Lithofacies interpretation from Core Studies of a Middle Jurassic reservoir, North Usturt Basin, Kazakhstan: New Insights on the Reservoir Quality of the Caspian Sea Coast
Sanchez Mendoza, Juan D.; Novikov, Alexey
Middle Jurassic, Callovian, sandstones are one of the primary producing formations in the North Usturt Basin close to the Caspian Sea Coast, Republic of Kazakhstan. Detailed facies and rock property analyses on 206 m of core reveal a number of lithofacies controlling the oil saturation distribution on intra-field levels. This new facies concept of reservoir distribution will be applied to all existing wells in order to predict the most suitable areas for the oil field development.
The sedimentological analysis identified major "depositional process related" facies with the Skolithos and Cruziana Ichnofacies which together display environmental settings varying between lagoonal, backshore, tidal, coastal and shallow marine deposits. The variations in environmental settings are marked by a number of flooding surfaces (FS) subdividing the reservoir into four major sedimentological units (layers A, B, C and D). When using the main distinctive FS as a datum for correlation between the four wells, the facies analysis suggests that the cored succession represents part of a progradational sequence and a gradual change from a more mud-prone lagoonal (NW-SE oriented) system to a parallel oriented mainly sand-prone shoreface system.
Facies related to predominantly fine to medium grained argillaceous sandstones correspond to restricted marine conditions and are characterized by strong bioturbation. The predominantly fine grained sandstones show lenticular, wavy well defined lamination and flaser lamination with ichnofossils such as Lockeia related to tidal settings. Subenvironments such as lagoons and bays display petrophysical properties below the reservoir cut-offs, due to the presence of laminated-homogeneous shales and silts enriched by organic matter and pyrite as lithological components.
The best reservoir properties are seen in the foreshore-upper shoreface related facies of predominantly medium grained, moderately to well sorted, mainly structureless / homogeneous, in places parallel bedded sandstones with low diversity suite of ichnofossils including Ophiomorpha, Macarronichnus and Skolithos (Skolithos Ichnofacies).Tidal deposits also form good quality reservoir within the sandy intervals. Further laboratory investigations have displayed a firm correlation of sedimentary facies with main reservoir properties such as porosity and permeability.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013