Publisher Summary The chapter uses conclusions of previous chapters, and briefly describes characteristics of the oil and gas bearing formations in the South Caspian basin. The main oil and gas bearing formation in the Eastern Azerbaijan and the offshore areas of Apsheron and Baku archipelagoes is in the middle Pliocene productive series, based on the characteristic mineral composition of both light and heavy minerals. The productive series is separated into two divisions, lower and upper, and into several suites according to lithology. Core data, paleontological studies, and log response suggest that sediments of the productive series were deposited in a shallow-water, fluvial-deltaic environment. The productive series is again divided into seven sedimentary sequences according to the transgression and regression cycles during the basin development. Reservoir-rock properties of the productive series, which are very good, vary both within the section and over the area. Paleogene to Neogene argillaceous rocks, which are widespread in the geologic section of the Azerbaijan and the South Caspian basin, make up to 50 to 95% of the section and play a key role in establishing lithologic, mineralogical, geochemical, and thermobaric characteristics of the basin. Undercompaction of argillaceous rocks, even at depths down to 6.5 km, is explained by the comparatively young age, a high sedimentation rate, their great thickness, and incomplete squeezing-out of pore water. Development of abnormally high-pore pressures may lead to the lateral variation of rock density, and under certain geologic conditions to folding, clay diapirism, mud volcanism, and earthquakes.