Eocene Volcaniclastics in the Kartli Basin, Georgia: A Fractured Reservoir Sequence

Abstract

In the broader Caucasus region, multiple extrusive volcanic units are present within the Jurassic, Cretaceous, Eocene and Miocene sedimentary successions. Partial reworking of volcanic material from various provenance areas into Eocene, Oligocene and Miocene reservoir units is commonly observed in the Eastern Black Sea and in the Rioni, Kartli and Kura Basins of onshore Georgia. Reservoir quality has in general been negatively affected by volcanic rock fragments which may have undergone complex diagenetic alteration. However, despite concerns regarding reservoir quality, oil at the Samgori field, the largest field in Georgia (~200 MM brl recovered), is hosted in altered Middle Eocene volcaniclastic sandstones interbedded with deep-water turbidites. Previous studies of core material from numerous wells in this field showed that most of the oil is contained in altered, microfractured, laumontite-rich tuffs which have fracture and cavernous net porosities averaging 12% and average permeability of 15 mD. The laumontite tuffs comprise only up to 20% of a tuffaceous sandstone section and occur as isolated lenses or pods on a sub-seismic scale (i.e. 5-10 m thick), causing highly variable oil productivity from one well to another.The petrographic analysis of samples of Middle Eocene volcaniclastic sandstones from outcrops in the central part of the Kartli Basin around Tbilisi broadly confirms the main conclusions of studies completed some 30 years ago which were based on the analysis of subsurface samples. However, the surface samples analysed show that zeolitization events typically did not improve, but actually reduced, reservoir quality due to extensive zeolite cementation. The poor reservoir properties of the plug samples, which are age-equivalent to the proven subsurface Middle Eocene reservoir interval, highlight fracturing as a key factor controlling the presence of exceptional producers (up to 9000 b/d) in the Samgori field complex. The study therefore underlines the critical role of fracturing of the Middle Eocene volcaniclastic reservoir sequence in the Kartli Basin.

Diagenetic Evolution Mechanism of the Jurassic Tuffaceous Sandstone Reservoir in Qikou Sag, Bohai Bay Basin, East China

“…Khalaf showed that volcanic ash dispersed in the Neoproterozoic terrestrial sedimentary rocks of the Wadi Queih Basin formed chlorite coatings that contributed to the preservation of pores [23]. Lenhardt et al and Tari et al showed that studies on the reservoir of volcanic and associated sedimentary rocks were mainly conducted in China [24,25], and a literature review by the authors showed that studies on the effect of dispersed pyroclastic on sandstone reservoirs were mainly conducted in China. The findings of studies from China showed that the influence of dispersed pyroclastic on sandstone reservoirs was reflected in the following two aspects: the coupling of pyroclastic and acid fluid controls the formation of secondary pores, and the devitrification of volcanic glass strongly affects reservoir formation [26][27][28][29].…”