for individual areas of the Skhidnosaratske deposit. The vaulted traps of the productive horizons of the Eifelian deposits tend to the area of borehole 2, and the Zhivetian deposits to the borehole 1. Consequently, in the first case, the most perspective horizons are e-2 and e-3, in the second – g-2 – g-4. Often more powerful packs of better-quality reservoir rocks are developed outside the vault, which leads to the formation of lithological traps in the e-1 and e-2 horizons. It has been shown that the complex structure (presence of layers of carbonate and terrigenous rocks) of fluid-resistant marl-sulfate packs could cause dispersion of hydrocarbons and thus reduce the prospects of individual horizons, in particular, e-2 and e-3. The established heterogeneity of the structure of the productive horizons, the pinching out of individual packs of both pore and fractured reservoirs, the spatial uneven development of the limestones anhydritization and dolomitization processes give possibilities to suggest the formation of separate deposits with different hypsometry of water-oil contacts.