Concretions in Akçakoca Sandstone Member of Kusuri Formation (Lower-Middle Eocene), Western Black Sea

Abstract

Akçakoca Sandstone member of Kusuri Formation is a channel fill sediments which were deposited in submarine fan system by turbidite mechanism during Early and Middle Eocene in the Western Black Sea, Turkey. They consist of channel axis (massive sandstone), channel margin, levee and their complex sequences. Submarine fans are important in the petroleum industry for their reservoir and source rock potential. In the Akçakoca region, natural gas has been produced from Akçakoca sandstones since 2007. In this region, the massive sandstones with various dimentions of concretions were deposited in the submarine fan system. These concretions have spherical, ellipsoidal and flattened ellipsoidal in shape and have variable diameters ranging between 0.1-2 m. Petrographically, they are classified as feldspathic litarenite (generally) and lithicarkose. The sandstone concretions generally are represented by clean sands (sand/clay ratios>=7:1; 9:1), and show poorly or moderately sorting. Whereas encompassed sandstones (host rock) are texturally (sand/clay ratios>=7:3; 9:5 ) and compositionally immature as indicated by significant feldspar, clay, volcanic, sedimentary and metamorphic rock fragments due to close provenance and rapid transport by short rivers and tectonically controlled basin margin. The massive sandstones are generally structureless, only having amalgamation surfaces but underlying unit is rich in bioturbations and dewatering products such as dish and ball-pillow and flame structures that indicate middle and upper fan deposition respectively. The environmental features imply that the concretions in the massive sandstones had resulted from dewatering process/soft sedimentary deformation that caused by remove of the clay matrix as suspension and subsequently precipitation of calcite cement or they were independently formed after rock formation in the late diagenetic period, resulted from ground water lenses involved precipitation of calcite cement, that occupied the intergranular and interparticular open spaces, leading to significantly reduced porosity and permeability of the massive sandstones. Based on the XRD analysis, calcite ratio is between 14% and 38% in the concretions. Porosity and permeability values of the host sandstones are generally high (5.6- 30.9% porosity, 0.02-320.67 md permeability). In conclusion; host sandstones with the high porosity and permeability values can be good reservoirs, but concretions within the host rock have not any reservoir features because of the low porosity and permeability values (3.2-7.8 % porosity, 0.02-0.09 md permeability). The concretions, embedded within the host rock, do not constitute the reservoir barrier and may have not a negative impact on the Akçakoca sandstones.