Middle Jurassic Estuarine Systems of the Southeast Part of the Uvat Region, Western Siberia

Konstantin Zverev*, Vladimir Fedorcov, and Elena Chuhlanceva
Tyumen Oil Co, TNK-BP, Russia
*[email protected]

Middle Jurassic deposits of the Tyumen Formation represent a phase of active fluvial to coastal-plain sedimentation during a relative sea-level rise. The main reservoir facies of the southeastern Uvat region is a series of bay-fill delta deposits. A typical sequence comprises bay-head delta sandstones at the base, which represent lowstand to early transgressive deposits, and onlapping transgressive bay-fill mudstones. Fluvial and tidal coarse to fine-grained sandstones of the J5 and J4 units are up to 135 m in thickness and were deposited during a slow rise in sea level. The major architectural feature of the estuarine deposits is the superposition of the multi-lateral and multi-story sandstone complexes, being 20–40 m in thickness and inter-bedded with finer grained coastal-plain deposits. The channel amalgamation rate varied with cyclic sea-level variations, which controlled the sandstone-bodies geometry. The J4 sedimentation was followed by a decrease in sediment supply, which resulted in the deposition of fine-grained sediments of the J-3 unit during an increase in the rate of sea-level rise. Heterolithic strata of J3 unit are interpreted as coastal-plain and central-basin mudstones, and are typically finely laminated or lenticular-bedded, sometimes rooted and weakly burrowed. Thin sandstone are developed mainly as elongate, laterally restricted sandstone bodies embedded in finer coastal-plain deposits and mudstones of brackish transitional origin. Mud and mixed sand-mud inter-tidal flat deposits of J3 are overlain by differently burrowed fine- to middle-grained sandstones of estuary-mouth deposits of the J2 unit, which in the turn are erosively overlain by heavily burrowed fine-grained shoreface sandstones.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain