Petroleum Geology of the Southern Pre-Uralian Foredeep with Reference to the Northeastern Pre-Caspian Basin

The southern Pre-Uralian Foredeep and the northeastern Pre-Caspian Basin of southern Russia and Kazakhstan are at the juncture of two major oil-producing regions, the Volga-Ural Basin and the new fields of the Northern Caspian Basin (e.g., Tengiz). The southern Pre-Uralian Foredeep has produced little oil; nevertheless, the Permian-Carboniferous stratigraphy and the general fold-thrust structure of the Pre- Uralian Foredeep, and adjacent Pre-Caspian Basin, afford the possibility for classic and largely untested sub-salt and sub-thrust plays.

Prior to the onset of Uralian orogenic activity, Late Devonian-Early Carboniferous rifting disrupted the East European continent, forming a series of rift basins including the Kama-Kinel troughs and the Pre- Caspian Basin. The Middle Carboniferous to Early-Middle Triassic Uralian Orogenic Belt consists of a complicated series of lower Paleozoic continental margin sequences, basement nappes, and accreted terranes, structurally interleaved via large-scale folding and thrusting. The orogen formed as a result of a progressive series of collisions between the East European continent and microcontinental plates and island arcs (the Tagil-Magnitogorsk and Eastern Uralian megazones), and the Kazakhstan and Siberian continents. N-S and W-E divisions of the Uralian Orogenic Belt and Pre-Uralian Foredeep reflect the basic tectonic structure of the orogen.

The Pre-Uralian Foredeep is not a simple flexural foreland basin, but the exact structural configuration is unresolved. In general, the regional stratigraphy and structure of the foredeep is more complicated than depicted in the literature and on published maps; the biostratigraphy critically needs to be updated. The foredeep developed as a series of regional depressions with up to fourth-order sub-basins. Within these sub-basins, both tectonic and eustatic mechanisms appear to control the sequence stratigraphy. Because of the tectonic influence, subsurface correlation based on sequence stratigraphic concepts may be valid only within each sub-basin. In part, the present structure of the Pre-Uralian Foredeep may reflect the structurally controlled Permian-Carboniferous paleogeography. This complex paleogeography also suggests that application of a simple “balanced cross-section” methodology could lead to erroneous results. Also unresolved are the paleogeographic, stratigraphic, and structural relationships between the Pre- Caspian Basin and the Pre-Uralian Foredeep.